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Numerical Simulation Study on the Microphysical Characteristics of Stratiform Clouds with Embedded Convections in Northern China based on Aircraft Measurements
2020,44(5):899-912, DOI: 10.3878/j.issn.1006-9895.1908.19114
Abstract:To characterize the microphysical characteristics and transformation process of stratiform clouds with embedded convections, a study was performed using the WRFV3 model and based on two aircraft measurements taken on May 1, 2009. The aircraft observation results showed that significant differences in the shapes and formation process of ice particles existed between the regions of stratiform cloud and embedded convection. Compared with the embedded convection region, the stratiform cloud featured more complicated shapes of ice crystals, including needle column, capped column, and dendrite types. However, the dendrite-type ice crystals dominated in the embedded convection region, and their growth was controlled by aggregation and riming processes. Overall, the results indicated that the basic characteristics of this stratiform cloud with embedded convections simulated by the WRF model agreed well with the aircraft observations, including cloud distribution, LWC, and numerical concentration on the flight route. The simulation results showed that in the stratiform cloud, with higher cloud water content and larger W, embedded convection could be developed because of a strong riming process. The hydrometeors of snow, graupel, and rainwater in the clouds accounted for 51.9%, 31.0%, and 16.0%, respectively, while cloud ice and cloud water accounted for very little. In the higher level, snow and graupel grew through deposition process. In the lower level, they grew through the riming process and melted into rain. Stratiform clouds with lower cloud water content and smaller W would remain as stratiform cloud. The hydrometeors of snow, rainwater, and cloud ice accounted for 90.4%, 6.1%, and 3.5%, respectively. The ice and snow grew through deposition process and melted into rain in the lower level.
2020,44(5):913-934, DOI: 10.3878/j.issn.1006-9895.1911.19132
Abstract:The real-time forecast accuracies of summer precipitation in Northeast China (NEC) from 1978 to 2018 were significantly low. Moreover, in the recent four years, when the prediction of the overall distribution of summer precipitation anomaly for the whole country was relatively accurate, the prediction of summer precipitation anomaly in NEC was contrary to the actual situation. Therefore, analyzing the cause for the low forecasting accuracy is necessary. In this paper, the forecasting ability of dynamic models and cognitive defects on forecasting summer rainfall in NEC are discussed. Moreover, by systematically reviewing the climatic characteristics, influencing factors, prediction methods of summer drought and flood in NEC, and the real-time forecasting skills, the causes have been obtained as follows: (1) The precipitation in early summer in NEC is mainly affected by the cold vortex activity in NEC, and in midsummer, it is mainly affected by the subtropical high in the West Pacific, the southerly wind in the Northeast, and the circulation pattern in the middle and high latitudes. However, the main dynamic climate models commonly used at home and abroad cannot accurately reflect the key circulation systems associated with precipitation in early summer and midsummer in NEC. (2) The relationship between the summer rainfall in NEC and the global sea surface temperature (SST) is weak and unstable. Especially, the influence of El Niño-Southern Oscillation (ENSO) on summer precipitation in NEC is complicated; the relationship between them varies from decade to decade. (3) The summer rainfall in NEC has remarkable multi-timescale variability (inter-seasonal, inter-annual, and inter-decadal timescales), influenced by different circulation systems, which makes accurately predicting summer precipitation in NEC more difficult. Finally, some scientific problems and possible solutions regarding summer rainfall forecasting in NEC are further discussed, which may be helpful for the future summer rainfall predictions in this area.
Numerical Investigation of the Effects of Boundary Layer Parameterization Schemes on Typhoon Meranti (1614) Landing Process
2020,44(5):935-959, DOI: 10.3878/j.issn.1006-9895.2004.19135
Abstract:To study the effects of different boundary layer schemes on the simulation of landing attenuation stage of typhoon Meranti (1614), a series of high-resolution (1.33 km) numerical tests were carried out using seven boundary layer parameterization schemes in the mesoscale numerical model WRF v3.8, namely, YSU, MYJ, QNSE, ACM2, UW, GBM, and Boulac, in terms of movement track, intensity, structure, rainfall, and near-surface physical variables. The results indicate the following. First, boundary layer schemes significantly influenced the simulation of typhoon Meranti’s track, intensity, and rainfall during its landing attenuation stage, and the maximum differences in the 24-h simulated typhoon track, lowest atmospheric pressure, maximum wind velocity, and 24-h cumulative rainfall extremum were 80 km, 11 hPa, 27 m s-1, and 241 mm, respectively. Second, simulation results of the Boulac scheme showed a typhoon track that is closest to real-time results, followed by GBM, YSU, and MYJ schemes, and then by ACM2 and UW schemes, whereas the QNSE scheme displayed the worst simulation. Meanwhile, the UW and QNSE schemes simulated the lowest atmospheric pressure values, and MYJ and QNSE schemes simulated the maximum wind velocity values that are closest to actual observations. All boundary layer schemes simulated the features of the typhoon. For example, the lowest atmospheric pressure increased gradually during the landing stage, and the rate of such increase after landing was greater than that before landing, which agreed with real-time results. However, the increasing rate of the lowest atmospheric pressure before the typhoon landing that was simulated by each scheme is greater than the real-time result, whereas such increasing rate after typhoon landing is less than the real-time result. Third, the Boulac scheme best simulated the 24-h precipitation distribution, heavy precipitation area, structure, intensity, and TS score of precipitation at each level, whereas the MYJ scheme was the second best. As simulated by QNSE, UW, and ACM2 schemes, the rain belt advanced so quickly northwestward that the TS scores of precipitation at various levels were poor. Fourth, in the overall simulation of track, intensity, and precipitation of the typhoon, Boulac and MYJ schemes showed optimal results, in which the Boulac scheme was superior in simulating the typhoon track and precipitation and the MYJ scheme was superior in simulating typhoon intensity. The YSU and GBM schemes had the second best simulation results, whereas QNSE, UW, and ACM2 schemes had worse simulation performance. Moreover, the boundary layer schemes significantly differed in calculating the latent heat flux and sensible heat flux of near-surface layer, thereby affecting the simulation of typhoon track, intensity, and rainfall, leading to significantly different simulation results. The QNSE scheme resulted in an abnormally high latent heat flux, the MYJ and Boulac schemes resulted in the most modest values, and other schemes resulted in slightly smaller values. On the other hand, the QNSE scheme had a slightly higher sensible heat flux, the MYJ scheme showed the most modest one, and other schemes resulted in significantly smaller values. Finally, the boundary layer schemes significantly differed in the simulated thermal and dynamical structure of boundary layer, and Boulac scheme had the obvious advantages, particularly for the structure of boundary layer in daytime.
Contribution of Tropical and Subtropical Circulation Anomalies to the Intensity of East Asian Winter Monsoon over Lower-Latitude Region
2020,44(5):960-974, DOI: 10.3878/j.issn.1006-9895.1911.19141
Abstract:The East Asian winter monsoon (EAWM) has two dominant modes: the in-phase and out-of-phase variations of wind anomalies over northern and southern China. Different from the first mode reflecting a uniform strong/weak situation of the EAWM throughout eastern China, the second mode indicates a situation that the intensity of low-latitude EAWM over southern China is independent of and even opposite to that of mid–high-latitude EAWM over northern China. The present study focuses on the characteristics of tropical and subtropical circulation anomalies associated with the variability of low-latitude EAWM under the background of the second mode by using empirical orthogonal function analysis, correlation analysis, and partial correlation analysis. The results reveal that the inter-tropical convergence zone (ITCZ) can be considered as an important circulation system that contributes to the variation of low-latitude EAWM. Corresponding to a stronger and northward-extended ITCZ, convective ascensions over the tropical western Pacific and South China Sea are strengthened. This anomalous ascension may induce low-level anomalous northerly wind, thus resulting in a stronger low-latitude EAWM. In addition, the subtropical upper-level jet can be regarded as another important circulation system affecting the low-latitude EAWM. An increase in wind speed along the axis of the jet may cause anomalous northerly quasi-geostrophic winds near the jet entrance. Associated with the forced positive secondary circulation anomaly with anomalous descent (ascension) to the north (south) of the jet, the low-level anomalous northerly wind appears under the jet, which in turn, facilitates a stronger low-latitude EAWM. Finally, both the individual and joint effects of tropical convective activities and upper-level subtropical jet on the low-latitude EAWM are further investigated. Relatively, the influence of the ITCZ seems more important. When the two circulation anomalies simultaneously increase (i.e., more active convective activity of the ITCZ and stronger wind speed along the subtropical upper-level jet), their joint effect can significantly reinforce northerly winds to the south of 35°N over southern China and vice versa. The abovementioned results imply that the variability of low-latitude EAWM is not only affected by cold air surges from northern China but also modulated by the joint effects of tropical and subtropical circulation anomalies.
A Vertical Second-Order Difference Scheme for Non-uniformly Distributed Layers and Its Application in GRAPES Model
2020,44(5):975-983, DOI: 10.3878/j.issn.1006-9895.1906.19145
Abstract:In the case of non-uniformly distributed layers, the vertical difference scheme used in GRAPES (Global/Regional Assimilation and Prediction System) model can only achieve first-order accuracy. A second-order scheme was designed and introduced into the GRAPES model in order to be used in the process of vertical discretization. An ideal test using the 1-D profile showed that the new scheme can improve the accuracy of difference computation obviously. An ideal density flow test was conducted to verify the correctness and stability of the new scheme in GRAPES model. A statistical evaluation of a medium-range forecast using the second-order scheme showed an improvement of forecast skill in large-scale fields, especially for the forecast after 120 h. Additionally, the second-order scheme was tested with a real-case experiment based on the extreme rainfall at South China, which again showed an improvement in the forecast of precipitation after 48 h.
Influence of Stochastically Perturbed Parameterization on Ensemble Forecasting of Winter Precipitation in China
2020,44(5):984-996, DOI: 10.3878/j.issn.1006-9895.2001.19157
Abstract:Precipitation ensemble forecasting is characterized by great uncertainty, and the uncertainty of the parameters in the physical that is closely related to the precipitation forecast is one of the sources of its numerical prediction error. As a frontier research field in international ensemble forecasting, the stochastically perturbed parameterization (SPP) method has been developed to address the uncertainty of representative model precipitation forecasts. To determine whether this method can reflect the uncertainty of numerical predictions of winter precipitation in China and provide a scientific basis for business applications, we used the China Meteorological Administration’s Global/Regional Assimilation and Prediction System (GRAPES) mesoscale regional ensemble prediction model and selected 16 key parameters from four parameterization schemes. These parameters, e.g., cumulus convection, cloud microphysics, boundary layer, and near-surface layer, greatly influence the uncertainty of model precipitation forecasts. In this paper, we introduce the stochastically perturbed parameterization (SPP) method and describe the results of an ensemble prediction experiment conducted from December 12, 2018 to January 12, 2019, a total of 31 days. We compare and analyze the effect of the SPP method on the winter weather situation and precipitation ensemble prediction. The results show that with the addition of a test for the SPP method, the results of probability prediction techniques for precipitation and isobaric elements are better than the control predictions without the SPP method, and the improvement of low-level and near-surface elements is better that of the iso-surface elements in the middle or upper floors. The precipitation prediction results obtained superior scores to those of the control prediction test, but because the improvement did not pass the test of significance, the differences were not statistically significant. The above results indicate that under the influence of the East Asian winter monsoon, the SPP method demonstrates no obvious improvement on the current prediction technique used for winter precipitation in China. The reason for this may be that the SPP method mainly represents the uncertainty of convective precipitation forecasting, whereas the winter precipitation process in China is mainly one characterized by the development of baroclinic instability. Because model precipitation is dominated by large-scale grid precipitation, and less convective precipitation, improvement in the winter precipitation forecast is not obvious. Thus, there is a scientific basis for applying the SPP method to the operation ensemble forecasting model.
Spatial Observation of Red Sprites over a Winter Mesoscale Convective System in North America and the Analysis of Its Parent Thunderstorm
2020,44(5):997-1012, DOI: 10.3878/j.issn.1006-9895.2002.19169
Abstract:Red sprites are large-scale transient luminous events (TLEs) that usually occur between about 40 and 90 km altitudes above thunderstorms, and they are caused by cloud-to-ground (CG) lightning strokes and subsequent continuous current. Compared with studies that focus on sprites that occur in summer, those focusing on winter sprites are fewer due to limited comprehensive synchronous observation data. Influenced by the upper trough and warm, moist airflow at low level, a thunderstorm occurred in Arkansas, North America, on December 27–28, 2008. The Imager for Sprites and Upper Atmospheric Lightning (ISUAL) aboard the FORMOSAT-2 satellite could record two red sprite events. Using the red sprites optical observation data obtained by ISUAL, Doppler weather radar data, National Lightning Location data, ultra-low frequency magnetic field data, and cloud-top brightness temperature data provided by the National Environmental Center/Climate Prediction Center of the United States and the sounding data, this paper presents a detailed study of the characteristics of the winter thunderstorm that produced the red sprites and the related lightning activity. The results show that ISUAL did not record the halo that accompanied the two red sprites. The first was a columnar sprite, and the specific morphology of the second could not be determined because of its dim light. The parent thunderstorm of the red sprites was a mesoscale convective system (MCS), which appeared around 1500 UTC on the 27th near northern Arkansas and moved from west to east. The thunderstorm became stronger at about 2359 UTC, and the area of maximum radar reflectivity (55–60 dBZ) reached 339 km2 and then began to weaken. At 0303 UTC, the thunderstorm intensity increased, then the cloud gradually spread, and the thunderstorm began to weaken and completely dissipated at 1100 UTC. The first recorded sprite occurred at 0446:05 UTC, and the second at 0447:17 UTC. They tended to be produced in the dissipation stage of the MCS, when the frequency of the positive and negative CG lightning was low and the Percentage Of Positive CG to total CG (POP) increased significantly, and they were mostly over the stratiform cloud area with a brightness temperature of -40℃–-50℃. The sprite production was accompanied by an increase in the echo area of 30–35 dBZ. The area of radar reflectivity larger than 40 dBZ decreased, and the area of 10–40 dBZ increased during the sprite time window, suggesting that the sprite production was the decay of the thunderstorm and that the area of the stratiform region developed, which is consistent with the results of previous studies on summer sprites. The parent CG flash of red sprites was positive and with a single return stroke, and it was located in the trailing stratiform region of the MCS, where the radar reflectivity ranged from 25 to 35 dBZ. The corresponding radar echo top heights were 2.5 km and 5 km, and the peak currents were +183 kA and +45 kA, respectively. Based on the ultra-low frequency magnetic field data, the impulse charge moment changes (iCMCs) of two parent lightning discharges were estimated to be +394 C km and +117 C km. The ultra-low frequency magnetic antenna recorded the internal current signal of the first red sprite, indicating that the red sprite was strongly discharged.
Correction for Cirrus Cloud Top Height of MODIS Based on CALIPSO Dataset in the Beijing–Tianjin–Hebei Region
2020,44(5):1013-1022, DOI: 10.3878/j.issn.1006-9895.1911.19181
Abstract:Cirrus clouds play an important role in atmospheric radiation and affect weather systems and climate change. Satellite remote sensing has considerable advantage in cirrus cloud detection, relative to traditional observation. As a passive remote sensing instrument, large deviations in the thin cirrus cloud top height data from the Moderate Resolution Imaging Spectroradiometer (MODIS) are detected. Comparatively, the Cloud–Aerosol Lidar with Orthogonal Polarization (CALIOP) aboard the Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO), which is an active remote sensing instrument, can acquire more accurate characteristics of thin cirrus cloud. In this study, MODIS cloud products in the Beijing–Tianjin–Hebei region from 2013 to 2017 are selected. Using the CALIPSO cirrus cloud top height data, a linear fitting method based on the cross-validation method is obtained, and the MODIS cirrus cloud top height data are corrected. The difference between MODIS and CALIPSO changes from -3 to 2 km to -2.0 to 2.5 km. Moreover, the maximum difference changes from approximately -0.8 km to approximately 0.2 km. In the context of different vertical levels and cloud optical depths, MODIS cirrus cloud top height data are improved after correction, which is more obvious at the lower cloud top height and optically thinner cirrus clouds.
The Properties of Convective Generating Cells Embedded in the Stratiform Cloud on Basis of Airborne Ka-Band Precipitation Cloud Radar and Droplet Measurement Technologies
2020,44(5):1023-1038, DOI: 10.3878/j.issn.1006-9895.2004.19185
Abstract:On the basis of airborne Ka-band precipitation cloud radar (KPR) and droplet measurement technologies (DMT), the dynamic and microphysical characteristics of convective generating cells (GCs) embedded in stratiform clouds initiated by the Huanghuai cyclone on April 22, 2018 were analyzed. First, a total of 36 GCs were observed by KPR in spring in Shandong Province. The results based on the echo intensity, horizontal scale, and echo top height of these GCs show that the average echo intensity of GCs is concentrated at 20 to 30 dBZ, accounting for 69%. The horizontal scale of GCs is concentrated at 15 to 30 km, accounting for 61%. The echo top height of GCs is concentrated at 6 to 8 km, which is 2 to 4 km higher than the surrounding stratiform clouds. Afterward, the microphysical parameters of GCs in mixed-phase cumulus clouds on April 22, 2018 were counted. The results showed that the inner part of GCs is dominated by updraft with the maximum wind speed of 1.35 m s-1 and average updraft of 0.22 m s-1. GCs have high supercooled water content with the maximum of 0.34 g m-3 and average of 0.15 g m-3. The ice particle concentration in the inner part of GCs is 5.5 times that of its outer part, and the mean diameter of the inner part of GCs is 1.7 times that of its outer part. The images sampled by the cloud image probe showed that the ice particles on the head and tail of GCs were mainly columnar and radial, respectively, whereas the ice particles in the core of GCs were polymers. The growth of ice crystals depended on the accretion and collision processes. The ice crystals formed columns when the supercooled water was insufficient; otherwise, they rapidly formed graupels. The microphysical formation mechanism of precipitation in GCs is different and strongly depends on the supercooled water content. When the supercooled water content of the cloud was sufficient, graupels were rapidly formed, and surface precipitation was formed after they passed through the melting layer. When the supercooled water content of the cloud was insufficient, the formation of precipitation depended on the water vapor deposition and aggregation processes.
2020,44(5):1039-1056, DOI: 10.3878/j.issn.1006-9895.2003.19186
Abstract:Atmospheric predictability research is the basis for weather and climate prediction. Under the background of global warming, meso/micro-scale extreme weather events such as heavy rain and severe convection have occurred more frequently in recent years, and their predictability has attracted wide attention. After briefly reviewing the history of atmospheric predictability research, this paper systematically reviews the latest advances in the predictability of heavy rain and strong convection over the last 20 years (1999–2018). The main research methods for meso/micro-scale predictability and their differences with traditional large-scale weather predictability methods are first discussed. Then, the primary initial error growth mechanism (error upscaling under deep moist convection) is elaborated in detail, and some arguments (error downscaling, error upscaling, and downscaling coexisting) are discussed. The effects of errors in NWP (Numerical Weather Prediction) models and convective environments on the practical predictability are also highlighted, and some recent mesoscale predictability experiments are reviewed. Finally, this paper briefly discusses the current problems, challenges, and future directions of the predictability research of heavy rain and severe convection.
Contrasting Salinity Interannual Variations in the Tropical Pacific and Their Effects on Recent El Niño Events: 1997/1998, 2014/2015, and 2015/2016
2020,44(5):1057-1075, DOI: 10.3878/j.issn.1006-9895.1912.19172
Abstract:Ocean salinity variation provides a new insight into related ENSO (El Niño–Southern Oscillation) expressed by climate variability. In this study, salinity variations and their related dynamic processes responsible for SSTA (sea surface temperature anomaly) were extensively compared and analyzed considering two strong El Niño events, 1997/1998 and 2015/2016, and one special El Niño, 2014/2015. The study shows that the development of ENSO is significantly associated with the occurrence and eastward diffusion of large-scale SSSA (sea surface salinity anomaly) in the western tropical Pacific. In April 1997 and 2015, corresponding to two strong El Niño events, there was a significant negative SSSA in the western–central Pacific. The anomaly moved eastward to the west of the dateline, which induced a shallower MLD (mixing layer depth), and a thicker BLT (barrier layer thickness), which enhanced the surface warming in the tropical central Pacific and the early warming in the equatorial eastern–central Pacific. Although a negative SSSA occurred in the April 2014/2015 weak event in the equatorial western–central Pacific, it did not develop eastward, resulting in a weakened thickening process of the BLT and a weak modulation effect on surface temperature. For the salinity change process corresponding to three El Niño events, surface advection and surface forcing caused by FWF (freshwater flux) were the major contributors to the salinity budget. Surface advection influenced the former variability of salinity tendency, inducing the occurrence of an ENSO signal. The precipitation in the tropical western Pacific had the most significant negative influence on FWF, which played a decisive role in the SSSA occurrence and ENSO development. Compared with the two strong El Niño events, the early FWF negative anomaly in 2014/2015 did not develop, did not move eastward, and weakened rapidly; this resulted in the slowing down of the negative salinity tendency in the western–central Pacific, deepening of the MLD, thinning of the BLT, and rapid cooling of the surface layer, which inhibited early warming in the equatorial eastern Pacific. The results of this study demonstrate that the salinity change was closely related to ENSO, and early SSS in the tropical western–central Pacific could be used as an index of SSTA. In particular, SSSA not only affects the strength of SSTA in oceans, it can also be used as a precursor to judge the development and strength of ENSO.
2020,44(5):1076-1092, DOI: 10.3878/j.issn.1006-9895.1911.19193
Abstract:To improve the analysis quality by incorporating the flow-dependent ensemble covariance into the variational data assimilation system, the new GRAPES (global/regional assimilation and prediction system) hybrid-3Dvar system was built. The new system is based on the GRAPES regional 3DVar system, which uses the statistic covariance, and was built by augmenting the state vectors with another set of control variables preconditioned upon the ensemble dynamic covariance. The new hybrid-3DVar system and the localization method were verified through a single-observation assimilation experiment with ensemble samples produced by the 3D-Var’s control variable perturbation method. The real observation assimilation and forecast experiment for Typhoon Soudelor yielded the following conclusions: (1) The background covariance, which is represented by ensemble samples, is flow-dependent, and the root mean square spread in the ensemble of momentum field and mass field is largest near the typhoon center. (2) The analysis increments of the new hybrid-3DVar have a more detailed structure and more medium- and small-scale information. (3) The analysis and 24 h prediction qualities of model variables in the new hybrid-3DVar are significantly improved compared with the 3DVar system, and the precipitation position predictions are more accurate. (4) The 24 h forecast track of Typhoon Soudelor is closer to the observational one, and the 48 h-predicted intensity also approaches the real observation.
Numerical Simulation of the Effect of Urbanization on a Single Extreme-High-Temperature Event in Beijing
2020,44(5):1093-1108, DOI: 10.3878/j.issn.1006-9895.2004.19229
Abstract:Urbanization has a significant influence on the frequency and intensity of heat waves, but the mechanism of the effect of urbanization on the high-temperature process is not fully understood. In this study, the authors used the Weather Research and Forecasting (WRF) model to simulate a summer high-temperature process on 2–6 July 2010 in Beijing. This paper reports the main results obtained regarding the urbanization effect on the surface air temperature of urban areas during the heat-wave process. The optimized WRF model was able to simulate the temporal characteristics of the five consecutive days of high temperature and the variation in the urban-heat-island intensity (IUHI) in Beijing. The impermeability of the underlying urban surface lowers the 2-m relative humidity of urban areas with respect to that of rural areas, which weakens the ability of urban areas to regulate the surface air temperature via latent heat. After sunset, the urban-sensible-heat flux decreases slowly, and the cooling rate in urban areas is slower than that in rural areas. At night, the structure of the boundary layer is stable, and its height is low, as is the wind speed. In this case, the energy transmitted between urban and rural areas is constrained, and the strong urban heat island is formed, resulting in the temperature in urban area is significantly higher than that in rural area at night. After sunrise, both the sensible and latent heat fluxes of urban and rural land surfaces increase rapidly, and the stability of the boundary layer decreases. In the afternoon, the underlying urban surface favors high and low value centers in the sensible and latent heat fluxes, respectively, with a weakened ability to regulate temperature via latent heat. This is conducive to vertical exchange of energy, which decreases the stability of the boundary layer. The IUHI is lower in the afternoon than in the evening. Therefore, the obvious urban-heat-island effect created by the underlying urban surface in Beijing increases the strength of extreme-high-temperature events. Furthermore, in this heat-wave process, most of the eastern part of China is controlled by continental warm high pressure with clear skies and few clouds, and the northwesterly winds flowing over the Taihang Mountains generate a Fohn effect, which is the synoptic situations of the heat-wave formation in Beijing.
The Causes of Variation in the Zonal Asymmetry of the Asian Westerly Jet and Its Impacts on East Asian Climate in Boreal Summer
2020,44(5):1109-1124, DOI: 10.3878/j.issn.1006-9895.2005.19232
Abstract:Based on CMAP (Climate Prediction Center (CPC) Merged Analysis of Precipitation) monthly mean rainfall data and NCEP/DOE (National Centers for Environmental Prediction/Design of Experiments) II Reanalysis data from NOAA (National Oceanic and Atmospheric Administration) and the monthly precipitation and average temperature data from NMIC (National Meteorological Information Center), the authors defined an index (IAja) that describes the zonal asymmetric variation of the Asian westerly jet in the upper troposphere, from which the authors investigated the characteristics of the intensity difference between the eastern and western parts of the Asian westerly jet and its impacts on the climate of East Asia from 1979 to 2019. The conclusions are as follows: There are prominent interannual variations in the zonal asymmetry of the Asian summer westerly jet, with significant quasi-periods of 6–8 years and 2 years. When the zonal asymmetry of the Asian summer westerly jet is typically strong (weak), the wave-like anomalous rainfall pattern generates positive (negative)—negative (positive)—positive (negative) signs in the lower to higher latitudes in the East Asia sector along with negative (positive) temperature anomalies in the Lake Baikal area, and simultaneously significant positive (negative) anomalies in regions in West China and North Japan. The divergent and convergent wind components by the anomalous diabatic heating as a potential vorticity source directly induce the circulation anomalies in the mid-latitudes. The anomalous anticyclonic circulation causes the intensity of the west Asian jet to increase and the eastern segment to decrease, which strengthens the zonal asymmetry of the Asian summer westerly jet. The formation of and support provided by the zonal asymmetric anomalies of the Asian jet are affected by their convergence and divergence in the tropics and mid-latitudes, as well as the eastward propagation of wave energy in the westerlies. This eastward propagation of wave energy may be related to the sea surface temperature anomaly of the Northern Atlantic. These results facilitate a better understanding of the formation mechanisms of the zonal asymmetry of the Asian summer westerly jet.
2020,44(5):1125-1140, DOI: 10.3878/j.issn.1006-9895.2003.19242
Abstract:Although the long-term trend of extreme temperatures has been extensively explored in previous studies, few studies have addressed the interdecadal variation of extreme temperatures. Based on the daily maximum temperature, minimum temperature, and daily temperature at 839 stations in China from 1961 to 2016, the authors analyzed the interdecadal variations in the winter extreme-low-temperature index in China. The first four wave components of the extreme temperature at each station were extracted by harmonic decomposition, which is regarded as the interdecadal component. A station is regarded as having undergone an obvious interdecadal variation if the cumulative variance explained by the interdecadal component is greater than 25%. The results show that the stations with obvious interdecadal variation in their winter extreme-low-temperature index are mainly located north of the Yangtze River, in northern Xinjiang, and in eastern Qinghai–Tibet Plateau. The interdecadal variations north of the Yangtze River and in northern Xinjiang are basically consistent after 1979. The years after 1979 can be divided into three periods: previously cold period (1979–1986), warm period (1987–2007), and later-cold period (2008–2016). The interdecadal variation in the extreme temperature indices of the stations located in the abovementioned two areas might be modulated by the interdecadal variation in the East Atlantic/West Russia (EAWR) teleconnection pattern, which corresponds to the interdecadal variation in both the frequency of the blocking-like circulation over the Ural Mountains and the amplitude of the planetary trough over East Asia.
Evaluation of Simulated Tropical Cyclones over the Western North Pacific with IAP AGCM4.1 Based on K-Means Method
2020,44(5):1141-1154, DOI: 10.3878/j.issn.1006-9895.2002.19252
Abstract:As the atmospheric component of CAS-ESM1 (Chinese Academy of Sciences Earth System Model, version 1), IAP-AGCM4.1 (Institute of Atmospheric Physics Atmospheric General Circulation Model, version 4.1) is being developed independently by Institute of Atmospheric Physics. In this study, the authors used TECA (Toolkit for Extreme Climate Analysis) to identify and evaluate tropical cyclones (TC) over the western North Pacific simulated by IAP AGCM4.1 from 1979 to 2012. The results show that IAP AGCM4.1 can reproduce the spatial distribution, track, and source of TC reasonably well compared to observation data, but it underestimates the number of TC, with only 36% of the observed tropical cyclones over the western North Pacific simulated. Further analysis using K-means clustering revealed that this underestimation is mostly due to the model’s inability to reproduce northwestward-turning and westward TC. For TC with westward–northwestward, westward-turning, and eastward-turning tracks, the numbers simulated are approximately 39%, 48%, and 85% of those observed, respectively. Moreover, the correlation coefficients of the seasonal variations between simulated and observed TC can reach 0.91, with duration biases of roughly 1–2 d. IAP AGCM4.1 performs well in simulating the tracks of the westward–northwestward and eastward-turning TC, with relative biases ranging between 1%–5% for the longitude of the centroid, 4%–16% for the latitude of the centroid, and 5%–15% for the latitudinal and meridional standard deviations. In addition, IAP AGCM4.1 reproduces the evolutions of environmental circulation and subtropical highs quite well during the lifetime of eastward-turning TC, with the simulated strength and area indexes of the subtropical highs highly correlated with the observations (the correlation coefficient is 0.89). The poor simulations of northwestward-turning and westward TC are likely due to simulated biases in the subtropical high.
Volume 44,2020 Issue 5
A study on the detection of decadal phase transition and early warning signals of PDO in recent and future 100 years
Available online:October 19, 2021 DOI: 10.3878/j.issn.1006-9895.2108.20127
Abstract:Based on the theory of critical slowing down, the present paper studies the decadal phase transition and early warning signals of PDO Series by using the historical monitoring data of PDO in the past 100 years (1900-2019) and the simulation data of future century model (2006-2100). First, the phase transition time of PDO is determined by Moving t-test technique, and then the early warning signal of phase transition is studied by means of variance and autocorrelation coefficient which characterize the critical slowing phenomenon. The results show that: (1) There have been four significant phase changes in PDO in the recent 100 years, and early warning signals can be given 5-10 years in advance; (2) In the next 100 years, PDO detection shows that PDO will have two decadal transitions around 2040 and 2080, and early warning signals can also be detected 5-10 years in advance; (3) Based on the study of phase transition detection and early warning signals of PDO sequences in recent and future 100 years, the reliability of the variance and autocorrelation coefficients characterizing the critical slowing phenomenon as an early warning signal of abrupt change is demonstrated, which provides an experimental basis for the wide application of this method .
Physically Consistent Atmospheric Variational Objective Analysis and Applications over Tibetan Plateau (II): Cloud-precipitation, heat and moisture structures in the Naqu region
Available online:October 19, 2021 DOI: 10.3878/j.issn.1006-9895.2110.21078
Abstract:This study sets up a long-term (2013-2017) dynamically and thermodynamically consistent atmospheric dataset over the Tibetan Plateau-Naqu analysis region. This dataset is derived by the constrained variational objective analysis with ground-based, sounding, and satellite measurements as well as ERA-Interim reanalysis data. Using averaged results from this five-year dataset, the annual evolutions of the atmospheric basic environments, cloud-precipitation, and large-scale dynamic and thermal structures in the Naqu analysis region are analyzed. The results show that: 1) the seasonal variation of wind speed above 350 hPa is significant, with a maximum (> 50 m/s) from November to February in the next year, and the vertical variation of wind speed is the weakest while that of temperature is the strongest from July to August. The high humidity area is located at 350 - 550hPa in summer and autumn but at 300 - 400 hPa in winter and spring. 2) The precipitation in the analysis region is rich from June to early July. In spring, autumn, and winter, the layer of 300 - 400hPa (as the junction of atmospheric ascending and descending motion) is the cloud concentration area. But in summer, the enhanced atmospheric ascending convection and water vapor lead to an increase of total and high clouds whereas a decrease of medium and low clouds. 3) The surface latent heat flux and the total air-column latent heat are the strongest whereas the air-column net radiative cooling is the weakest in summer. The strong surface sensible heating in the plateau leads to the horizontal warm advection below 500hPa, while the strong westerly and radiative cooling cause the cold advection above 500hPa. In addition, the analysis region is characterized by dry advection in the whole year, however, there is a weak moist advection in summer. 4) The apparent heat source Q1 has obvious vertical stratification characteristics, that is, showing diabatic cooling below 500 hPa and diabatic heating in 300 - 500 hPa and 100 - 150 hPa in the whole year; while the layer of 150~300 hPa has diabatic cooling in the dry seasons (winter and spring) and diabatic heating in the wet seasons (from the end of spring to autumn). In summer, the entire air column is almost dominated by diabatic heating because of the enhanced ascending motion, net latent heating, the transport of sensible heat by rising turbulence, and the existence of high clouds.
Microphysical Characteristics of Winter Mixed-phase Stratiform Clouds and Summer Convective Clouds in the Rocky Mountain Region using Airborne Measurements
Available online:October 18, 2021 DOI: 10.3878/j.issn.1006-9895.2107.21046
Abstract:Microphysical characteristics in mixed-phase stratiform and convective clouds are very different, but have not been well considered in numerical models. This is one of the sources leading to uncertainties in modelling clouds and precipitation. In order to improve our understanding on the difference in microphysics between mixed-phase stratiform and convective clouds, and to provide quantitative results for model evaluation and parameterization, The microphysical characteristics of continental mixed-phase winter stratiform and summer convective clouds in the mid-Rocky Mountain region are compared using data collected during the Ice in Clouds Experiment—Layer Clouds (ICE-L) and the High Plain Cumulus (HiCu) project. The particle images and particle size distributions (PSD) were measured using 2D-Cloud and 2D-Precipitation probes, the liquid water content (LWC) was measured using the King hot-wire probes, and the ice water content (IWC) is calculated based on the particle spectrum. The main findings are: (1) Between -30° and 0°C, the LWC in the summer convective clouds is an order of magnitude higher than that of the winter stratiform clouds, and the IWC in the summer convective cloud is 1-2 orders of magnitude higher. More supercooled liquid water was observed near the convective cloud top. The LWC in summer convective clouds increases with decreasing temperature from 0°C to -20°C, while the LWC in winter stratiform clouds varied in an opposite way. The liquid fraction in the summer convective clouds is smaller than that of the winter stratiform clouds, suggesting more rapid ice production. (2) Both the winter stratiform and summer convective clouds had large spatial variability in their phase distribution. As the temperature decreases from 0°C to -30°C, ice in the winter stratiform clouds grew through the Bergeron process, and the water-dominated zones transform to the mixed-phase and ice-dominated zones. The phase distribution was more complicated in summer convective clouds, suggesting complicated liquid-ice interaction. (3) The ice PSD in summer convective clouds was broader than that in winter stratiform clouds between 0°C and -30°C. As the temperature decreases, the ice PSDs in both winter stratiform clouds and summer convective clouds broadened. (4) The observed particle images in winter stratiform clouds were irregular at temperatures lower than -20°C, while between -20~-10°C the ice were dendrites and irregular, and at temperatures warmer than -10°C the ice were mainly needles, columns and irregular, indicating the ice grew through vapor diffusion and coalescence in winter stratiform clouds. In summer convective clouds, the ice mainly formed through drop freezing, riming and coalescence. (5) In stronger updrafts of summer convective clouds, higher LWC and liquid fraction were observed. The IWC had no obvious correlation with vertical velocity, indicating the efficiency of glaciation in HiCu clouds was not dependent on vertical velocity.
Available online:October 18, 2021 DOI: 10.3878/j.issn.1006-9895.2106.20245
Abstract:Wind profiler radar data was used to retrieve cloud dynamic characteristics such as vertical velocity, droplet terminal velocity, and microphysical parameters such as cloud water mixing ratio, rain water mixing ratio. A summer precipitation was observed and analyzed by retrieval results on 7-8 May 2020 in Beijing Haidian, combined with weather radar, sounding, automated meteorological readings, disdrometer and microwave radiometer. Results indicated that the vertically-scanning instruments as well as retrieval results obtained precipitation dynamic and microphysical information that can be applied to further studies. The station was located at the edge of the main precipitation reflectivity factor, was stratiform cloud precipitation type, and the overall precipitation reflectivity factor was weak (mainly at 0-20dBZ), vertical shear of horizontal wind at 4km ran through the entire precipitation. The precipitation was divided into two stages: the earlier stage (7th 20:00-8th 02:00) existed shallow convective structures at lower height; the convective precipitation cloud top was relatively high (average height 8207m), horizontal wind shear at low-level promoted the development of convection, the proportion of 10-20dBZ was large, particle spectrum was narrow, diameter <1mm, the rain rate was weak, but number concentration was large, the maximum value was 26305 m-3, there existed warm advection at 2-3km, and the value of water vapor and liquid water was large , rain water mixing ratio range was 0.02-0.15g/kg, cloud water mixing ratio range was 0.5-2g/kg, and large value had a wide distribution, droplet terminal velocity was 3.2-4.2m/s, vertical velocity was in range of 0.6m/s, updraft and downdraft alternated obviously; the later stage (8th 02:00-10:00) turn into typical stratiform cloud precipitation, cloud top was relatively low (average height 7831m), the proportion of <10dBZ was large, the strong value center of bright band was formed at 3100m, the particle spectrum broadened, and the maximum diameter was close to 1.5mm, but number concentration decreased, the maximum value <3000 m-3, the rain water and cloud water value was one order of magnitude smaller than convective stage, and the intensity range was narrowed, droplet terminal velocity reduced to 2.8-3.6m/s, the vertical velocity was also an order of magnitude smaller than that in convective stage, and appeared obvious horizontal orientation upward and downward areas below the height of the bright band (2.5-2.8km).
Relationship between vertical convection structure and precipitation simulation bias in the tropical atmosphere: An analysis based on GAMIL3
Available online:October 12, 2021 DOI: 10.3878/j.issn.1006-9895.2109.21098
Abstract:Based on the LASG/IAP developed general circulation model GAMIL2 (G2) and GAMIL3 (G3), the simulation ability of tropical precipitation and convective vertical structure was evaluated, and the reasons for the improvement of precipitation simulation and the relationship between the vertical structure of tropical convection and precipitation simulation deviations were explored. Both versions of GAMIL precisely capture the main characteristics of tropical precipitation, while G3’s simulation are globally accurate than G2. The main improvement of the new version is to significantly reduce the positive precipitation bias in the tropical northwest Pacific Ocean. The water vapor budget diagnosis shows that the precipitation deviation mainly come from the evaporation term and the vertical advection dynamic term, and the latter comes from the combined effect of intensity of vertical motion and the vertical motion profiles. The vertical structure deviation of convection mainly exists in equatorial Indian Ocean and equatorial Atlantic Ocean areas, which mainly corresponds to weaker convergence component in lower atmosphere and higher altitude of detrainment. In the tropical northwest Pacific and equatorial Eastern Pacific, the typical “top-heavy” and “bottom-heavy” characteristic of the vertical motion profile are well represented, but the deeper convection than reanalysis data is still eminent. The wet static energy (MSE) budget shows that the excess net energy flux over the tropical Northwest Pacific is the main source of the modeled vertical motion deviation. However, the deeper vertical convective structure results in a larger gross moist stability (GMS), which offset the net energy flux deviation and inhibited the simulated convective intensity. the significant improvement of precipitation simulation in the tropical northwest Pacific in G3 Is mainly due to the reduction of positive deviation of convective intensity. The down-regulation of convective threshold and stratus threshold in G3 increase the frequency of convection and inhibits excessive intensity of vertical motion. The vertical structure of tropical convection has various and close relationships with precipitation deviation, which should be paid more attention to in future model development.
Available online:October 12, 2021 DOI: 10.3878/j.issn.1006-9895.2105.19246
Abstract:As an important feature in the macroscopic characteristics of clouds, the vertical structure characteristics of clouds determines different cloud types, and further affects the energy budget balance of earth atmosphere system through absorbing and emitting radiation. Therefore, the research about the vertical structure characteristics of clouds is always an important subject of cloud physics. As an instrument for observing the vertical structure characteristics of clouds, sounding balloon can penetrate clouds to achieve temperature and humidity profiles in high resolution. And based on this information, the vertical structure of clouds can be precisely identified by some method. In this paper, observation data of our national operational radiosonde sites are used to identify the vertical structure of clouds by relative humidity threshold method, and cloud boundary are further tested by comparing with ceilometer and “FengYun-4” satellite. It is on this basis that mean vertical distribution of one-, two-, and three-layer clouds is obtained from 2015 to 2017 and analyzed in daily, seasonal and regional variations. The results indicate that (1) one-layer clouds are generally located at altitudes that fall somewhere between the altitudes of the two- and three-layer cloud configurations, and with the increase of the number of cloud layers, the uppermost layer is more higher and the lowest layer is more lower. (2) In diurnal variation, the lowest cloud base height of single and multilayer clouds at noon is higher than those at morning, and the uppermost cloud top height of single and multilayer clouds at evening is higher than those at morning and noon, the variation of the mid layer in multilayer clouds is less than the uppermost layer and lowest layer. (3) In seasonal variation, there are a greater number of clouds with more vertically developed during summer than other seasons, suggesting that development of cloud is favorable under warm atmospheric conditions. (4) The regional variation of cloud vertical distribution in China shows obvious latitudinal variation trend. Thin clouds with higher cloud base centered on the Tibetan Plateau in southwest gradually transform to thick clouds but lower cloud base in coastal area of southeast, which indicates that different cloud type directly affected by the difference of the geographical condition and climate zone.
Vertical characteristics of water vapor transport in rainy season in eastern China based on Lagrangian method
Available online:October 12, 2021 DOI: 10.3878/j.issn.1006-9895.2105.20236
Abstract:The Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) platform and the areal source–receptor attribution method is used to simulate Lagrangian trajectories of air parcels in East China during the summer monsoon from 1961 to 2010 in different vertical levels. And the water vapor transport pathways and moisture contribution on different vertical layers in each rainy season are determined quantitatively. The result indicated that during the pre-monsoon in South China (SC), the most important vapor transport channel in lower level (under 1500m) is West Pacific Ocean channel, with the proportions of trajectories reaches 52.3%, and in middle level (1500m to 5000m) is Indian Ocean channel, the proportions of trajectories is larger than 37%. However, the most important moisture source is the East China region and West Pacific Ocean in the lower level, and the contribution rate for the two sources are both higher than 20%. After the onset of the South China Sea summer monsoon, during the monsoon in SC, the Indian Ocean channel is the strongest moisture channel for lower level, middle level and the upper level (higher than 5000m), and the trajectories rate in middle level is 65.6%. Meanwhile the moisture contribution of Indian Ocean also has a great increased, it is the most important moisture source in middle level and upper level. But the most important moisture source in lower level is East China and South China Sea region. During the Meiyu period, the West Pacific channel is the most important channel in the lower layer, and the Indian Ocean channel is the most important channel in the middle and upper level. Compared with the monsoon in SC, the Indian Ocean channel is weaker in the middle and upper levels, while the mid-latitude westerly channel is stronger. During the rainy season in North China, the most important water vapor channel in the lower layer is the West Pacific channel, while the most important water vapor channel in the middle and upper layer is the mid-latitude westerly channel. During Meiyu and North China rainy season, the main source areas are eastern China and the West Pacific region. Especially in the rainy season in North China, the water vapor from East China in the lower level reaches 43.1%, which indicates that the local evaporation at the low level plays a crucial role in the rainy season precipitation in North China.
Analysis on the causes of atmospheric fine particle pollution in winter in Hohhot-Baotou-Ordos of Inner Mongolia
Available online:October 09, 2021 DOI: 10.3878/j.issn.1006-9895.2107.21049
Abstract:Using WRF and IAP-AACM developed by Institute of Atmospheric Physic, Chinese Academy of Sciences, several typical pollution episodes of fine particulate matter (PM2.5) over Hohhot-Baotou-Ordos area of Inner Mongolia in winter of 2016 were analyzed. The resultes indicated that the air quality changes in Hohhot-Baotou-Ordos area were mainly affected by the large-scale synoptic pattern. In the stage of pollution accumulation, at 500 hPa, this area was controlled by the flat westerly airflow in front of the blocking high pressure or weak high pressure ridge; at the ground, this area was located in the weak high pressure or uniform pressure field. The low wind speed and the low height of boundary layer were unfavaorable for the dispersion of pollutants, meanwhile, the air temperature and relative humidity were high, which was conductive to the formation of secondary particles. In the stage of pollution dissipation, the synoptic patterns had significantly changed. Below the 550 hPa, there was strong cold advection, causing the gale weather on the ground, which were beneficial for the elimination of pollutants. Accompanied by cold air moving southward, pollutants over downstream areas were also removed. Local emission was the main source of PM2.5 over Hohhot-Baotou-Ordos area. The contribution of local emission to Ordos was more than 60%, and Hohhot more than 80%, Baotou more than 90%. The change of air quality in this region could reflect the change of regional air pollution meteorological conditions. Cross correlation analysis showed that the PM2.5 concentrations in Shanxi, Hebei and Henan regions were correlated with that in Hohhot-Baotou-Ordos (P < 0.0001) with a phase difference of 6-24 hours. The improvement of PM2.5 pollution in Hohhot-Baotou-Ordos depended on the control of local source emissions. In winter, the air quality change in this region can be used as a precursor factor for the air quality change in the downstream region, which is helpful for the prediction and early warning of air quality in the downstream region.
Numerical Simulation on Formation and Dissipation of a Cold Air Pool in Chongli Winter Olympic Games Area
Available online:October 09, 2021 DOI: 10.3878/j.issn.1006-9895.2109.21070
Abstract:Based on the mesoscale regional numerical modelS(WRF) and spectral nudging method, this study simulates a cold air pool (CAP) process during the 2021 Winter Olympics test competition. In this paper, the vertical change of the wind temperature field during this process has been analyzed, and the specific reasons for the formation and dissipation of the CAP has been revealed. The results show that the stationary synoptic situation is the general background for the maintenance and development of the CAP. During the development of the CAP, the temperature inversion layer was rapidly established from top to bottom, and the southeast cold air flow appeared at the bottom of the valley. Affected by the downward gravitational wind, the cold air accumulated to the bottom of the valley continuously, and the depth of the CAP increased. After sunrise, the large-scale system winds over the mountain was reestablished. The temperature inversion layer was eroded from the bottom, and the structure of the CAP was destroyed. The strong radiation cooling at night time is the main reason for the formation of the CAP. The difference in the intensity of the radiation cooling will cause the difference in the cooling range of the CAP. The sudden enhancement of the radiation cooling after midnight created favorable conditions for the maintenance and development of the CAP in the middle and later periods. By analyzing the evolution of the potential temperature profile, friction velocity and boundary layer height during the process, it can be confirmed that the development of turbulent activity is an important factor in influencing the dissipation of temperature inversion and the destruction of CAP structure.
Available online:October 09, 2021 DOI: 10.3878/j.issn.1006-9895.2109.21119
Abstract:Stable isotopes in atmospheric water vapor, which can track moisture sources and water vapor transport, are widely used as an important tracer of present-day water cycle. ‘‘Amount effect” is invoked to interpret water vapor stable isotopes at mid-low latitude monsoon region. However, recent studies have shown that non-local factors such as moisture sources and water vapor transport have significant influence on the stable isotopes. Therefore, based on Lagrangian Particle Dispersion Model and Satellite remote sensing δD in water vapor, the main factors affecting water vapor δD are analyzed in the region with abundant Chinese stalagmite δ18O records. On the seasonal scale, water vapor δD is more depleted in late summer and early autumn and enriched in winter and spring. We find this characteristic cannot interpret by "temperature effect" or "amount effect". On the contrary, accumulated rainfall over water vapor transport paths is the dominant factor of water vapor δD and there is a significant negative correlation between them. On the interannual scale, water vapor δD is enriched in the summer of El Ni?o year and depleted in the summer of La Ni?a year. The contribution of moisture sources to water vapor δD is small, but the accumulated rainfall over water vapor transport paths increases significantly in La Ni?a year compared with El Ni?o year. This suggests that tropical convection and depletion in water vapor transport paths are strong in La Ni?a year, resulting in depleted water vapor δD in the study area. In conclusion, upstream convection, measured by accumulated rainfall, is the primary driver of water vapor δD variations on seasonal to interannual scale. Enhanced convection will deplete δD in the study area, while the weakened convection is the opposite.
Numerical simulation of the effects of thunderstorm charge distributions on the discharge characteristics of positive intracloud lightning flashes
Available online:October 09, 2021 DOI: 10.3878/j.issn.1006-9895.2109.21079
Abstract:In this study, a stochastic lightning parameterization scheme is coupled into the dipole charge structure, and the parameters and positions of the main negative charge region are fixed. Positive intracloud flashes (+IC) initiating at different altitudes are simulated by adjusting the parameters (charge concentration and horizontal range) of the upper positive charge region, and then the relationships between the characteristics of +IC flashes and the distributions of thunderstorm charge are discussed. Simulation results indicate that under dipole charge structures, the elevation of the upper positive charge region can generate high-altitude +IC flashes consistent with observations. Different from normal IC flashes dominated by upward negative leaders and horizontal or slightly downward positive leaders, IC flashes initiated at high altitudes are characterized by long-distance downward positive leaders and horizontal or slightly upward negative leaders. With the elevation of the upper positive charge region, the initiation altitudes of +IC flashes increases. When the upper positive charge region is lifted to a certain altitude (in this paper, the lower boundary altitude of the upper positive charge region is 12 km), IC flashes are usually initiated from the main positive charge region, and the concentration and horizontal radius of the upper positive charge area have no significant effect on the initiation altitude of IC flashes. In addition, the length of positive or negative leader channels has a significant positive correlation with the concentration, horizontal radius of the charge region, and the distance between the initiation point and the negative or positive charge region.
Relationship between two types of east-west oscillation of the South Asian High and their different influences on weather
Available online:October 09, 2021 DOI: 10.3878/j.issn.1006-9895.2106.21075
Abstract:The South Asian high (SAH) center is characterized by bimodality distribution (i.e., Tibetan Plateau (TP) mode and Iranian Plateau (IP) mode), showing an east-west oscillation pattern. In addition, east edge of the SAH also regularly extends eastward to East Asia or retreats westward to the TP, manifesting another type of east-west oscillation. By using NCEP1 daily reanalysis data, APHRODITE daily precipitation data and daily precipitation data in India, this paper investigates the relationship between the two types of east-west oscillation of the SAH, and the differences in their impacts on the circulation and weather in Asia. The results show that bimodality east-west oscillation of the SAH center can significantly affect the occurrence and amplitude of eastward extension/westward retreat of the east edge of the SAH. Although the east edge of the SAH can extends eastward when the SAH center is in the TP or IP mode, the frequency of eastward extension of the east edge of the SAH in the TP mode is significantly higher than that in the IP mode; In the IP mode, east edge of the SAH is more prone to retreat westward. And magnitude of eastward extension of the east edge of the SAH in the TP mode is larger than that in the IP mode. Further analysis reveals that bimodality east-west oscillation of the SAH center is closely related to the rainfall anomaly pattern in the northern India and the TP region, and is coupled with the variation of thermodynamic effect related to rainfall anomaly. While the eastward extension/westward retreat of the east edge of the SAH is related to the dipole rainfall anomaly pattern in East Asia (i.e., rainfall anomaly in the central and eastern TP, and in the middle and lower reaches of the Yangtze River and Yellow river, is opposite to that in the region south of the Yangtze river), by resulting in the westward extension/eastward retreat of the western Pacific subtropical high. Furthermore, when the SAH is in the TP mode and its east edge extends eastward, and when the SAH is in the IP mode and its east edge retreats westward, rainfall anomaly in the western TP is always opposite to that in the central and eastern TP.
Cloud liquid water path retrieval products over the Pacific Ocean and their climate change characteristics
Available online:October 09, 2021 DOI: 10.3878/j.issn.1006-9895.2105.21063
Abstract:Based on the multi-year brightness temperature observation data of the AMSU-A (Advanced Microwave Sounding Unit-A) on the polar-orbiting satellite NOAA-15, a product of cloud liquid water path on the global ocean has been established and compared with ERA5 and FNL/NCEP. We analyzed the ability of retrieval products to reproduce the climate change characteristics of cloud liquid water path, and further analyzed the linear and nonlinear climate change trend characteristics of cloud water path in the Pacific region through linear regression and EEMD (Ensemble Empirical Mode Decomposition) methods. The results show that the CLWP inversion data can well reproduce the average spatial distribution characteristics of the years and the corresponding climate change trend. The CLWP climate trend shows obvious latitude zone characteristics, and the increasing and decreasing trends appear with the latitude zone interval. Moreover, in the eastern Pacific region of the northern hemisphere, the latitude zone characteristic of the climatic trend of the CLWP has a phenomenon of migration northward. In contrast, the climatic trend of the inversion product has better similarity with the ERA5 reanalysis data. While the latitude zone characteristics of the trend, especially the northward movement of the latitude zone characteristics, cannot be reproduced well by the FNL data. It is characterized by a decrease in water vapor in the equatorial region, and a significant increase in cloud liquid water paths on both sides.
The Relationship between Interdecadal Variation of Rainy Season Precipitation and Water-Vapor Transport in North China
Available online:October 09, 2021 DOI: 10.3878/j.issn.1006-9895.2107.21059
Abstract:Based on the homogenized daily precipitation in North China and ECMWF-ERA5 reanalysis data for 1961-2018, and a new monitoring standard that takes precipitation and the position of the Western Pacific subtropical high ridge into consideration, we calculated the rainy season precipitation in North China (RSPNC) and onset/ending date with the new monitoring method, and discussed the climatic characteristics of the water-vapor transport and associated interdecadal variations in precipitation and moisture budget. The temporal and spatial variations in water-vapor transport and associated impact on RSPNC were further investigated. The main results can be summarized as follows: (1) The onset/ending dates of the rainy season in North China are distinct in each year, so as the periods of occurrence of the rainy season and the intraseasonal variation. (2) Precipitation is determined by large-scale atmospheric moisture transport and associated convergence. The critical four water-vapor pathways maintained the RSPNC including Indian monsoon, East Asian monsoon, trans-equatorial airflow between 110°E and 120°E, and mid-latitude westerlies near 40°N. (3) The RSPNC and water-vapor budget displays similar interdecadal variations, and abrupt climate changes occurred in 1977, 1987 and 1999, respectively, featuring a "reduction-increase-reduction" phase. The RSPNC is strongly correlated with the net water-vapor budget within the domain of North China. (4) The intensity of water-vapor flux and the arriving timing exert significant impacts on the amount of precipitation. The distribution patterns of water-vapor flux anomalies in rainy decades and rainless decades are distinct: in the rainy decades, the anomalous anticyclonic circulation dominates the Northwest Pacific, and the northward water-vapor transport is strong, which converges with the eastward water-vapor transport over mid-high latitude westerlies in North China, and the water-vapor diverges more strongly than that in normal years. In terms of intraseasonal process, water-vapor fluxes are stronger in amplitude, reach North China earlier, weaken later, converge stronger, and have a longer lifetime. While in the rainless decades, the anomalous cyclonic circulation dominates the Northeastern China, Korean Peninsula, and the area around the Sea of Japan, and it turns into a weaker-than-usual northward water-vapor transport, and the water-vapor divergence is obviously strengthened; accordingly, the intraseasonal process shows the opposite characteristics. (5) Considering the four boundaries of water vapor transport, the southern and weastern boundary water-vapor inputs are the largest and the second-largest, respectively. Their interdecadal variations are critical for the interdecadal variation of the RSPNC. In rainy decades, there are stronger inputs of water-vapor from the southern and western boundary, but strong output from the north boundary; however in rainless decades, water-vapor inputs are weak from the southern and western boundaries, and the output swithes to input from northern boundary, which is essentially distinct from that in the rainy decades.
The Relationship between on Snowfall in Yanqing Zone of Winter Olympic Games and Easterly Wind in Boundary Layer
Available online:October 09, 2021 DOI: 10.3878/j.issn.1006-9895.2106.21055
Abstract:Affected by special geographical environment, winter snowfall in Beijing is often accompanied by easterly wind in boundary layer. The water vapor transport and dynamic convergence effect caused by wind in boundary layer easterly are of great significance to the occurrence and development of snowfall. Different from the existing snowfall studies on boundary layer easterly wind in plain areas, based on the topographic characteristics of Yanqing Zone of 2022 Winter Olympic Games, this paper compared the mechanism of easterly wind in boundary layer with different thermal and humidity properties and development heights on snowfall under similar weather conditions.The results show that: (1) A longer route through the Bohai Bay is beneficial to the obvious humidification of the boundary layer easterly wind, and vice versa. (2) The "dry and cold" easterly wind can form a cold pad to lift the warm and humid air in Beijing plain area. When the easterly wind develops deep in the vertical direction (over 600m), it can overturn Jundu Mountain with lower altitudes in eastern Yanqing and form a confluence on leeward slopes. At the same time, it is blocked Haituo Mountain with higher altitudes in the west, forming the forced uplift of the windward slope. The combined two effect leads to the convergence of the east is strong and the west is weak, which causes the distribution of snowfall is more east than west. (3) The "warm and humid" boundary layer easterly wind cannot cross Jundu Mountain westward due to its low vertical extension height, which has little effect on snow in Yanqing. (4) When the air is affected by the northwest airflow of 500hPa, closer to the terrain, the saturation area near the 700 hPa height and the lifting movement will make the high altitude mountain area of Yanqing experience obvious snowfall.
South Asian Summer Monsoon Simulated by FGOALS-g3 Climate System Model: Climatology and Interannual Variability
Available online:October 09, 2021 DOI: 10.3878/j.issn.1006-9895.2105.21042
Abstract:In this study, based on observation and JRA55 reanalysis data sets, we systematically evaluate the performance of South Asian Summer Monsoon (SASM) simulated by FGOALS-g3 climate system model. We draw attention to the simulated differences between FGOALS-g3 and FGOALS-g2, coupled model and atmospheric model. Results show that compared with FGOALS-g2, the simulation of climatological Indian Ocean trade winds and the sinking branch of the Walker circulation during El Nin ?o are improved in the FGOALS-g3 owing to the change of local sea surface temperature (SST). However, the systematic cold biases in the middle and upper troposphere persist, causing the reduced meridional temperature gradient and weakened SASM, leading to the biases of descend motion and moisture divergence, and the thus still dry biases over the terrestrial monsoon trough in the FGOALS-g3. Meanwhile, the negative correlation between El Nin ?o—Southern Oscillation and Indian summer rainfall captured by FGOALS-g3 stays weaker than observation, resulting from the weaker descending motion caused by SST biases. Furthermore, results also show that the air-sea interaction-induced climatological SST biases compensate the wet biases in the SASM region via the change of atmospheric circulation and water vapor transportation. At interannual timescale, the inclusion of negative feedback process of SST—precipitation—cloud shortwave radiation in the coupled model effectively improve the biases intensity of rainfall and atmospheric circulation simulated by the atmospheric model; however, the westward biases of sinking branch of the Walker Circulation caused by the SST biases in the coupled model lead to greater wet biases in the Indian Peninsula.
Difference of New Particle Formation and Its Influence on Cloud Condensation Nuclei Activity in Beijing and Xingtai
Available online:October 09, 2021 DOI: 10.3878/j.issn.1006-9895.2107.21013
Abstract:Taking three aerosol field measurement experiments carried out in 2016 summer at Xingtai (XT) site, 2016 winter and 2017 winter at Beijing (BJ) site as examples, typical new particle formation events during these three experiments are selected to analyze their effects on aerosol hygroscopicity and cloud condensation nuclei (CCN) activity. BJ and XT were considered as two typical cities located in a northern megalopolis area and a central-southern industrial area, respectively, in the North China Plain. The formation mechanism of new particles in different seasons at two sites were different, and the corresponding condensation sink (CS), growth rate (GR), and aerosol chemical composition were also different. The dominant chemical species formed during NPF events was organics at BJ site, but which were sulfate and organics at XT site. The further study found that aerosol hygroscopicity and CCN activity were significantly stronger at XT site than those at BJ site, especially for the nucleation mode particles. The above results indicate that the difference in aerosol hygroscopicity and activation ability should be fully considered when estimating the influence of new particle formation on CCN number concentration.
Establishment and Application of Physical Inspection Method for Artificial Precipitation Enhancement Effect
Available online:October 09, 2021 DOI: 10.3878/j.issn.1006-9895.2105.20237
Abstract:For physical inspection of artificial precipitation enhancement effect based on multi-source detection data,we establish the similarity measurement coefficient APC of contrast area selection,and the dimensionless PIDI index method for physical inspection of artificial precipitation enhancement effect.Results show that:(1)The PIDI index method of physical inspection for artificial precipitation enhancement effect can be used to minimize variability influence of seeding cloud body and precipitation with the similarity coefficient APC,and synthesize a variety of dimensionless cloud physical detection parameters with dimensionless method.Finally,a percentage change rate is used to comprehensively measure the overall variation trend and degree of various cloud physical parameters.(2)The PIDI index method was applied to inspect the precipitation enhancement effect of 24 aircraft from 2014 to 2019.The average change rate of 7 indexes (cloud top temperature,effective particle radius,optical thickness,liquid water path,combined reflectivity,≥30dBZ echo area,vertical cumulative liquid water content) caused by artificial precipitation enhancement was 3.4%~19.6%.The precipitation change rate of 18 operations was 0~58.3%,the precipitation change rate of 6 operations was -37.5%~0.The changes of cloud physical parameters caused by most precipitation-increasing operations are obviously smaller than the changes of precipitation.(3)For the 18 operations with positive effect of precipitation enhancement,cloud top temperature,combined reflectivity and vertical cumulative liquid water content for most operations were increased due to artificial catalysis,effective particle radius,optical thickness and liquid water path for most operations were decreased due to artificial catalysis.(4)The difference and similarity between the PIDI index method and the K-value method were compared by using an aircraft precipitation enhancement operation.For the test of precipitation variation trend,the two are consistent.The difference between the two is that the PIDI index method can reflect the average change rate of all inspection indexes caused by artificial catalysis.
Aircraft measurements on summer vertical distributions of aerosols and transitions to CCN and cloud droplets in Central Northern China
Available online:October 09, 2021 DOI: 10.3878/j.issn.1006-9895.2104.20255
Abstract:The spatiotemporal distributions of aerosols and their transition to cloud droplets are a key issue in studies of clouds and precipitation and climate change. The properties of vertical distribution, size distribution and origin, as well as their transition relationships with cloud condensation nuclei (CCN) and cloud droplet number concentration are investigated using nine flights of aircraft measurements under clear and cloudy conditions in Shanxi, Central Northern China. Results indicate that the structure of atmospheric boundary inversion layer has an important impact on the vertical distributions of aerosols and CCNs. The size distributions under different weather conditions are similar in the upper atmospheric layers but different at lower levels. The high correlation between aerosols and CCNs can be found in vertical. The linear relation of y=1.3x-616.3 can be found between aerosols and cloud droplets with a determination coefficient of 0.96, and transition rate from aerosols to cloud droplets may reach 47%. The linear relation of y=1.6x-473.8 can be found between CCNs and cloud droplets with a determination coefficient of 0.96, and transition rate from CCNs to cloud droplets may reach 69%.
Simulation study on the impacts of the solar radiation on the formation and development of Tibetan Plateau vortex
Available online:October 09, 2021 DOI: 10.3878/j.issn.1006-9895.2105.20215
Abstract:In this study, the influence of the diurnal cycle of solar radiation on the development of the Tibetan plateau vortex (TPV) was examined by using WRF-ARW model. The results showed that the solar shortwave radiation has a significant effect on the occurrence and development of TPV. The control run (CTL; with diurnal cycle of solar shortwave radiation) well reproduced the development process of the TPV. In the experiment with turning off the shortwave radiation (All_night), the TPV developed much faster at the early stage, whereas in the daytime experiment (All_day), the shortwave radiation greatly suppressed the development of TPV. The diagnostic analysis indicated that the longwave radiation cooling steepened the tropospheric lapse rate and thus weakened the atmospheric static stability; Additionally, the decreases in temperature increased the relative humidity at night, which was conducive to potential instability in the lower troposphere and thus promoted the formation and development of TPV, Conversely, solar shortwave radiation warmed the upper troposphere and strengthened the static stability, which inhibited the development of convection. The convergence at lower layer is stronger at night than day, which is beneficial to the enhancement of the ascending motion and the formation of the TPV; The unbalance term indicated that, with the center of the TPV corresponding to the positive area of the unbalanced term and the outer edge of the TPV with the negative area of the unbalanced term. Numerical results showed that the development of TPV bears many similarities to tropical cyclogenesis in terms of dynamics and thermodynamics. The above conclusions are consistent with previous studies on the effect of solar radiation on strong convection.
Feature Analysis of Dynamic Condition and Hydrometeor Transportation among Zhengzhou “7·20” Superheavy Rainfall Event Based on Optical Flow Field of Remote Sensing Data
Available online:September 28, 2021 DOI: 10.3878/j.issn.1006-9895.2109.21155
Abstract:In this paper, aiming at the “7·20” superheavy rainfall event in Zhengzhou City, Henan Province on July 20, 2021, the FY-4A stationary satellite imager data and ground-based weather radar data are respectively used to produce optical flow field. Compared with the horizontal wind speed of FNL data and ground observation data, the weather satellite and radar optical flow field can approximately reflect the motion characteristics of the upper-level and lower-level atmosphere and cloud system. On this basis, the feature analysis of dynamic condition and hydrometeor transportation related to the superheavy rainfall event is presented. The results show that, there was a "southwest to northeast" transporting zone of water vapour and cloud on the afternoon of July 20, extending from southern China to northern China through Henan Province. There was actively convective in this transporting zone, extending to the existing cloud system on northern-central parts of Henan, providing favourable transporting condition for superheavy rainfall event. In Zhengzhou region, the anticyclonic vorticity on upper atmosphere increased and the divergence on lower atmosphere converted to strong cyclonic convergent on the afternoon of July 20 and before the severest precipitation, indicating that the updraft in the precipitation system in Zhengzhou region was increasing. The hydrometeor input at south boundary of Zhengzhou region greatly increased ahead of the stage of severest precipitation. These results indicate that there was not only a large amount of water vapour, but also hydrometeors in severe convective cloud transported to the updraft area in large-scale precipitation system. It may greatly accelerate the microphysical process of water vapor transforming into cloud water and finally forming the precipitation, which may be an important cause of the rapid enhancement of the superheavy rainfall. The analysis method based on optical flow field of remote sensing data proposed in this study has significant application potential in short-term and impending rainstorm forecast and early warning.
Microphysical characteristics of a stratiform precipitation with embedded convection based on multi-source data
Available online:September 27, 2021 DOI: 10.3878/j.issn.1006-9895.2107.21043
Abstract:Based on ground-based micro-rain radar and cloud radar, combined with aircraft observation, a stratiform precipitation with embedded convection is analyzed to study the cloud-precipitation microphysical structure accurately. The results show that: 1) The selected precipitation process is divided into stratified cloud and convective cloud. Above zero layer, especially on the height of 5~6 km, doppler velocity and spectrum width of convective precipitation are greater than that of the stratiform cloud precipitation, which indicates that vertical wind of environment, the size range of the particle occurred in convective precipitation are greater than the stratiform precipitation. 2) At the period of convective precipitation, there is a V-shaped gap caused by attenuation in radar reflectivity of cloud radar and micro rain radar in time and height profiles. The attenuation of cloud radar is grater than that of micro rain radar, the higher the height, the greater the attenuation. 3) At the period of stratiform precipitation, near the bright band,the?leap?increase height of radar reflectivity factor is 80m higher than the Doppler velocity, and the leap?increase height of Doppler velocity is 20m higher than the spectral width. 4) The precipitation mechanism near the zero degree layer of is complex. When the negative temperature is close to 0℃, the particle shape includes aggregated dendritic ice crystals, needle ice crystals and cloud droplets. The Doppler velocity and spectral width of convective cloud precipitation are greater than that of stratiform precipitation above the zero degree layer, especially at the altitude of 5 and 6km. The vertical airflow and the scale range of small and large particles in convective precipitation are greater than that of stratiform cloud precipitation.
Available online:September 09, 2021 DOI: 10.3878/j.issn.1006-9895.2104.20225
Abstract:In recent years, deep learning models has been increasingly used in solving nowcasting problems that have a large impact on disaster prevention and mitigation. In this paper, we took nowcasting as a spatio-temporal sequence prediction task, and use radar reflectivity factor as the test object. We use TAGAN deep learning model based on GAN frame to predict the radar echo image of the future 1h, and compared it with Rover optical flow Method and 3DUnet model based on convolutional neural network. The radar echo data set of the 2018 Global Weather AI Challenge is selected for training and testing. The test results show that the TAGAN model shows advances by multiple scores such as the hit rate (POD), false alarm rate (FAR), critical success index (CSI), and correlation coefficient. The TAGAN model performs well in the above test scores and increases with the prediction time compared to the traditional optical flow method and the comparative deep learning model. Compared with the traditional optical flow model, the improvement effect is more significant. The results may shed some light on expanding and improving the application of deep learning models in near-weather forecasting.
Statistical Characteristics and Formation Mechanisms of Night Warming Events at Chongli-Yunding Winter Olympic Stadium
Available online:September 09, 2021 DOI: 10.3878/j.issn.1006-9895.2107.21057
Abstract:Based on the observation data from the automatic weather stations during November 2018–March 2019 and November 2019–March 2020 at Yunding Winter Olympic Stadium in Chongli, Hebei Province, the night warming events in the stadium are statistically analyzed in this paper, and then several types of possible formation mechanisms for these events are further discussed through the microwave radiometer, the three-dimension laser radar and the wind profiler data, and the reanalysis data from NCEP/NACR. The results show that the occurrence probability of night warming events in Yunding Stadium reaches 76.9% from November to the March next year, indicating that warming event is a common phenomenon there. The frequency and amplitude of warming decrease with the increase of the station altitude. The triggering mechanisms of night warming events in Yunding Stadium can be classified into four categories such as the mixed warming in the inversion layer caused by vertical wind shears, the foehn warming, the whole-layer sinking warming and the warm advection warming in mid and low level. During the warming process of the second to the fourth types, the warming amplitude at the bottom of the valley may be significantly larger than that at the top of the mountain when there is a temperature inversion in the valley before the start of warming, thereby the warming amplitude decreases with the increase of altitude. The warming at the top of the mountain is only affected by the third and fourth types of formation mechanisms, while all the four types can form warming events at the valley. That is why the occurrence frequency of warming events decreases with the increase of altitude.
Available online:September 09, 2021 DOI: 10.3878/j.issn.1006-9895.2108.21069
Abstract:The spectra measured by the Atmospheric Carbon dioxide Grating Spectrometer (Atmospheric Carbon dioxide Grating Spectrometer - ACGS) carried by the China’s global carbon dioxide observation satellite (TanSat) in the band of 0.76μm, 1.61μm and 2.06μm can be used for the retrieval of carbon dioxide (CO2) concentrations by fitting the observations and simulations using the optimal estimation algorithm. Accurately detecting the change of the center wavelength is highly important because of its very high spectral resolution and accuracy requirement for product retrieval. The variations of center wavelength for all three bands of ACGS have been calculated on the locations of the individual solar absorption lines by comparing the solar-viewing measurements and the high-resolution solar reference spectrum. The variations with magnitudes less than 10% of the spectral resolution for each band have been detected. The changes are probably caused by vibration and the instrument status difference between the ground and space, especially temperature variation on orbit. The scheme described here can be used not only for monitoring spectral stability but also to gain spectral knowledge prior to the level-2 product processing. These minor temporal changes of wavelength on orbit should be corrected in the product retrieval.
Numerical simulation of a large-scale snowstorm in northern China using different cloud microphysical parameterization schemes.
Available online:September 09, 2021 DOI: 10.3878/j.issn.1006-9895.2107.21064
Abstract:Using ERA5 reanalysis data as the initial field, and the WRF model was used to conduct the numerical simulation study of a large-scale snowstorm weather process from April 19-20, 2020. we adopt 5 microphysical parameterization schemes (Bulk scheme: Thompson, Morrison, WDM6 and NSSL; Bin scheme: SBM FAST) for sensitivity experiments, and compare with the observation data (precipitation data collected at automatic weather stations, radar base data). The temporal and spatial evolution characteristics of precipitation, radar reflectance, dynamic thermodynamics and water condensate in heavy snow weather are analyzed. The results showed that the intensity and range of the snowfall were basically simulated by the above different microphysical parameterization schemes, but there were significant differences in the simulated dynamics and fine cloud microphysical structures, which were reflected in the vertical velocity and spatial and temporal distribution of water condensates simulated by different cloud microphysical schemes. The cloud water simulated by the bin scheme is strong, with obvious sinking movement, concentrated convection range, and obviously weak graupel and ice crystal process. However, the rainwater and graupel simulated by Bulk scheme are strong, showing a wide range of upward movement and a wide range of convection. The above dynamic forces and the distribution of cloud water condensates are related to the characteristics of cloud microphysics scheme. Bulk scheme adopts the overall cloud group design, binding the movement of different particle types, so it cannot describe the sinking and dragging process of different particles in detail, reflecting that there is no obvious vertical sinking movement, which also results in the significant increase of graupel particles. For different Bulk schemes, although the space-time distribution of the total content of hydrocondensates is similar and all have a wide range of space-time distribution, the components of hydrocondensates (cloud water, rain water, ice, snow and graupel) are significantly different, which is related to the description of different cloud microphysical processes.
Dynamic and Thermodynamic Effects on the Evolution of the Transverse Shear Line over the Tibetan Plateau in Boreal Summer
Available online:September 09, 2021 DOI: 10.3878/j.issn.1006-9895.2108.21053
Abstract:The Tibetan Plateau transverse shear line (TPTSL) is one of the main weather systems over the Tibetan Plateau. Based on the ERA-5 reanalysis datasets provided by the European Centre for Medium-Range Weather Forecasts (ECMWF), 14 cases of TPTSL which causes heavy rainfall and generates in June-August with a lifetime of 38 hours are selected and composited to reveal the impact of dynamic and thermodynamic forcing on the intensity evolution of TPTSL s. The results are as follows. (1) At 500hPa, the TPTSL generates in the saddle field between the Iran high and the western Pacific subtropical high, and is located in the low-pressure center contoured by 584dagpm and warm center contoured by 272K, to the north of the specific humidity center. At 200hPa, the TPTSL is located in the northern margin of the South Asian High and to the south of the entrance region of the westerly jet stream. (2) The intensity of TPTSL at 500hPa shows an obvious diurnal variation with the strongest at 23LT and the weakest at 13LT (LT=UTC+6h). (3) The vorticity budget reveals that the variation of the divergence at the upper and the convergence at the lower layers over the Tibetan Plateau is indicative of the intensity of TPTSL, and the vorticity at 500hPa gets maximum 3 hours later than that of the convergence. (4) Potential vorticity (PV) increases with the development of the TPTSL. The PV budget shows that the water vapor and diabatic heating play an important role in the generation and evolution of the TPTSL. The enhanced sensible heating leads to an intensified ascent. With sufficient water supply, the latent heating releases and the diabatic heating center rises to the middle layer, which is favorable for the generation and development of the TPTSL.
Available online:September 09, 2021 DOI: 10.3878/j.issn.1006-9895.2104.20232
Abstract:Using WRF model and three-dimensional precipitation diagnostic equation, the high-resolution simulation and diagnosis analysis of the physical process of heavy precipitation in the main precipitation period of the heavy rain process in Beijing happened on July 20 has been carried out. The results show that before the peak of precipitation, the strong water vapor convergence supports the strong precipitation, while humidifying the atmosphere. In the later stage, the water vapor convergence is significantly weakened, and the precipitation causes the obvious reduction of water vapor content in the local atmosphere. Before the peak time of precipitation, water vapor convergence, condensation and liquid-phase condensate convergence jointly contribute to the rapid development of heavy precipitation cloud system. In the later stage, the weakness of the dynamic convergence effect and the continuous consumption and divergence of water condensate lead to the significant decrease of water condensate content and the gradual disintegration of precipitation system. During the main precipitation period, the intensity and range of vertical upward motion gradually increased, and reached the maximum at the peak of precipitation, and then weakened and contracted. The peak height of the ascending motion is located on the zero level at the initial stage, and then decreases to the lower part of the zero level, accompanied by a "weak-strong-weak" precipitation intensity change. Under the control of ascending motion, the change range of water condensate is obvious, but the change range of different water condensate is different. Graupel particles and raindrops increase most significantly, and the contents reach the maximum at the peak of precipitation, and then decrease. The variation range of other water condensates is weaker than the above two due to the process of microphysical transformation and dynamic divergence. This paper also points out that the possible influence of different microphysical parameterization schemes on the physical process of heavy rain happened on July 20 and the differences of physical processes of precipitation with different intensities are worthy of further study.
Using Adjoint-Based Forecast Sensitivity Method to Evaluate Observations of WPRD&MWR Impacts on Model Forecast
Available online:September 09, 2021 DOI: 10.3878/j.issn.1006-9895.2107.20222
Abstract:A large number of observations assimilated can effectively improve the results of model forecast. However, there are significant differences in the effects of different observations on the forecast. It is one of the most challenging diagnostics in numerical models to reasonably evaluate the contribution of observations to the forecast. In this paper, the WRFDA-FSO system is constructed by the method of adjoint-based forecast sensitivity to observation(FSO). Based on the wind profile radar detection(WPRD) and ground-based microwave radiometer(MWR) data obtained by the mega city project in Beijing in September 2019, the experiments on the impact of observations on the 12h forecast of WRF model are carried out by using WRFDA-FSO system, and the contribution of wind, temperature and humidity observations to the forecast is analyzed. The results show that: (1) In general, the observations(MWR, WPRD, Sound, Synop and Geoamv) assimilated all reduce the 12h forecast error of WRF model, and make positive contribution to the forecast. Among them, MWR observations have the greatest impact on the forecast, and the improvement of WPRD observations on forecast is better than that of wind field observations of Sound. (2) Among the U and V observations of WPRD and temperature and Specific humidity observations of MWR, the positive contribution value of V observations and temperature observations to the forecast is higher, and the effect of improving the forecast is better. (3)The observations of WPRD and MWR at most levels reduce the forecast error and are positive contribution to forecast, and the positive contribution of temperature observations is mainly below 800 hPa near the ground.
The Driving Force of Land Surface Air Temperature Variability Studied Based on the Slow Feature Analysis Method
Available online:September 09, 2021 DOI: 10.3878/j.issn.1006-9895.2106.20213
Abstract:Slow Feature Analysis (SFA) can extract slowly varying external forcing information from non-stationary time series. In recent years, the SFA method has been applied to the field of climate change research to explore the potential driving forces of climate change and related dynamic mechanisms. Based on the slow feature analysis method, this paper extracts the slowly varying external forcing information of the global land surface air temperature (LSAT) and studies the spatial structure characteristics of the global LSAT slow varying driving force and the main driving factors of low-frequency variability. The LSAT slowly varying driving force extracted by the SFA method has a significant correlation with Global Radiative Forcing (GRF) and the main modes of global sea surface temperature (SST) (Atlantic Multidecadal Oscillation AMO, tropical Pacific ENSO variability, and Interdecadal Pacific Oscillation IPO), indicating that the variability of LSAT in most parts of the world is significantly affected by GRF and the three SST modes. The influence of GRF on LSAT variability has the characteristic of global consistency, while the influence of the three SST modes on LSAT variability has obvious regional characteristics. In addition, because the SFA method can effectively reduce the interference of random noise in the original LSAT sequence, the interpretation variance of the LSAT variability of the GRF and SST modes is significantly improved, which further shows that the GRF and SST modes are the main driving factors of the global LSAT low-frequency variability. Finally, the results of the historical sea surface temperature-driven Atmospheric General Circulation Model (AGCM) test which is also named as Atmospheric Model Intercomparison Project (AMIP) test, used to verify the significant influence of the three SST modes on the regional LSAT variability.
Characteristic analysis of the cold and warm conveyor belts in an idealized extratropical cyclone simulation
Available online:September 09, 2021 DOI: 10.3878/j.issn.1006-9895.2105.20190
Abstract:The idealized moist baroclinic wave simulation was performed to describe the rapid development of extratropical cyclone. Then the warm and cold conveyor belts were identified with the use of Lagrangian trajectory selection method and physical quantities along their trajectories were diagnosed, followed by the analysis of their impacts on the precipitation. The study reappears the previous conclusions of conveyor belts and find some more refined structures, especially for the cold conveyor belts. As the result shows that, the warm conveyor belts could generally split into “forward-sloping ascent” and “rearward-sloping ascent” branches. They originate in the warm sector ahead the cold front and low-level, and move respectively forward and rearward relative to the cyclone center when they spiral upward to the upper and middle level, generating negative PV disturbance upon the outflow region and promoting the development of upper system. They also transport abundant moisture upward and influence significantly the formation and maintenance of the precipitation extremum around the front. The cold conveyor belts are confirmed to represent two branches of ascent and remaining low-level as described by the previous researches, and then could be classified finely into four branches. The “forward-sloping ascent” and “rearward-sloping ascent” branches originate near the warm front and curve separately forward and rearward relative to the cyclone center when rising upon the middle level, benefiting to the rainfalls around the warm front. While the branches of “wrapping around the cyclone and forward-sloping” and “wrapping around the cyclone and rearward-sloping”, always staying in the lower level, move from east of the surface cyclone far from the warm front toward the cyclone center whenever their vapor content increases, and then descend respectively anticlockwise and clockwise to the downstream and upstream of the cyclone after travelling slowly upward west of the cyclone around the center, inducing the little precipitation northwest of the cyclone.
Temporal and Spatial Distribution of Precipitation Related to Northeast Cold Vortex in Warm Season in Jilin Province during 1981—2019
Available online:August 27, 2021 DOI: 10.3878/j.issn.1006-9895.2107.20217
Abstract:On the basis of quality control of hourly rain-gauge dataset in 39 years (1981—2019) from 51 meteorological stations, provided by the Meteorological Information Center of Jilin province, the spatial and temporal distribution characteristics of precipitation during Northeast cold vortex process in warm season (PNCVPWS) were analyzed. Results showed that: (1) Overall, precipitation in Jilin province showed a clear diurnal variation. The peaks for diurnal precipitation and frequency occurred between 16:00 and 17:00 BJT (Beijing time). The frequency with 0.1—5mm per hour precipitation (P < 5mm h-1) displayed large value in the mountainous area in eastern Jilin province, but the high value center moved to the south-central while the precipitation larger than 5mm per hour (P > 5mm h-1). (2) There was little difference in spatial distribution for the contribution of P < 5mm h-1 to warm seasonal precipitation in Jilin province during daytime, but the value center showed more clearly in mountainous area in eastern Jilin province at night. The contribution of the gauge record between 5mm h-1 and 10mm h-1(5mm h-1 < P < 10mm h-1) was greater in central Jilin province, moreover, the contribution showed greater in daytime than night. But for P > 10mm h-1, the high value center moved to the Midwest of Jilin province, similarly, the contribution value showed greater in daytime than night also. (3) The rainfall amount of P < 5mm h-1 accounted for 61% to total precipitation, and the value of 5mm h-1 < P <10mm h-1 and P > 10mm h-1 was nearly 20%, respectively. The contribution of P < 5mm h-1 had a decreasing trend in recent years, but the value for P > 10mm h-1 increased obviously in contrast.
Study on the influence model of meteorological elements on summer vegetation coverage in North China
Available online:August 27, 2021 DOI: 10.3878/j.issn.1006-9895.2102.20233
Abstract:Vegetation coverage is extremely sensitive to climate change. North China located in a semi-humid area in my country. Meteorological factors have an important impact on vegetation coverage in this area, but there is a lack of effective mathematical models to quantitatively describe the impact of meteorological elements on vegetation. Therefore, we based on the vegetation coverage data of the Moderate-resolution Imaging Spectroradiometer (MODIS) and main meteorological element data in North China during 2000 to 2018, carried out research on the influence of meteorological elements on vegetation coverage model, and initially established the relationship between summer vegetation coverage and meteorological elements in North China. Research shows: (1) Summer vegetation coverage in North China is positively correlated with precipitation and relative humidity, and negatively correlated with temperature, sunshine hours and ground temperature. The order of correlation is the sunshine hours of the same period, the ground temperature of the previous month, relative humidity of the same period, temperature of the previous month and precipitation of the same period. (2) Combining the two statistical methods of multiple regression method and least square method, the influence model of meteorological elements on vegetation coverage was constructed from the perspective of regional average and spatial distribution, and the possible influence of changes in meteorological elements to vegetation coverage was quantitatively described. Among, the 5-variable model has a slightly higher correlation coefficient for the simulation of vegetation coverage, and the model based on the least square method has a better fitting effect on vegetation coverage than the multiple linear regression model. (3) The model that removes the relative humidity has the largest change in the ability to simulate vegetation cover change in North China, that is, the synergistic effect of temperature and precipitation on vegetation cover in North China is more important. Research is conducive to understanding how meteorological elements affect the vegetation ecosystem, and then provides a theoretical basis for the construction of national ecological civilization.
Summer Regional Daily-Precipitation Extreme Events in Yangtze-Huai Rivers Region of China and Their Relationships with Rossby Wave Activities
Available online:August 27, 2021 DOI: 10.3878/j.issn.1006-9895.2108.20226
Abstract:Based on NCEP/NCAR reanalysis and datasets of daily basic meteorological elements from China national meteorological stations, the statistical characteristics of The regional daily-precipitation extreme events (RDPEs) in the Yangtze-Huai Rivers (YHR) region in eastern China from 1979 to 2016 and their relationships with Rossby wave activities are investigated. The results show that in the summers (June-July) of 1979－2016, the 95th percentile threshold of regional extreme daily-precipitation in YHR region is 33.95 mm/d. There are 63 RDPEs in total in the past 38 years. When RDPEs occur in YHR region, it is controlled by an anomalous cyclonic circulation in the middle and lower troposphere and an anomalous anticyclonic circulation in the upper troposphere. The water vapor from the Bay of Bengal and the South China Sea has a strong concentration in YHR region. Baroclinic circulation and sufficient water vapor are conducive to the occurrence and development of RDPEs. The eddy enstrophy in the upper troposphere over YHR region reaches its maximum on the day before RDPEs occur and rapidly decreases from then on. Meanwhile, the eddy enstrophy in the middle and lower troposphere reaches its maximum when RDPEs occur. This is dominated by processes including the advection term of the time average flow to the disturbed vorticity and the horizontal divergence term of the disturbed flow. Moreover, Rossby waves originate near the Caspian Sea and the Black Sea and obviously disperse downstream-ward. It takes about 3－5 days to move to YHR region, providing disturbance energy for the formation of RDPEs in this region. Overall, these results have deepened the understanding of the reasons for RDPEs in YHR region and could provide clues for effective predictions.
Available online:August 27, 2021 DOI: 10.3878/j.issn.1006-9895.2107.21001
Abstract:Based on 32-yr daily precipitation observation data from Thailand"s meteorological department during 1981-2012, by using linear trend and ensemble empirical mode decomposition (EEMD), this study focused on the changes in the proportion of stations with abnormally high (95 percentiles and above) of precipitation and precipitation days in multiple grades in Thailand as a whole and its five geographical regions, and further analyzed the annual precipitation and the variation trend of relative contribution of annual persistent and non-persistent precipitation-heavy to annual precipitation-heavy. The main conclusions are as follows : 1) Annual precipitation is strongest in East and South Thailand, while weakest in North Thailand. And during the study period of 32 years, it shows an increasing trend on 87% of stations (with 22 stations reach the significance level of 90% T test), showing the fastest increase in South Thailand and the least increase in East Thailand. In Thailand, annual precipitation in Bangkok station and Nakhon Si Thammarat station on the Coast of the Gulf of Thailand shows the most significant increasing trend. 2) The proportion of stations with abnormally high of annual precipitation, annual precipitation days and mean precipitation intensity increase significantly in North Thailand, which indicates the range of extreme precipitation is expanding, while in South Thailand, only the range of extreme precipitation for precipitation-storm increases significantly. 3) In Thailand, precipitation-heavy varies significantly in different regions. In North, Northeast and Central Thailand, precipitation-heavy is more of non-persistent form, while in East and South Thailand, two forms (non-persistent and persistent) of precipitation are balanced. The proportion of stations with abnormally high of persistent precipitation-heavy has a significant expansion trend in North and South Thailand, which indicates that the range of heavy precipitation under the influence of the stable system has a significant expansion trend.
Available online:August 27, 2021 DOI: 10.3878/j.issn.1006-9895.2106.21030
Abstract:The performances of 19 models from the Coupled Model Intercomparison Project Phase 6 (CMIP6) in simulating the water cycle over East China are evaluated based on observations and reanalysis data using Brubaker model in this study. The sources of model bias are also investigated. The CMIP6 multi-model ensemble (MME) can reasonably simulate the climatic distribution and annual cycle of precipitation and evaporation with pattern correlation coefficient of 0.92 and 0.87, respectively. Compared with observations, MME overestimates precipitation in North China (0.55 mm day-1) but underestimates precipitation in coastal areas of South China (-0.3 mm day-1). All 19 models overestimate evaporation with biases of 0.03-0.98 mm day-1. Thus, the differences between precipitation and evaporation simulated by most of the models are smaller than the observation and reanalysis data. The MME can well simulate the annual cycle of the contribution of each moisture source to precipitation, but underestimates the contribution of remote moisture via southern boundary, resulting in dry bias over the study region. We find that the southerly wind speed over southern boundary determines the difference of water vapor transport among CMIP6 models. The stronger the southerly wind speed is in the model, the higher water vapor flux incomes via the southern boundary, and the more precipitation the model simulates. The position of the convergence zone over the Northwest Pacific is one of the important systems affecting the southerly wind speed over the southern boundary. The eastward shift of the convergence position in the model results in weaker southerly winds, leading to weaker moisture transport to the study region and less precipitation, and vice versa. This study systematically evaluates the performance of CMIP6 in reproducing the East Asian water cycle, and demonstrates the limitation of the models in simulating the convergence zone over the Northwest Pacific and its impact on the East Asian water cycle.
Available online:August 27, 2021 DOI: 10.3878/j.issn.1006-9895.2106.21041
Abstract:This review summarizes the research progresses of tropical synoptic systems and mesoscale convective processes during warm season (May－Octorber) over the South China Sea in recent decades. Basic characteristics of the tropical atmospheric circulation and summer monsoon related to the mesoscale convective processes are briefly reviewed. Furthermore, the activity regularities, structure features, and formation mechanism for mesoscale convective system are emphatically summarized. Opportunities and challenges regarding the mesoscale convective processes over the South China Sea confronted currently are proposed, and future research directions in this field are then highlighted.
Available online:August 27, 2021 DOI: 10.3878/j.issn.1006-9895.2105.21062
Abstract:In this paper, the coupling Noah/Single Layer Urban Canopy scheme coupled with WRF (V3.9.1) model is used as a control case to study the effects of land use type (Md04 scheme), land surface process (NoUCM scheme) and lake (Nolake scheme) on the intensity of urban heat island and the horizontal and vertical spatial distribution characteristics of urban meteorological elements in Kunming. The main conclusions are as follows: (1) The average daily variation of urban heat island intensity in four cases is basically the same. The UHI intensity is weak in the daytime and strong at night, and reaches the maximum at about 2000 BT. In the case of Control-NoUCM (Nolake), the average daily maximum difference is -0.79oc (+1.07oc). (2) From the energy balance equation analysis of Control-Md04 case, the difference of sensible heat (latent heat) energy is +46.18 (-79.71) w/m2，and the release of latent energy is greater than the absolute value of sensible energy. In the case of Control-NoUCM,the difference of sensible heat (latent heat) energy is -40.88 (+29.60) w/m2. NoUCM scheme does not consider the heat storage and shielding of geometric buildings, and most of them are absorbed by the surface, resulting in a large absolute value of sensible heat flux. (3) In the four cases, the boundary layer height reaches the maximum (minimum) value at 1500(0700) BT. The height of urban boundary layer in NoUCM (Nolake) decreased by about 103m (32m), while that in Md04 case increased by about 102m. (4) The experimental results of the influence of lakes on the circulation of urban heat island show that the vertical movement over the lake is weak, while the horizontal lake land wind is strong, which is conducive to the transportation of water vapor to the city, increasing the humidity of dry air and making the water vapor content higher, thus increasing the release of latent heat energy, reducing the sensible heat flux and reducing the temperature gradient.
Physically Consistent Atmospheric Variational Objective Analysis and Applications over Tibetan Plateau (I): Method and Evaluation
Available online:August 27, 2021 DOI: 10.3878/j.issn.1006-9895.2106.21068
Abstract:A physically consistent atmospheric objective analysis model based on the constrained variational analysis (CVA) method was applied to the Tibetan Plateau for large-scale atmospheric structure analysis. This objective analysis model can deal with multi-source measurements with different spatial and temporal resolutions, and satisfy the conservation of column-integrated mass, heat, moisture, and momentum by using surface precipitation and flux data at the surface and top of the atmosphere to constrain the sounding measurements. An experiment during August 2014 around Naqu in the Tibetan Plateau shows that those state variables generated by the model can retain observational characteristics. The analyzed large-scale derivatives such as vertical velocity, divergence, temperature and water vapor advection, apparent heat source and apparent moisture sink by the objective analysis model can reasonably demonstrate dynamic, thermal, and moisture structures during the analysis period, which is conducive to the precipitation process studies. It shows that the layer of 350~400hPa is an important change center of dynamics, heat, and water vapor in the analysis region during August 2014. In this model, different sources of measurements have different impact on the final analysis fields. Sounding measurement has a significant impact on the upper-level wind. However, the amplitude of this impact is small within 1 m/s. Precipitation and flux measurements mainly affect the large-scale derivatives such as vertical velocity, in which precipitation mainly affects the upward movement during precipitation periods, and flux data mainly affect the downward movement during light/no rain periods. In general, the physically consistent atmospheric variational objective analysis model has high stability and strong validity.
Available online:August 27, 2021 DOI: 10.3878/j.issn.1006-9895.2108.21071
Abstract:Based on the in-situ gradient observations from the wind profile radars, WindCubeV2 and masts during the landfall of Typhoon Minguk (1822) and Lekima (1909), combined with the simulated winds of typhoon Minguk (1822) with a finest horizontal resolution of 2 km and vertical resolution of 50-model levels, in which the lower levels are densified, the strong wind structure in the lower level (below 300 m height over sea level or terrain) were analyzed. It showed that within the range of 0-200 km from the typhoon center, (1) the maximum wind speed height and the wind shear index increased outward along the radial direction, and the wind shear index on the land underling surface was generally higher than 0.12. Because of the weak drag effect on the ocean underling surface, the wind shear index was usually small with the exception of island areas; (2) the strong wind shear on the right front quadrant of the moving direction of typhoon remained stable at about 0.17, which was not sensitive to distance and altitude. There existed the wind profile similar to the jet stream on the left rear quadrant, and previous study pointed out that the changes of super-gradient/ sub-gradient wind in the vertical direction should be responsible for the jet type profiles (Tan et al., 2013). The vertical variations of the strong winds on the left front quadrant showed nonlinear characteristics, indicating the more complex strong wind structure over this area; (3) the gust factor and turbulence intensity decreased with the mean wind speed and altitude; (4) the maximum wind direction variation during the landfall of typhoon decreased outward along the radial direction, and exhibited statistically significant spatial asymmetry with the largest variation near the right rear quadrant. Over some areas of the right rear quadrant, the wind direction changed more than 30° in half an hour, and most of them occurred before or during the typhoon"s landfall. It was hoped that these information could be helpful for microscale wind simulation as well as the prevention and mitigation of typhoon disasters over offshore wind farms in China.
Available online:August 17, 2021 DOI: 10.3878/j.issn.1006-9895.2104.21010
Abstract:Meteorological satellites have provided useful information for improving weather forecasting, environmental monitoring and short-term climate prediction. In the field of weather forecast, it provides a more powerful means for the forecast of typhoon, rainstorm, hail, sandstorm and other severe weather. In this study, the microstructure of hail clouds were analysed by satellite observation data based on nearly a decade of hail events record of Shaanxi, Shandong, Guizhou, and Xinjiang. The comparison between the hail cloud and deep convective precipitation cloud characteristics which retrieved by polar orbit satellites shown different cloud properties such as cloud top temperature/effective radius, cloud glaciation temperatures. Based on the distinct cloud properties between hail clouds and convective clouds, we summarized the characteristics and further apply them on the FY-4A geo-stationary satellite which captures the life cycle of hail which happened on August 16, 2019, Shandong area. The results shown satellite has the potential to capture a hail cloud during its developing stage and use it as an application of early warning. The hail cloud shows the following characteristics: (1) There are significant differences in cloud physical characteristics between hail clouds and deep convective precipitation clouds. The cloud microphysical characteristics of hail clouds observed by satellites are shown in three aspects: Tg is cooler, with an average value of -33℃. The hail cloud reaches glaciation temperature with smaller effective radius (<40mm) with an average of 37.2?m when the clouds are fully glaciated. It also shows the smaller the re-g was, the stronger the hail cloud was. Additionally, hail clouds tops often have a reduction zone of re with increasing height. (2) All the studied areas have consistent cloud properties such as lower Tg, smaller re-g and decreased re at in comparison to adjacent convective clouds. However, it still showed regional variabilities which indicates we should establish different indicators for identifying hail clouds for early warming purpose. (3) The case study of FY-4A geo-stationary satellite shows geostationary satellite can track the evolution of hail clouds. By tracking the hail cloud, we found the geostationary satellite has a response consistent with that of the polar orbit satellite, providing a method for monitoring and early warning service of hail weather. The geostationary satellite can be used to track the development and evolution of the cloud cluster, at any time when the satellite detects strong hail signal because of the high time resolution. Combining satellite early warning with radar observation, the location of hail occurrence can be determined precisely. (4) Combining the indicators summarized by polar orbit satellites with FY-4 to track hail cloud evolution. Four hail storms occurred in Shaanxi and Shandong were applied for early warnings. Ground observations reported 22 hail events in the two regions, of which the satellite successfully warned 20 times in advance and missed two times. The average early warning time is about two hours before the hail disaster.
Characteristics of air-sea interaction associated with large-scale SST warm anomalies over the North Pacific in winter on submonthly timescales
Available online:July 19, 2021 DOI: 10.3878/j.issn.1006-9895.2106. 21047
Abstract:Using the National Centers for Environmental Prediction/Department of Energy (NCEP/DOE) reanalysis 2 and the National Oceanic and Atmospheric Administration (NOAA) Sea Surface Temperatures (SSTs) during the period of 1985－2015, eight warm events in the North Pacific are selected based on the definition of large-scale SST anomalies. The dynamic composite method following the SST anomaly center is used to study the large-scale SST warm anomalies with a lifespan of 50 days over the wintertime North Pacific and associated characteristics of the air-sea interaction on submonthly timescales before and after their peak stages. The results show that: (1) the early stage of the large-scale SST warm anomalies is mainly characterized by the forcing of the atmosphere on the ocean, while the forcing of the ocean on the atmosphere dominates the late stage. (2) The atmospheric structure associated with the SST warm anomalies changes significantly from the early to late stages. The early stage shows an equivalent barotropic dipole pattern of pressure anomalies above the warmer SSTs, with an anomalous high in the northeast and an anomalous low in the southwest, which corresponds to the anomalous easterly wind over SST anomalies. At the late stage, the equivalent barotropic anomalous cyclone is located to the north of warmer SSTs, with a weak anomalous anticyclone to the south, which corresponds to the anomalous westerly wind over SST anomalies. (3) The cyclonic circulation anomaly occurs at the late stage due mainly to the high frequency transient eddy feedback forcing, in which the forcing of transient eddy vorticity feedback acts as the major contributing factor. (4) The structure of ocean current is also different between the early and late stages. At the early stage, the ocean dynamic process is not conducive to maintaining the SST warm anomalies. At the late stage, both anomalous warm advection and anomalous downwelling act to maintain the SST warming and thus its influence on the atmosphere.
Available online:July 14, 2021 DOI: 10.3878/j.issn.1006-9895.2106.21004
Abstract:Based on the aircraft directly entering the snowfall cloud with the data of micpas, radar and satellite, the macro and micro structure characteristics of heavy snowfall cloud under the influence of Reflux system in Shanxi Province on November 29th, 2011 were analyzed. It is found that the radar echo of this snowfall process was mainly between 10 and 20dBZ large stratiform cloud echo, inlaid with more than 30dBZ massive strong echo.The zero line of radar radial velocity had a strong "s" curve with a bull"s-eye structure. There was a strong wind vertical shear from the lower level to the upper level. The liquid water content was mainly below 3.2km and the maximum value was 0.0697 g m-3. The ice and snow crystal number concentration N50, N200 and ice water content were mainly produced in the upper part of the stratiform mixed snowfall cloud, and the maximum values appeared near -9.3℃, which were 188.4L-1, 33.5L-1 and 0.121g m-3 respectively. From -14.4 to -19.7 ℃ the images of ice crystals were mainly needle, columnar and irregular and ice particles grew mainly by deposition. The images of ice and snow crystals near -9.3℃ were mainly dendritic and irregular. The aggregation, collision and fracture of radial dendritic ice crystals may be the main reason for the high concentration of ice and snow crystals. The distribution of ice and snow crystal spectrum can be well fitted by the exponential form. The spectrum fitting parameters can be expressed by the power function Nos=1.021λ1.684 and the correlation coefficient R2 was 0.86. There were three inversion times below 3.2km. The appearance of inversion layer made the cloud microphysical characteristics and fitting parameter Nos decrease and inhibited the growth of riming and deposition. As a result, the variation of fitting parameters Nos and λ with temperature in this observation was inconsistent with the previous observation. The greater the inversion intensity was, the greater the inhibition effect was.
Diurnal variation of cloud macro parameters in three important weather systems over the Tibetan Plateau using Ka-band Cloud Radar
Available online:June 21, 2021 DOI: 10.3878/j.issn.1006-9895.2106.21061
Abstract:The diurnal variation of cloud macro parameters over the Tibetan Plateau is affected by the combined effects of large-scale circulation, local solar radiation and surface processes, and has an important influence on the radiation budget, transmission of radiation and the distribution of sensible and latent heat. Due to the lack of continuous quantitative observation means, the understanding of the diurnal variation characteristics of cloud macro parameters in various weather systems is still very insufficient. Ka-band cloud radar of APSOS (Atmospheric Profiling Synthetic Observation System) is the first radar to realize long-term cloud observation in the Tibetan Plateau. In this paper, data of APSOS Ka-band cloud radar in 2019 are used to study the time-domain and frequency-domain diurnal variation characteristics of cloud frequency, single-layer cloud top height, cloud bottom height and cloud thickness under the influence of westerly trough, shear line and vortex system. Main conclusions are as follows: (1) The daily mean cloud frequency is 56.9% for the westerly trough system, 50.8% for the shear line system and 73% for the vortex system; (2) Although the genesis of westerly trough and shear line system is different, the diurnal variation trend and main harmonic period of cloud macro parameters of the two systems are similar: the diurnal variation trend is sinusoidal, the minimum value appears before sunrise and the maximum value appears before sunset. The main harmonics of cloud frequency are diurnal (24 hours) and semidiurnal (12 hours) harmonics, and the diurnal harmonics have the largest amplitude among the main harmonics of cloud top height, cloud bottom height and cloud thickness; (3) The diurnal variation characteristics of cloud macro parameters in vortex system is different from that in the first two systems. The diurnal variation of cloud parameters in the vortex system is multi-peak type. Although the harmonic amplitude of diurnal period is the largest among the main harmonics of cloud frequency, single layer cloud top height and cloud bottom height, the spectrum distribution is discrete, and the maximum harmonic period of cloud thickness amplitude is 4.8 hours; (4) The statistical regression equations of diurnal variation of cloud frequency, single layer cloud top height, cloud bottom height and cloud thickness are given.
Available online:June 21, 2021 DOI: 10.3878/j.issn.1006-9895.2105.21048
Abstract:The Indian Ocean Dipole (IOD) is the strongest interannual variability in the tropical Indian Ocean in autumn. It will influence the climate in many parts of the world through atmospheric teleconnection. The current coupled climate model has very limited IOD forecasting skills, which are far lower than the forecasting skills of El Ni?o events in the tropical Pacific. Due to super capability of deep learning in processing data, we use the convolutional neural network (CNN) of the deep learning and the multi-layer perceptron (MLP) of the artificial neural network, respectively, to perform IOD prediction. In order to explore the forecasting capabilities of CNN, this article only uses three initial conditions in the boreal spring which has the low prediction skill, namely January- February-March (JFM) and February-March-April (FMA)), March-April-May (MAM), to forecast the Indian Ocean Dipole Index (DMI), East Pole Index (EIO), and West Pole Index (WIO) for the next seven months. The results show that the CNN model can make useful prediction for the DMI, EIO and WIO at least 6-month ahead. Compared with the current state-of-the-art general coupled model, the CNN model can significantly improve the prediction skills of DMI index and EIO index, but has little improvement for WIO prediction skill. The CNN model is able to predict the strong IOD events in 1994, 1997 and 2019 well for the lead time longer than 7 months. In general, because of the CNN is better than the traditional neural network MLP for the IOD prediction due its strong capability in capturing the spatial structure characteristics of the Indian Ocean SST.
Available online:June 15, 2021 DOI: 10.3878/j.issn.1006-9895.2103.20256
Abstract:According to the NFSV-type SST forcing errors of 12 TCs, the optimal target observation deployment of sea surface temperature is identified through OSSEs. The sensitive area of NFSV-type SST forcing errors occurs along the track of TCs and during the intensification phase of TCs. The results of OSSEs show that, the additional observations deployed in the sensitive area with 90 km intervals improve the simulation of TC intensity effectively. Compared with observations deployed in non-sensitive areas, the target observations in the sensitive areas of NFSV-type SST forcing errors can improve the simulation of TC intensity more effectively. When the local target observation area in non-sensitive area reaching to the sensitive area, the additional observations will improve the simulation of TC intensity gradually. Especially, the further experiments show that the simulation of TC intensity improve more significantly when the target observations deployed during the intensification phase corresponding to the periods of the occurrence of the large value region of NFSV. Therefore, the additional observations in the target area identified by NFSV-type SST forcing errors can improve the simulation of TC intensity most effectively, and the suitable deployment of observations in the sensitive area during the sensitive periods is the most economical observation strategy.
Available online:June 15, 2021 DOI: 10.3878/j.issn.1006-9895.2104.20241
Abstract:This study focuses on the bias characteristics of multi-model ensemble mean in precipitation basing on the grid datasets of the fifth phase of the Coupled Model Intercomparison Project (CMIP5) models and the observation (CRU TS 4.0). Three bias correction methods are tested and then a projection on precipitation is made for coming 30 years. Results shows that the model precipitation is overestimated in northern Asia and underestimated in the South, with 30%-40% more precipitation in Qinhai-Tibetan plateau, Inner Mongolia and Mongolia, and 20%-30% less in southern coast of China mainland and Vietnam, and 30%-40% less in South Asia than the observation. The similar bias pattern is found in 2006-2015 precipitation projected under RCP4.5 with CMIP5 historical climate simulation, implying that the bias pattern is almost stationary, belonging the model climate drift which can be removed through the difference between a period-mean projection and the historical simulation averaged over a period. Bias correction test confirms that the logarithm regression (LR) is better in the North than year-to-year increment regression (YYIR), whereas the YYIR is better in the South than LR. The combination of the YYIR and LR is applied in the bias correction of the 2021-2050 precipitation projection under RCP4.5. The projection shows a change in precipitation pattern versus 1976-2005, such as 6%-10% increase in warm season (May-October) precipitation in Arabian Peninsula, northwest part of South Asia, southern Iron, northern part of Central Asia, the Far East of Russia, the east part of Northwest China, the northeast part of China and the coast belt around the Bohai Sea; the precipitation would decrease by about 10% in the south part of Central Asia and northern Iron, about 3% reduction in Southwest China mainland. As for the cold/cool season (November to next April) the precipitation projected would increase in the North, especially in high latitude area, and decrease in the South, such as about 6%-10% decrease in South Asia, about 10% drop in Northwest China, North China, Northeast China, Mongolia and high-latitude Asian continent. Hence, climate warming would expand the area of South Asian monsoon westward leading more frequent rainfall with flood in the dry land of Northwest South Asia, like Pakistan, whereas severe drought would likely appear in the area around southern Caspian Sea. It must be noticed that the weak decrease of the precipitation in South China mainland in the projection may imply a severe drought for coming 30 years due to the increase in surface evaporation with temperature warming. So, a preparation should be made against the potential risk of the drought for coming 30 years.
Multi-scale topographical attribution detection for the development of an eastward rainstorm vortex in southwest ChinaLi Xiang1,2, Yang Shuai2, Yang Shuyun1
Available online:June 10, 2021 DOI: 10.3878/j.issn.1006-9895.2106.21072
Abstract:Based on observation analyses and numerical simulation, we reveal that three vortexes, north Plateau Vortex (PLV1), south Plateau Vortex (PLV2), Southwest Vortex (SWV), develop successively during the disaster-causing rainstorm event in Southwest China from 5 to 6 August 2019, which lead to the intensification and eastward propagation of the rainstorm. By utilizing of numerical experiments, the effects of multi-scale topographic factors (Tibetan Plateau, Hengduan Cordillera and Sichuan Basin, denoted as TP, HC and SB) on the evolution of vortexes are studied. The results show that, HC plays a key role in the formation of the SWV, while SB influences the location and intensity of SWV. As for the propagation of Plateau vortex (PLV2 herein), SB only affects the vortex intensity, but does not change the propagation path of PLV2. Once HC is removed, the propagation of the Plateau Vortex disappears. The influence of slope change of the steep terrain at the boundary between TP and SB on the development of vortex is further analyzed. It shows that the steeper the slope is, the faster the propagation speed of the Plateau vortex is, and the stronger the SWV is after the two vortexes (PLV2 and SWV) merge. Finally, the impact of varied slopes on the evolution of vortex intensity is explained based on the theory of slantwise vorticity development. As the slope becomes steeper, the coefficient of slantwise vorticity development (CD) decreases sharply. In the case of steep slope, the forcing effect of rapidly-decreasing CD along the track of vortex glide on the local tendency of vertical vorticity contributes to the rapid intensification of the vortex.
Numerical Simulation and Diagnosis of a Nocturnal Warming Process in Hebei Winter Olympic Games Area
Available online:June 02, 2021 DOI: 10.3878/j.issn.1006-9895.2103.21031
Abstract:Nighttime Warming is a frequent weather phenomenon in Hebei Winter Olympic Games area, and its accurate simulation and forecast is very important for the construction of the competition area and the guarantee of the competition. By introducing the higher resolution terrain data, the mesoscale regional numerical model WRFv4.1.5 was used to reproduce the nocturnal warming process, and the meteorological characteristics and causes of the process were explored. The results show that: the process is affected by the northeast cold vortex, the cold advection is significant at the middle and upper levels, the wind shear is strong at the high and low levels, and there are obvious advection differences. At the same time, the variation characteristics of meteorological elements near the surface are obvious, including the decrease of relative humidity, the increase of wind speed, the decrease of sea level pressure, the enhancement of surface heat flux and long wave radiation, etc. In addition, the strong wind shear and advection difference between the upper and lower levels are easy to produce vertical mixing. The isentropic surface fluctuation is obvious, and the turbulence is strengthened, which further strengthens the turbulent vertical mixing movement, enhances the moving heat flux transport, and produces the abnormal increase of night temperature.
Examination of mechanisms underlying the variations of microphysical properties in different fog phases from the perspective of entrainment
Available online:June 02, 2021 DOI: 10.3878/j.issn.1006-9895.2102.20189
Abstract:As one of the main processes affecting cloud and fog, entrainment-mixing process has an important impact on the cloud/fog life cycle, precipitation formation, radiative transfer, aerosol indirect effect evaluation and so on. In this study, entrainment-mixing mechanisms in a radiation fog was discussed from the microphysical and dynamical perspectives, which not only improved the theoretical understanding of entrainment-mixing mechanisms, but also revealed the development and dissipation of radiation fog from a new perspective. By using the comprehensive field observational data in Nanjing during the 2006 and 2007 winter, entrainment-mixing mechanisms in nine fog cases were analysed. First, a radiation fog event during 10 - 11 December 2007 was studied to understand microphysical relationships and entrainment-mixing mechanisms during different phases in detail. Results showed that the extreme inhomogeneous entrainment-mixing was found in the mature phase, in which volume-mean radius changed slightly as number concentration and liquid water content decreased. The homogeneous entrainment-mixing was found in the rapid dissipation phase, in which all microphysical properties decreased simultaneously with positive correlations. Except for microphysical properties, the scale number was calculated as a dynamical measure for entrainment-mixing mechanisms. In the mature (rapid dissipation) phase, the scale number was small(large), indicating that the extreme inhomogeneous(homogeneous) entrainment-mixing was most likely to occur. Then the microphysical relationships of the other 8 fog events were examined, which indicated that volume-mean radius had positive correlations with liquid water content in general, i.e., homogeneous entrainment-mixing dominated. The research results were helpful to the development of parameterization schemes of entrainment-mixing mechanisms, and provided reference for the simulation and prediction of radiation fog.
Key Circulation Characteristics of Spring-to-summer Seasonal Transition Process over the Mid- and High Latitude Asia
Available online:June 02, 2021 DOI: 10.3878/j.issn.1006-9895.2104.21028
Abstract:The spring-to-summer seasonal transition over the Mid- and High Latitude Asia (MHASST) is an important part of the several spring-to-summer seasonal transitions in different regions of the Asian continent. It provides the necessary circulation conditions in the mid- and high latitudes for the establishment of the Meiyu rainfall in the Yangtze and Huaihe River basins. However, so far there is no systematic summary on its uniqueness and key characteristics. In this paper, we analyze and summarize the key characteristics of the MHASST process, based on the daily data of NCEP / NCAR reanalysis data I. The MHASST is symbolized by the establishment of the Northeast Asian ridge at 500 hPa and then the "double blocking" circulation pattern. The formation of the Northeast Asian ridge and its related land sea temperature difference is mainly attributed to the snow melting process and thus the local strong warming process in Northeast Asia. The establishment of the northern East Asian low (850 hPa) is another important sign of the MHASST. When the MHASST occurs, the 200 hPa Asian jet axis over the Tibetan Plateau jumps northward from ~ 35oN to ~ 37oN, while the Asian temperate jet disappears completely. With the seasonal change, the meridional gradient of near surface temperature in the mid- and high latitude Asia weakens, thus causing the attenuation of high frequency transient baroclinic disturbances. In contrast, low-frequency weather systems, including the Asian Blocking high and the northeast China cold vortex system, become the dominant weather systems in the same region. From the perspective of the early and late timing of the MHASST, this paper also discusses the evolution features of the circulation and weather system over the mid- and high latitude Asia, and the results further supplement the key information of the climatic MHASST.
Research on Sensitivity of Microphysical Parameterization on Numerical Simulation on a Meiyu Front Heavy Rainfall Process
Available online:June 02, 2021 DOI: 10.3878/j.issn.1006-9895.2105.21025
Abstract:Microphysical processes in Meiyu front rainfall have an important effect on evolution of precipitation. Based on WRF (version 3.4.1) model, one Meiyu front heavy rainfall case from 29 to 30 June is analyzed with 3 different microphysics schemes (Morrison, Thompson and MY). The main findings are as follows. (1) The general large-scale circulation of the Meiyu rainfall case could be reasonably reproduced by all the three experiments with different microphysics schemes, which was consistent with ERA5 reanalysis data. The local circulation during Meiyu front heavy rainfall was significantly influenced by microphysical processes and the differences in the local features between different experiments were evident. The local circulation and updraft in the Thompson experiment were stronger than those in the other two schemes. Precipitation in all the model output was overestimated and the hourly rain rate was always greater. The overestimation of melting of ice phase hydrometeors or accretion of cloud droplet by raindrop was one of the most important causes to the overestimation of modeled precipitation. On the whole, Morrison run performed relatively better. (2)Melting of ice phase hydrometeors and accretion of cloud droplet by rain drop were the key source terms to the growth of rain drop. And evaporation process was the most important sink term. On the whole, raindrop collecting cloud droplet contributed more than melting of ice phase hydrometeors to the growth of raindrop. However, for each scheme, differences of these microphysical process terms leaded to the difference of modeled precipitation in distribution. (3) Thompson run produced the largest amount of melting of ice phase hydrometeors and evaporation (especially in low level). At the same time, it produced the largest amount of condensation which leaded to more collection of cloud droplet by raindrop. Therefore, Thompson run produced most raindrop and rainfall. The predominant product through deposition and riming process was snow and the largest amount of snow was produced. (4) Through Bergeron process, Morrison run produced more snow than graupel (ice particles nearly could be neglected), Thompson run produced predominant snow and MY run produced more snow than ice particles (graupel nearly could be neglected). The largest amount of produced ice particles in MY run through the process leaded to more ice particles than that in other schemes. (5) Cloud droplet contributed more than raindrop in riming process. In Morrison and Thompson schemes, the amount of graupel collecting cloud droplet was larger than that through other riming processes. Other riming processes contributed the growth of graupel in different degrees in Morrison run, while other riming processes nearly could be neglected compared to graupel collecting cloud droplet. And MY run produced larger amount of snow growth by deposition. Therefore, differences of Bergeron and riming processes in all three schemes lead to the differences in ice phase hydrometeors distribution.
Available online:June 02, 2021 DOI: 10.3878/j.issn.1006-9895.2103.21021
Abstract:With the construction of about 70,000 automatic weather stations across China, comprehensively automatic meteorological observation has been realized. However, the real application of this kind of observations always surfers from their low quality. Large number of error data seriously affect the practical application of observation. Therefore, it is a particularly important work to repair those abnormal observations. Using a total of 168 times of hourly surface temperature observations of automatic weather station during December 1-7, 2019 provided by Jiangsu meteorological bureau, a restoration method based on the Empirical Orthogonal Function method is proposed. The accuracy analysis of the ideal restoration experiments shows that the new restoration method can well repair the wrong observations, and the error of the restoration method is about 0.48 ℃. The methods based on Cressman interpolation, which rely on single point observation information, are more vulnerable to small-scale signal interference and introduce unnatural observation information, and the surface temperature repair error can reach 1.55 ℃. The analysis of the actual repair results also proves that the new repair method makes full use of the time-space separation and modal orthogonality of EOF analysis method, and gradually eliminates the influence of wrong data through iterative method, so as to obtain better space-time continuity repair results with the surrounding observation data.
Characteristics of Water Vapor Transport Associated with Abnormal Precipitation over the East of Southwestern China in June and July, 2020
Available online:June 02, 2021 DOI: 10.3878/j.issn.1006-9895.2105.21002
Abstract:In this paper, the anomalous characteristics of precipitation in the east of southwestern China (ESWC) in June-July,2020 and the related large-scale characteristics of water vapor transport, water vapor budget and water vapor source are analyzed by using correlation, regression, clustering, Lagrange trajectory model HYSPLITv5.0 simulation and other statistical methods based on the daily precipitation data of 118 stations and other reanalysis data. Then some indexes of water vapor intensity in key areas are defined, and the relationship between water vapor intensity in key areas and sea temperature and snow cover in Qinghai-Tibet Plateau in early winter is investigated. The results show that the average precipitation in the ESWC in June-July, 2020 is 50% more than that in normal year, which is the highest since 1961. Precipitation in most areas is obviously higher than that in normal year, except for some areas in central Guizhou and northeastern Sichuan. The configuration of tropospheric atmospheric circulation field in June-July, 2020 is a typical rainy circulation situation in the ESWC. at 200hPa, the position of the upper jet stream leans to the south, resulting in frequent and southern cold air activity. At the same time, the ESWC is located just south of the jet axis, with strong divergence outflow from the upper layer and strong convergence inflow from the lower layer, which provides favorable dynamic conditions for precipitation. In addition, the Western Pacific Subtropical High (WPSH) obviously extends westward, and the warm and humid airflow in the southwest side of the WPSH is transported to the ESWC, which is conductive to more precipitation in this region. The qualitative water vapor source calculated by Euler method and the quantitative water vapor trajectory tracking results calculated by Lagrange method both show that: the water vapor associated with precipitation in the ESWC in June-July, 2020 mainly comes from the warm and humid airflow in the southern ocean, such as Bay of Bengal, South China Sea and Arabian Sea, while the dry and cold air in the north also transports water vapor to this region, but the water vapor content is much smaller than that in the southern ocean. In the previous winter, the snow cover area of the Qinghai Tibet Plateau was relatively large, and the SST of the Indian Ocean and the equatorial Middle East Pacific was relatively high, resulting in the weakening of the west wind in the bay of Bengal, the strengthening of the east wind in the South China Sea, and the strengthening of the south wind in the northern part of the Indochina Peninsula. This feature is just the typical wind field feature of the strong and westward WPSH, which is beneficial to strengthening of the westward water vapor transport in the Bay of Bengal and the South China Sea, and the strengthening of northward water vapor transport in the Indochina Peninsula. This makes the water vapor in the southwest side of WPSH continuously be transported into the ESWC, resulting in more precipitation in this region.
The Cooperation of High and Low Latitudes-Wave Trains in the occurrence of Extreme Winter Precipitation over South China
Available online:June 02, 2021 DOI: 10.3878/j.issn.1006-9895.2102.20246
Abstract:By using the ERA-Interim reanalysis data and daily precipitation observation from 756 stations over China, the extreme winter precipitation over South China during 1979-2015 are identified. They could be classified into five groups according to their spatial distribution via k-means clustering method. That is, the extreme precipitation occurs over the Yangtze River valley, central South China, southeastern South China, Huaihe River Valley and Southwest China. To disclose the causes of the widespread extreme winter precipitation over South China, the extreme precipitation with an eastward migration from Southwest China to the coastal Southeast China have been further analyzed, by comparing with that occurs locally over Southwest China. The results show that the main differences in the trigger and maintenance mechanisms of winter extreme precipitation between the local and eastward migrating cases, are the confliction between warm and moist advection by India-Burma trough and intensity of cold air activity. For the local cases, active cold air activity inhibits the development of India-Burma trough, resulting in the constraint of extreme precipitation over Southwest China. On the contrary, the warm and moist advection by India-Burma trough move eastward continuously in the eastward migration cases along with weak cold air activity. The cooperation of high-latitude wave train and South Asian jet wave train is crucial to the confliction between cold air activity and warm advection. When these two wave trains develop synchronously, the cold air activity over South China will be strong and the warm advection from the Bay of Bengal will be confined over Southwest China, so does the resulting precipitation. However, when the high-latitude wave train developed ahead of South Asian jet wave train, and the cold air activity is weak when the India-Burma trough is enhanced by the South Asian jet wave train, the warm advection from the Bay of Bengal could move eastward continuously, resulting in the eastward migration of precipitation from Southwest China to the coastal Southeast China.
A case study of raindrop size distribution characteristics and orographic impact in spring stratiform precipitation over the Qilian Mountains
Available online:June 02, 2021 DOI: 10.3878/j.issn.1006-9895.2103.20231
Abstract:Locating in the northeastern of the Tibetan Plateau, Qilian Mountains is an important ecological protective screen and conservation zone, as well as a key water source for Yellow River basin. However, studies on cloud and precipitation in this region are seriously lacking. Using the observations of 11 Parsivel2 disdrometers in Qilian Mountains, this paper studied the raindrop size distribution characteristics and orographic impact in a spring stratiform precipitation case over the Qilian Mountains. This precipitation process was initiated under the influence of a short wave trough and had significantly temporal and spatial variation. The disdrometer observations show that the mass equivalent diameter (D_m) was small, and the total number concentration of the raindrops (N_T) and D_m would increase and decrease as the elevation become higher. The relationship between 〖logN〗_w and D_m had featured stratiform precipitation characteristics, and in the same D_m there was higher N_w in Qilian Mountains. For lower elevation sites, there were less smaller raindrops (＜1 mm) and more larger raindrops because of the evaporation of small raindrops and the coalescence. Because the sites of higher elevation were close to the cloud base or in the cloud, the size scale of raindrops became smaller and D_m varied little with rain rate (R). The parameter values of the Γ distribution fit results were larger than other regions, so as to the N_0 of M-P distribution fit results. Γ distribution fits better for smaller raindrops, however, the Γ distribution and M-P distribution have underestimate and overestimate for larger raindrops, respectively. Influenced by the topography and the relative position between observational site and cloud base, the fit results of raindrop size distribution, the μ-Λ relationship, and the Z-R relationship all showed quite different characteristics with other regions or researches.
Available online:June 02, 2021 DOI: 10.3878/j.issn.1006-9895.2105.20206
Abstract:The WRF model was used to numerically simulate the heavy snowfall process in the Ili River Valley and the Northern slope of Tianshan Mountains on November 30, 2018, and analyze the vertical velocity and vertical kinetic energy change mechanism of heavy snowfall under complex terrain. Studies have shown that the passage of a cold front causes an increase in surface pressure and an increase in the dry air mass in column, which leads to changes in the vertical pressure gradient force and the dry air column buoyancy, which in turn causes the development of vertical motion. The local time variation of vertical velocity mainly depends on the perturbation vertical pressure gradient force, drag force of the water substance and perturbation dry air buoyancy. When the airflow crosses the Tianshan Mountains North Slope, the perturbation vertical pressure gradient force on the windward slope is larger, and the perturbation dry air buoyancy is smaller, which promotes the upward movement; On the leeward slope, the perturbation vertical pressure gradient force and the perturbation air buoyancy form a downward total force produces sinking acceleration, which causes the leeward slope strong sinking gale. The work done by the perturbation vertical pressure gradient force is basically opposite to the work done by the perturbation dry air buoyancy. On leeward slope, the work items of the perturbation vertical air pressure gradient force and the comprehensive force are to suppress the vertical kinetic energy, when the work item of the perturbation dry air buoyancy and the drag force of the water substance enhances the vertical kinetic energy. In addition, the perturbation vertical pressure gradient force and the perturbation dry air buoyancy work items mainly appear in the middle and low layers, the water material drag force work items are mainly located in the lower layers, and the comprehensive force work in flat terrain is significantly less than that in complex terrain.
Impact Studies of Introducing the Large-Scale Constraint into Km-Scale Regional Variational Data Assimilation
Available online:June 02, 2021 DOI: 10.3878/j.issn.1006-9895.2009.20176
Abstract:Since the framework design and observation selection are mainly for meso- and small-scale analysis, kilometer-scale data assimilation (DA) systems often suffer from insufficient large-scale analysis capabilities. This work adds an extra large-scale constraint to the cost-function of the GRAPES (Global/Regional Assimilation and Prediction System) regional 3km variational DA framework, to study the impacts of introducing the large-scale information of the global system on the kilometer-scale DA and forecast. The results of numerical experiments in one month show that, the introduction of large-scale constraint can significantly improve the analysis and forecast capabilities of the synoptic situation field, increase the precipitation forecast scores, and reduce analysis and forecast error of 2m temperature and 10m wind. Furthermore, results of the quantitative precipitation sensitivity tests show that the large-scale constraint of the temperature and humidity field is a very important factor for improving the precipitation scores. The results also indicate that the humidity field constraint is important for reducing precipitation false alarm and improving the TS scores for short-term precipitation forecast, while the temperature field constraint is important for improving the TS scores for longer forecast ranges. In addition, under the condition of introducing the large-scale constraint, the analysis and prediction results of the experiment with the full cycling scheme (no cold start during one month cycling) are equivalent to that of the experiment with partial cycle (daily cold start). This laid a good foundation for the GRAPES kilometer-scale system to adopt the full cycling scheme to further simplify the cycle process and reduce the calculation consumption.
Numerical simulation study on microphysical formation mechanism of a hail process in Yunnan, southwestern China
Available online:June 02, 2021 DOI: 10.3878/j.issn.1006-9895.2104.20152
Abstract:Abstract The microphysical formation mechanism of hailstones is the scientific basis of hail suppression operation, however, the relevant study on hail formation in southwestern China is few. A hail cloud case on 11 July 2016 in Yunnan is numerically simulated and studied by using the three-dimensional cloud model with hail-bin microphysics developed by the Institute of Atmospheric Physics, Chinese Academy of Sciences, and the microphysical formation mechanism of hailstone is investigated. This hail cloud has a rapid development and high intensity, which is a typical summer hail cloud in the southwest mountainous area. The simulated rainfall, hailfall and reflectivity are generally consistent with the corresponding observations. The simulated maximum updraft velocity is 28.5 m/s. The analysis of the microphysical process of hail formation shows that the main source of hail/graupel embryos is produced by the probability freezing process of supercooled rain drops, which is accounted for 97.8%, whereas the collision between ice crystals and supercooled rain drops is only accounted for 2.2%, and the results are significantly different from the source of hail/ graupel embryos and the proportion of frozen droplets obtained in other regions in both China and other countries. The diameters of the hail/graupel embryos are mainly ranging from 0.5-1.0 mm. The growth of hail/graupel embryos depends on the collision process with the supercooled cloud water, in which, the embryos with diameter less than 0.5 mm is hard to grow, and those with diameter larger than 1.0 mm grows easily. Larger raindrops can directly freeze into larger hail embryos, which can contribute to the formation of hail in a short period of time. The short-lived accumulation zone of supercooled water occurred during the development process of the hail cloud has less contribution to the growth of hail/graupel embryos. The location and evolution of the zonal wind velocity "zero line" and hail formation “channel area” suggest that hail cloud should be suppressed at the top of the strong center in the early stage of hail clouds, and at the strong center in the hail developing and strengthening stage.
Available online:June 01, 2021 DOI: 10.3878/j.issn.1006-9895.2104.21007
Abstract:In this paper, based on multi-source observation and reanalysis data and combined with a variety of analysis methods, the two torrential rain processes in Beijing on July 21, 2012 (hereinafter referred to as "721") and July 20, 2016 (hereinafter referred to as "720") are analyzed to compare and reveal the differences between them from multiple perspectives. The results show that the total amount of precipitation of the two processes is similar, but the precipitation duration and hourly rainfall intensity are quite different which represents that he duration of "7.21" is shorter and the rainfall intensity is stronger that is corresponding with the dominant weather system and evolution, convective system evolution and local sounding conditions of the two processes. The convective effective potential energy is significant in "7.21" main precipitation period resulting in the dominant convective heavy precipitation in warm area, while the convective effective potential energy is small in "7.20" main precipitation period and it is dominated by low vortex systematic precipitation. Therefore, there are significant differences in the statistics of hourly rainfall intensity and short duration rainfall events between the two processes. The proportion of medium intensity hourly rainfall stations of "7.20" is significant, while the proportion of short duration heavy rainfall stations is obvious. The differences in accumulated rainfall, duration, 5-minute and 1-hour maximum rainfall between the two short duration precipitation events are really significant. The "7.21" short duration heavy rainfall events (the short duration extremely heavy rainfall events with an hourly rainfall of more than 50 mm accounted for a significant proportion) exceeds half, as well as the maximum 5-minute and 1-hour precipitation were 20.4 mm and 103.6 mm, respectively, which is extremely notable. While the short duration medium intensity precipitation events of "7.20" accounts for the largest proportion, and the maximum 5-minute and 1-hour precipitation of only 10.7 and 59.3 mm. Compared with "7.20", "7.21" is more disastrous. The contribution of water vapor from central and eastern China and coastal areas is the largest in both processes, which is more prominent in "7.21". However, the contributions of the Indian Peninsula - Bay of Bengal - Central South Peninsula, South China Sea, Northwest Pacific and sea of Japan are also obvious in the "7.20". The above conclusions contribute to understanding the reasons of the different disaster of the two torrential rain processes.
Available online:May 31, 2021 DOI: 10.3878/j.issn.1006-9895.2105.20214
Abstract:At present, the cloud detection based on high-resolution optical images, with deep learning methodology, cannot provide adequate and accurate information about the cloud, the cloud shadows or their edge details. The main reason lies in the insufficient fusion of semantic information in different scales of classification techniques. In response to this problem, this paper combines Res.block module that can prevent network degradation, the multi-scale convolution module (MCM) that can increase the receptive field of the network and the multi-scale feature module (MFM) that can extract and integrate information from different scales and proposes a detection algorithm based on Multi-scale Feature Fusion Network (MFFN) based on deep learning. The experimental results show that rich spatial information as well as semantic information can be extracted by the algorithm. Cloud and cloud shadow masks with higher level of accuracy can also be acquired. The accuracy of the cloud detection is 0.9351, and the accuracy of the cloud shadow detection is 0.8103. Meanwhile, the study provides theoretical support and technical reserve for the application of deep learning techniques on operational cloud detection.
Seasonal differences in the trend transition characteristics of the surface sensible heat on the central and eastern Tibetan Plateau
Available online:May 27, 2021 DOI: 10.3878/j.issn.1006-9895.2105.21026
Abstract:In this paper, the Piecewise Linear Fitting Model (PLFIM) was used to analyze the seasonal differences of the Surface Sensible Heat (SSH) trend evolution characteristics at 70 meteorological stations on the Central and Eastern Tibetan Plateau (CETP) during1982-2018, and the key meteorological factors influencing the changes of the SSH in different seasons were quantitatively evaluated using the analysis of variance method. Results show that: (1) the seasonal average SSH fluxes in the CETP have a trend transition feature in all four seasons, as a whole, the trend transition time of autumn and winter is earlier (1999), and spring and summer is later (2000); regionally speaking, the turning time is earliest in Zone II (Eastern part of TP), followed by Zone IV (Southeastern part of TP) and Zone I (Northern part of TP), and the turning time is latest in Zone III (Southwestern part of TP). Before the trend transition time, the weakening of the SSH is most prominent in summer, followed by spring and autumn, and weakest in winter; after the trend transition time, the enhancement of the SSH is strongest in winter, and the enhancement trend is similar in other seasons. In winter and spring, the key areas for the trend transition of the SSH are in the eastern and southern of TP, respectively, while in summer and autumn, the key areas are mainly in Zone II and III. (2) Before the trend transition time, the decrease of the surface wind speed has an important contribution to the decreasing trend of the SSH in four seasons, but the interannual variation of the SSH in winter is dominated by the change of ground-air temperature difference; in spring and autumn are affected by both the variations of ground-air temperature difference and surface wind speed; and in summer is dominated by the surface wind speed. After the trend transition time, the increases of the surface wind speed and surface-air temperature difference in autumn and winter jointly lead to the trend turning of the SSH, but the interannual variation of the SSH is dominated by the variation of ground-air temperature difference; the increase of ground-air temperature difference in spring is the main cause of the trend turning of the SSH, and the ground-air temperature difference also plays a dominant role in the influence of the interannual variation of the SSH; the significant increase of surface wind speed in summer leads to the trend turning of the SSH, but the interannual variation of the SSH is affected by both the ground-air temperature difference and the surface wind speed.
Available online:May 27, 2021 DOI: 10.3878/j.issn.1006-9895.2102.20238
Abstract:In the long course of human beings to recognize climate, the study has involved a wide range of subjects, a great number of important persons, and a complicated set of incidents. All of them have made it extremely difficult for us to figure out the logical chain of the climate development. However, the development process of climate for thousands of years is very important, especially for the construction of climatology discipline. Based on a large number of previous studies, this paper attempts to review the significant incidents and persons along the development course of climatology in the temporal context. It will finally work out how human beings understood climate, and discuss the contribution to contemporary climatological development.
Available online:May 27, 2021 DOI: 10.3878/j.issn.1006-9895.2103.21029
Abstract:Whether perturbation can accurately describe the uncertainty of atmospheric motion is the core issue of ensemble forecasting research. A reasonable perturbation structure and amplitude should be able to reflect the characteristics of forecast error about the state of atmospheric motion. For solving the problems of insufficient spread, recognizability and reliability of perturbation in GRAPES regional ensemble forecast, this paper designs and develops different schemes to constrain initial ensemble perturbation with analysis error extracted from data assimilation system, in the light of the relationship of structure and evolution characteristics between ensemble perturbation and the forecast errors of weather systems at different scales. This study analyzes the spatial physical structure and spatiotemporal evolution characteristics of ensemble perturbation, spread and perturbation energy, in order to comprehensively assess the quality and performance of the analysis-constrained schemes. The results show that the schemes can partly identify and adjust the false perturbation in the original forecast. After constraining, the structure and evolution of the perturbation are in better agreement with the development of weather systems at different scales, with higher accuracy to describe the forecasting uncertainty. The feature that spread and perturbation energy increases with the forecast time is also more significant, especially in the early period.
Available online:May 27, 2021 DOI: 10.3878/j.issn.1006-9895.2105.21036
Abstract:In this study ,we evaluated the ability of four versions of Flexible Global Ocean-Atmosphere-Land System model (FGOALS) developed at State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics(LASG), Institute of Atmospheric Physics (IAP), Chinese Academy of Sciences in simulating climatology and seasonal cycle of tropical pacific precipitation and SST.FGOALS-f3-L and FGAOLS-g3 participated the Coupled Model Intercomparison Project (CMIP) Phase 6 while FGOALS-s2,FGOALS-g2 participated the CMIP Phase 5 (CMIP5)).This paper compared historical and AMIP simulations in terms of atmosphere-ocean feedback mechanisms and heat budget analysis to investigate the formation of precipitation and SST biases. The results exhibited notable improvements in FGOALS-g3 and FGOALS-f3-L compared with the previous versions ,such as a reduction of 50% of RMSE of SST decreasing from above 2 °C in FGOALS-g2 and FGOALS-s2 to 1 °C in FGOALS-f3-L. However, there are negligible improvements in precipitation. FGOALS-f3-L reproduces the meridional asymmetry of SST and precipitation in East Pacific well because of the reduction of errors in the mean state of shortwave radiation and its reasonable representations of ocean dynamic heat transport and surface shortwave radiation feedback. Moreover, excessive rainfall biases in the northern side of the equator become more severe than previous models, which is greater than 4mm/day. This overestimated convections bring latent heat which enhances the latitudinal ?diabatic heating gradient and strengthen the cross equatorial south wind. Errors in wind actually cancel out part of the warm biases of SST resulting from overestimation of shortwave radiation. Similar error sources existed in the simulated seasonal cycle , characterized by an improved annual cycle but a weaker overall amplitude in FGOALS-g3 and FGOALS-f3-L.This improved annual signal of SST results from stronger amplitude of annual cycle of meridional wind and latent heat flux. Actually, errors of wind and latent heat are bigger but cancel out the annual biases in short wave radiation flux. Therefore, the annual signal in SST seems to be enhanced than former versions. While the false semi-annual cycle of SST in FGOALS-g2 and FGOALS-s2 is mostly result from the semi-annual cycle of latent heat flux.
Objective Clustering of Spatial Patterns of Summer Extreme Precipitation Frequency over Huaihe River Basin and Their Formation Mechanisms
Available online:May 17, 2021 DOI: 10.3878/j.issn.1006-9895.2105.20223
Abstract:Based on the daily precipitation over Huaihe River basin and reanalysis datasets from 1961 to 2016, the spatial patterns of the summer extreme precipitation frequency over Huaihe River basin are objectively classified through K-means cluster, rotated empirical orthogonal function analysis (REOF), and their formation mechanisms are investigated via observational diagnosis and numerical simulations. The results show that: (1) the spatial distribution of summer extreme precipitation frequency over Huaihe River basin can be objectively divided into three types: S-Type, C-type, and N-type, in which the extreme precipitation mainly falls in south of 33°N, 31°-36°N and north of 34°N, respectively. (2) The S-Type is related to the anomalous southwestward shift of north western Pacific subtropical high (NWPSH). The cyclonic anomalies north of NWPSH hinder the water vapor further to the north, resulting in more extreme precipitation in south of Huaihe River basin. For the C-Type, the Huaihe River basin is controlled by saddle circulation anomalies pattern, therefore, extreme precipitation is mainly confined to the central Huaihe River basin. The N-Type is associated with the northwestward shift of the NWPSH. The southerly wind could transport water vapor to the north of Huaihe River basin, resulting in more extreme precipitation in the region. (3) The formation and the related circulation anomalies of S-Type and N-Type are closely associated with the El Ni?o-like and La Ni?a-like SST anomalies, whilst the C-Type related saddle circulation pattern is linked to two southward-propagated stationary Rossby waves over Eurasian Continent induced by the Arctic sea ice anomalies over Barents Sea/Kara Sea.
Air vertical motion and raindrop size distribution retrieval with Ka/Ku dual-wavelength cloud radar and its preliminary application
Available online:May 14, 2021 DOI: 10.3878/j.issn.1006-9895.2104.20200
Abstract:A new Ka/Ku dual-wavelength (Ka and Ku band wavelengths are 8.9mm and 2.2cm, respectively) cloud radar (DWCR) upgraded form the Ka band cloud radar was used to observe the vertical structures of cloud and precipitation in Southeaster China to improve the observational ability of air vertical motion and microphysical parameters. Aiming at researching the dynamic and microphysical parameters of precipitation in Southeaster China by the DWCR, a retrieval algorithm (DWSZ) for air vertical motion (Vair), raindrop size distribution (DSD), liquid water content (LWC) and rain rate (R) and radar wave attenuation correction in rain area with the DWCR’s reflectivity spectral density data (SZ) were presented in this paper. A precipitation case in Longmen, Guangdong Province, was used to analyze the vertical air motions with DWSZ and single Ka and Ku band CR. The DSD data from disdrometer were used to examine the retrieved Vair and microphysical parameters by the DWSZ. The results showed that the retrieval algorithm gave a reasonable Vair，microphysical parameters and their vertical distributions. The attenuation correction for Ka and Ku band reflectivity data reduced the bias between both bands of reflectivity. The DWCR could be used to observe cloud and precipitation with reflectivity of 0-30dBZ. In the mixed cloud observed by the DWCR, the updraft and downdraft were found in front and back of the convective cells. From the averaged profiles, Vair and mass-weighted mean diameter (Dm) reached their maximum at 2 km, drop number concentration (Nw), LWC and R increased below this level. Water vapor condensation, rain drop collection cloud drop are possibly main process. This work revealed the relationships between microphysical and dynamic processes in this precipitation case, verified the DWCR observational ability and provided the base for development of Ka/Ku band dual wavelength cloud radar, future improvement of single wavelength cloud radar retrieval algorithm and observation of more fine and accurate structures of the microphysical and dynamic parameters.
Variation Characteristics of the Westerly flow around the Tibetan Plateau in Winter and its Impact on Climate in China
Available online:May 12, 2021 DOI: 10.3878/j.issn.1006-9895.2103.20244
Abstract:Using the NCEP/NCAR reanalysis data from 1979 to 2019 and the temperature and precipitation of the daily data set of basic meteorological elements in China (V3.0), the indexes that objectively characterizes the changes of the northern and southern branch westerly flows around the Tibetan Plateau in winter are defined firstly. And then, the different characteristics of their changes are analyzed, and the influence mechanism of the abnormal changes of the two branch flows around Tibetan Plateau on the temperature and precipitation in China is preliminarily studied by means of correlation analysis and composite analysis. The main results are: (1) There are obvious different characteristics of changes between the northern and southern branch westerly flows around Tibetan Plateau in winter. When the northern branch (southern branch) flow is stronger and the southern branch (northern branch) flow is weaker, there is a large-scale anomalous anticyclone (cyclone) circulation system from the western Tibetan Plateau to the eastern coast of China in the middle-low latitudes of troposphere, and the central Tibetan Plateau is an anomalous anticyclone (cyclone) system center over 500hPa, which extends vertically to the height of 300hPa (inclining to the Northwest with the altitude). There is a significant negative correlation between the north and the south branch westerly flows of the troposphere. (2) The anomalous changes of the south/north branch westerly flows around Tibetan Plateau in winter have a significant impact on the winter weather and climate in China. When the northern branch flow of the Tibetan Plateau is stronger (weaker), the Hetao, Tibetan Plateau and the South China have relatively higher (lower) temperature and less (more) precipitation, but the lower (higher) temperature and more (less) precipitation in Northeast China. However, when the southern branch flow is stronger (weaker), the temperature is generally lower (higher) in the whole China, the precipitation is less (more) in northeastern China and northern Xinjiang, the more (less) precipitation in most parts of southwestern and southern China. (3) A comprehensive analysis the abnormal changes of the vertical structure of atmospheric elements over the East Asian troposphere influence on the China weather and climate anomalies shows: when the northern branch flow is stronger corresponding to the weaker southern branch flow, there are abnormal westerlies in the troposphere to the northern area of 35°N, and abnormal easterly winds above 850hPa to the southern area. The troposphere atmosphere over eastern China affected by the abnormal zonal flows around the Tibetan Plateau of the "Barotropic structure". The corresponding troposphere below 850hPa has a consistent anomalous southwest wind from southern to northern areas, the anticyclonic shear between the anomalous northwest wind and northeast wind above 850hPa in near 35 ° N, the abnormal descending motion and the warmer below 300hPa, these conditions are not conducive to the cold air southward movement and the anticyclone shear enhanced the effect of subsidence motion on temperature increase, as a result, the temperature is higher and the precipitation is less in most areas of eastern China. On the contrary, when the southern branch flow is stronger corresponding to the weaker northern branch flow, the abnormal zonal wind in the troposphere is an obvious "Baroclinic feature", and the anomalous westerly wind shows a slope zonal feature from lower to upper troposphere and from low latitude to high latitude. The Abnormal easterly wind of the lower troposphere is "triangle shaped 200hPa of North China, matching the meridional wind anomaly and the abnormal ascending movement from South China to North China, the "triangle shaped" abnormal cold air mass cuts southward into the South China Sea in the lower layer, the warmer southwest airflow climbs from south to north over the cold air mass to the middle and high latitudes in the upper layer, resulting in a large range of abnormally lower temperature and more than precipitation in China.
Microphysical Characteristics of Fog with Different Intensities and Their Relationship with Visibility in Shouxian County
Available online:April 25, 2021 DOI: 10.3878/j.issn.1006-9895.2103.20230
Abstract:Fog has adverse effects on transportation, especially extremely dense fog. In this paper, the fog droplet spectrum data measured by FM-100 fog drop spectrometer at Shouxian National Climate Observatory in January 2019, together with the contemporary conventional meteorological observation data, were used to investigate the microphysical characteristics of fog with different intensities. Based on the analysis of the relationships between visibility (V) and liquid water content (L), number concentration (N) of fog droplets and relative humidity (RH), various visibility parameterization schemes were established. The results show that: (1) With the increase of fog intensity, the water content in fog increased significantly, with average values of 0.003, 0.01 and 0.09 g·m-3 during the periods of fog, dense fog and extremely dense fog, respectively. When the L was greater than 0.02 g·m-3, the proportion of extremely dense fog reached 95%. (2) The N and droplet size increased with the increase of fog intensity. From fog to dense fog, the N increased significantly (increased by 67%), while from dense fog to extremely dense fog, the droplet size increased significantly, and the average diameter (D) and effective radius (Re) increased by 62% and 135% respectively. When the Re was greater than 4.7 μm, the proportion of extremely dense fog reached 95%. (3) All the spectra distributions of droplet number concentration for fog, dense fog and extremely dense fog were bimodal structure, with the major peaks close to the end of small particles. The spectrum type of extremely dense fog was Deirmendjian distribution; while it was Junge distribution for dense fog and fog. As for fog water mass concentration spectrum it was characterized by multi peaks for extremely dense fog with the maximum peak appeared at 21.5 μm, bimodal distribution and single peak type for dense fog and fog respectively, with the maximum peak at 5 μm. (4) Both L and N were inversely correlated with visibility, and L showed the highest correlation coefficient with visibility. Four kinds of visibility parameterization schemes were established by using full sample and segmented method respectively, and the test results indicated that the visibility subsection fitting scheme based on L was the best.
Available online:April 20, 2021 DOI: 10.3878/j.issn.1006-9895.2104.21011
Abstract:Record–breaking rainfalls occurred over Yangtze–Huaihe River Basin (YHRB) in China during Meiyu Period (June–July, JJ) of 2020, causing severe floods and disasters. These rainfall anomalies were closely linked to the extremely strong anomalous anticyclone over the western North Pacific (WNPAC), which favored convergence of water vapor over YHRB. This study argued what have drived the record–breaking rainfalls and WNPAC in Meiyu periods of 2020. A weak Central–Pacific El Ni?o rapidly decayed in spring and developed to a La Ni?a in late summer, while sea surface temperature (SST) in the tropical Indian Ocean (TIO) and tropical northern Atlantic (TNA) was considerably high from previous winter to summer. We revealed that the weak decaying El Ni?o alone was not sufficient to maintain the strong WNPAC in JJ of 2020, whereas the long-lasted warm SST anomalies in the TIO and TNA prominently contributed to the enhancement and westward shift of the WNPAC. The TIO warming intensifies the WNPAC through the eastward propagation of Kelvin waves and/or modulating the Hadley circulation. The TNA warming can force a westward–extending overturning circulation over the Pacific–Atlantic Oceans, with a sinking branch over the central tropical Pacific, which suppresses the convection activity over there and in turn gives rise to the WNPAC. The TIO and TNA warming contributed significantly to the extremely strong WNPAC in JJ of 2020.
Available online:April 20, 2021 DOI: 10.3878/j.issn.1006-9895.2101.20228
Abstract:In this study, we investigate the relative contributions of interdecadal Pacific oscillation (IPO) , Atlantic interdecadal oscillation (AMO) and global warming (GW) to decadal variation of land precipitation in North America from 1934 to 2018. Firstly, through the singular value decomposition (SVD) analysis for North American land precipitation and sea surface temperature (SST) in middle and low latitudes, it is found that IPO (42.33%) and AMO (23.21%) are the main SST modes in winter, and AMO (32.66%) and IPO (21.60%) in summer, to affect the interdecadal variation of the land precipitation. Secondly, the linear regression model is used to analyze the relative contribution of the three signals. The results show that AMO contribute dominantly of North America in summer. IPO plays a role only secondary to IPO, and winter is the opposite. GW also plays an important role. In summer, AMO is the primary contributor to the changes of precipitation in Alaska; In Canada, GW dominates. In USA and Mexico, GW, AMO and IPO are of equal importance; In winter, GW is the primary contributor to the changes of precipitation in Alaska and Canada; IPO dominates in USA and Mexico. In terms of information flow, we have presented the regions of sensitivity to the three modes. We also apply ECHAM 4.6 model to further verify the above results, show that the Indian Ocean is pivotal in having AMO and IPO in effect in causing the precipitation variabilities.
Available online:March 30, 2021 DOI: 10.3878/j.issn.1006-9895.2103.20254
Abstract:The sea-level pressure (SLP) in the East Asia-Northwest Pacific region directly reflects the circulation characteristics of the lower atmosphere, and its dynamical characteristics have significant effects on the atmospheric circulation situation, the evolution of the pressure system and the development of weather and climate systems. Therefore, an in-depth analysis of the spatial and temporal evolution characteristics of the sea level pressure field in the East Asia-Northwest Pacific region is of great significance to improve the weather and climate forecasting in China. In order to investigate the dynamical characteristics of the daily sea-level pressure field from the perspective of nonlinear dynamics, a new method is used to quantitatively estimate two instantaneous indicators of the sea-level pressure attractor: the instantaneous dimension and the instantaneous stability. The instantaneous dimension characterizes the dispersion of the attractor orbit in local space, and the instantaneous stability characterizes the stability of the orbit in local time, which together characterize the instantaneous (daily) dynamical properties of the sea-level baroclinic attractor. In this paper, different spatial and temporal characteristics of the sea-level pressure field in the East Asia-Northwest Pacific region are screened by the correspondence between the indicator values of different sizes and the daily sea-level pressure circulation field. The main conclusions are: (1) When both indicators of sea-level pressure attractor are low, the spatial characteristics of the corresponding circulation field are as follows: the pressure structure is single, usually with several strong high and low-pressure centers facing each other at east-west direction, while the time characteristics show that the circulation mode can be stable for about 10 days. (2) On the contrary, when both indicators are high, the spatial characteristics of the circulation field are as follows: multiple weak pressure centers exist at the same time, with a chaotic spatial structure. As for the temporal characteristics, the circulation field is extremely unstable, and the duration is only about 1 day. (3) The two indicator anomalies in the East Asia-Northwest Pacific region have a consistent interdecadal trend, both showing a clear downward trend from the 1970s to 1990s, a rapid rising trend in the late 1990s and fluctuating changes after 2000.
Study on the microphysical structure characteristics and cloud-rain autoconversion threshold function of stratiform warm clouds in Jiangxi
Available online:March 30, 2021 DOI: 10.3878/j.issn.1006-9895.2102.20166
Abstract:This study analyzes the microphysical properties of precipitating and non-precipitating warm clouds based on seven-flight cloud measurements from November 6 to December 25, 2014, in JiangXi Province, China. The autoconversion threshold function (T) represents the probability that the collision-coalescence process occurs in clouds, which is critical to determine the initial time and intensity of the precipitation. We found that, in general, the T increases with height above the cloud base, with the maximum value occurring in the middle and upper parts of clouds. The occurrence frequency with T > 0.6 in the precipitating clouds is larger than that in the non-precipitating clouds, indicating a stronger collision-coalescence process and a greater probability of raindrops generated by the condensation and collision-coalescence processes in the precipitatng clouds. There is a negative relationship between the relative dispersion of cloud droplet size distribution (ε) and the number concentration of cloud droplet (Nc); meanwhile, the negative relationship becomes more evident with the increasing T. Compared with the average radius of cloud droplet (ra), the standard deviation of cloud droplet size distribution (σ) dominates the enhancement of the degree of the negative relationship between ε-Nc.
Fenwei Plain Air Quality and the Dominant Meteorological Parameters for Its Daily and Interannual Variations
Available online:March 03, 2021 DOI: 10.3878/j.issn.1006-9895.2101.20240
Abstract:Due to the special terrain conditions and coal-based energy structure, air pollution in the Fenwei Plain has been a serious issue. In 2018, the Fenwei Plain was listed as a key area for air pollution prevention and control. This study used the observed concentrations of PM10, PM2.5, SO2, NO2, CO, O3 over 2015–2019 and the Air Quality Index (AQI) to analyze the temporal and spatial distributions of AQI and mass concentrations of pollutants in the Fenwei Plain. We applied the multiple linear regression model to identify the meteorological conditions that influenced the daily and interannual variations of PM2.5 in winter and the maximum daily average 8-hour O3 (MDA8_O3) in summer in the Fenwei Plain. We found that air quality of the Fenwei Plain deteriorated year by year from 2015 to 2017 but improved from 2018 to 2019. The most polluted cities were Xi’an, Weinan, Xianyang, Linfen, Yuncheng, Sanmenxia, Luoyang, which were located in the junction of the Fenhe plain and the Weihe plain. The primary air pollutants in the Fenwei Plain were PM2.5, PM10 or O3, which accounted for about 90% of the polluted days. Severe pollution occurred mainly in the winter heating period when the weather conditions were unfavorable and the emissions of pollutants were large. In summer, concentrations of O3 in the Fenwei Plain increased over the past years. The most important meteorological parameter for daily variations of both PM2.5 in winter and MDA8_O3 in summer was 2-meter air temperature (T2M), with relative contributions of 45.5% and 35.3%, respectively. T2M was positively correlated with PM2.5 in winter and MDA8_O3 in summer. The second important meteorological parameter was 2-meter relative humidity (RH2M) for both PM2.5 in winter and MDA8_O3 in summer, with relative contributions of 41.5% (positive correlation) and 25.4% (negative correlation), respectively. With respect to interannual variations in PM2.5 in winter, the two most dominant meteorological parameters were T2M (43.6%) and RH2M (31.9%), which were both positively correlated with concentrations. Changes in meteorological conditions in winter over 2015–2019 had an effect of increasing PM2.5, which offset to some extent the decreases in emissions. With respect to interannual variations in summertime MDA8_O3, the two most dominant meteorological parameters were T2M (71.7%, positive correlation) and wind speed at 850 hPa (WS850, 16.3%, negative correlation). Changes in meteorological conditions in summer over 2015–2019 had an effect of increasing O3 (1.2 μg m-3 yr -1), which was a smaller effect compared to the increases in O3 (7.5 μg m-3 yr -1) caused by changes in anthropogenic emission. Our results indicate that air pollution in the Fenwei Plain is severe. Which the particulate pollution has not yet been resolved yet, it now also faces new challenges of ozone pollution. Considering that the Fenwei Plain is under influenced by Shaanxi, Shanxi, and Henan, it is necessary for the three provinces to joint prevention and control to improve the air quality in the Fenwei Plain.
Available online:March 03, 2021 DOI: 10.3878/j.issn.1006-9895.2101.20201
Abstract:As the key precursors leading to ozone pollution, atmospheric volatile organic compounds (VOCs) are indispensable parts of urban air quality modeling. Due to their complex composition and the lack of monitoring data, the understanding of their simulation accuracy is still poor. In this paper, the nested grid air quality model prediction system (NAQPMS) is used to simulate VOCs in the Pearl River Delta region from September 21 to November 20, 2017. The VOCs concentration monitoring data from 8 ground stations of photochemical monitoring network is used to evaluate the accuracy of key VOCs components. The results show that the model has high simulation accuracy for toluene, ethylene and xylene with concentration deviation ratio of 0.4-26.6%, which can well reproduce the trend of daily average concentration and the double-peak characteristics of diurnal variation. However, the model has a large simulation deviation for isoprene with strong chemical reaction activity and closed relation to plant emissions. The deviation ratio is nearly 100%, which cannot reproduce the diurnal variation characteristics of high concentration in daytime and low concentration at night. It is found that the total amount of VOCs emitted by plants in the Pearl River Delta region is relatively large. However, the ignorance of biological VOCs emissions in the current simulation system may be the key reason for this simulation deviation. Besides, the results of simulation evaluation also show that the model still exists great uncertainty in the VOCs spatial distribution. This paper shows that there is an urgent need to combine VOCs observation data in the model to reveal and reduce the uncertainty of VOCs simulation.
Available online:March 02, 2021 DOI: 10.3878/j.issn.1006-9895.2008.20157
Abstract:Using the analysis and predictions of leading 35 days related to Global and Regional Assimilation Prediction System (GRAPES)-Global Forecast System (GFS) during the period from September 2018 to August 2019, we diagnosed the prediction errors and evaluated the extended forecast capability to improve a numerical weather guidance for the sub-seasonal timescale. Result show that, GRAPES-GFS could capture the spatial distribution characteristics of 2m temperature and 500hPa geopotential height during winter in 2018 and summer in 2019, however there exists large system bias related to 2m temperature analysis in the desert plateau areas which have the thermal forcing effect significantly, especially in arid areas of Africa. Related to 2m temperature, the Root-Mean-Square Errors (RMSE) of leading 1 to 3 weeks predictions approximate to the linear growth. GRAPES-GFS posses the high prediction skill in the East Asia and Austria but have the lower prediction skills in the ocean areas compares with the land areas. Related to 500hPa geopotential height, when leading 1 to 3 weeks predictions, there exist higher prediction skills in low latitude than in high latitudes of East Asia. Also, the prediction skills of the tropics is much lower than other regions and the northern hemisphere is higher than the southern hemisphere. Related to the Madden-Julian Oscillation (MJO), it is found that, GRAPES-GFS can reproduce the propagation characteristic of spatial-temporal variations related to the upper and lower zonal wind and can capture the location of strong convective activity signals. However, the Outgoing Long Wave Radiation (OLR) positive anomaly is much weaker and the negative anomaly is much stronger. GRAPES-GFS have a skillful MJO forecast for 11 lead days when it is useful for ACC and for the selected two strong MJO cases, GRAPES-GFS could describe the MJO propagation process exactly but have a stronger signal during MJO developing and decaying period.
Available online:March 02, 2021 DOI: 10.3878/j.issn.1006-9895.2102.20173
Abstract:This paper mainly focuses on numerical weather prediction problem in the steep orographic region of China. The step terrain vertical coordinate (known as Eta) is introduced into the dynamic core of Weather Research and Forecasting model (WRF) to improve numerical forecasting in the complex orographic region. We firstly design a mathematic transformation scheme and give the Eta-coordinate WRF core the same formulation of model equation system as the Sigma WRF core, which can facilitate discretization and programming of the numerical model. The comparison between two dynamic frames shows that the difference just lies in representation of the column mass of atmosphere between the reference level and the top level of the model. We carry out 2D mountain wave simulation experiment and it is indicated that the inherent isolation of the step topography can result in the airflow separation, which accounts for inadequate simulation of the mountain wave with coarse vertical layering using the new Eta-coordinate WRF core. Fortunately, the mountain wave simulation will be improved using the new dynamic frame either with fine vertical resolution or with long simulation time, which is believed as alleviation on the airflow isolation of the step mountain.
Numerical Simulation Analysis on the Generation and Evolution of the Dynamic and Thermodynamic Processes of an Extreme Rainfall in Jilin Province
Available online:March 02, 2021 DOI: 10.3878/j.issn.1006-9895.2011.20183
Abstract:The analysis of circulation background and trigger mechanism of an extreme precipitation weather process in Jilin area on July 13-14, 2017 was conducted by using the ERA-Interim reanalysis data , conventional meteorological observation data, CMORPH fusion precipitation data and high-resolution numerical simulation results of WRF. The results are as follows. (1) The northward movement of the subtropical high and the cold vortex over Northeast China and mid-latitude front zone are the large-scale circulation background of precipitation. Precipitation occurs in the straight zonal circulation between the bottom of the cold vortex and the subtropical high. Clod trough, low-level shear line, upper-level jet stream and low-level jet stream are important weather systems affecting precipitation. (2) Under the background of high-level divergence and low-level convergence, extreme water vapor transport and the blocking and forced uplift of the low west and east high terrain in Jilin area are one of the factors that cause extreme precipitation. (3) Dry and cold air intruded into the middle and upper levels, and the high-level momentum transmitted to the low altitude, which strengthened the low-level jet and generating an ultra-low-level jet near surface, resulting in the enhancement of the vertical ascent movement. The intersection of north-south meridional momentum transport strengthens low-level wind convergence shear, and the development of convection on the shear line led to extreme precipitation in Yongji County.
Decadal variability in the relationship between May rainfall over Southwest China and Arabian Sea Monsoon
Available online:March 02, 2021 DOI: 10.3878/j.issn.1006-9895.2012.20195
Abstract:Based on the observed precipitation data at 115 stations in Southwest China over the period of 1960-2017 and Japanese 55-year reanalysis data, this study investigated the dominant mode of the May rainfall over Southwest China and its relationship with the Arabian Sea monsoon. The results show that the leading mode of the May rainfall over Southwest China exhibits consistent variability over the region. The variability of the leading mode shows a close relationship with the Arabian Sea monsoon, but the relationship experienced an interdecadal change around the late 1970s. Over the period of 1960-2017, the anomalous atmospheric circulations and water vapor transportation associated with the Arabian Sea monsoon are mainly over the Arabian Sea to Bay of Bengal, which cannot reach Southeast China; consequently the Arabian Sea monsoon has a weak influence on the May rainfall over Southeast China during the period. In contrast, over the period of 1981- 2017, the anomalous Arabian Sea monsoon is related to larger-scale atmospheric anomalies over the region from the northern Indian Ocean to South China Sea. Such atmospheric anomalies can lead to anomalous water vapor and vertical motion over Southeast China; therefore, the Arabian Sea monsoon can significantly influence the May rainfall over Southeast China during the period. Further preliminary analysis indicates that the change in the relationship between Arabian Sea monsoon and May rainfall over Southwest China could be related to the change in decadal variability of Arabian Sea monsoon. Over the period before the late 1970s, the variability of Arabian Sea monsoon is relatively weak and its related atmospheric circulations are also weak. On the contrary, the variability of Arabian Sea monsoon is stronger after the late 1970s. The stronger variability of Arabian Sea monsoon is related to stronger atmospheric circulations, which can extend to Southwest China, consequently favoring the influence of Arabian Sea monsoon on May rainfall over Southwest China during the period. This result indicates that the variability of monsoon could play an important role in its influence.
Impact Study of Spectral Nudging Technique Drived by ECMWF Data on the Fine Numerical Prediction of Super Typhoon Lekima (2019) in Zhejiang Province
Available online:February 25, 2021 DOI: 10.3878/j.issn.1006-9895.2101.20193
Abstract:To improve the performance of high-resolution regional numerical model, based on both advantages of high accuracy forecasting data from European Centre for Medium-range Weather Forecasts (ECMWF) and spectral nudging (SN) technique, the impact from spectral nudging (SN) technique drived by ECMWF data on the fine prediction of super typhoon Lekima (2019) in Zhejiang province are investigated by the Weather Research and Forecasting (WRF) model. The results show that: (1) The contribution of SN to track of Lekima is obvious, for instance, the maximum hourly track errors can be reduced about 80 km. (2) In Zhejiang province, the Fine predictions of gale and heavy rainfall induced by Lekima (2019) are largely improved through SN technique. Compared to the control experiment, the increased percentage of ETS score of strong wind larger than 17.2 m/s is about 8% in mean and 20.8% in maximum. At the same time, the increase rate of TS scores of heavy rainfall with threshold as 50 mm/24h (100 mm/24h) reaches 11.8% (26.2%). (3) Further diagnosis illustrates that wind field spectral nudging can amend efficiently the west deviation of typhoon track and the over-prediction of strong wind as well as the heavy rainfall in southwest of Zhejiang province, which are related with the improvements in all atmospheric elements in troposphere, steering flow of Lekima, low-level wind field and resulted rainfall enhancement effect by local topography.
Abstract:In this paper, we redefine and calculate boreal summer intraseasonal oscillation(BSISO)index by making empirical orthogonal decomposition (EOF) using the OLR data and the 850 hPa zonal wind velocity u data over 10°S-50°N, 40-160°E from May to October in 1981-2010. The evolution characteristics of BSISO and its influence on summer precipitation in North China are analyzed. The results are as follows: (1) there are two obvious low frequency signals in the Indian Ocean-Northwest Pacific region in summer. One is BSISO1, which is inclined from northwest to southeast and propagates from the tropical Indian Ocean to northeast with an oscillation period of about 45 days. The other is BSISO2, which is inclined from southwest to northeast and propagates from the Northwest Pacific to the northwest with an oscillation period of about 15 days. (2) BSISO affects the summer precipitation process in North China mainly by influencing circulation anomalies and water vapor transport anomalies. At 500hPa, BSISO signal will cause the North-South movement and intensity change of the subtropical high in the east of North China to affect the summer precipitation process in North China. At 850 hPa, BSISO signal will affect the water vapor transport to North China and the convergence and divergence through the associated cyclonic or anticyclonic anomalous circulation to influence the summer precipitation process in North China. (3) Although MJO signal exists throughout the year, its amplitude is the largest in winter half year, especially in winter, and the minimum in summer. The amplitude of BSISO signal is the largest in summer half year, especially in summer. Therefore, using the low frequency signal of tropical atmosphere to predict the extension period, winter half year can focus on consideration the impact of MJO and summer half year focus on consideration the impact of BSISO.
SSimilarities and Differences of Monsoon Circulation during Meiyu in the Middle and Lower Reaches of the Yangtze River between 2016 and 1998 and the Physical Mechanisms
Available online:February 24, 2021 DOI: 10.3878/j.issn.1006-9895.2101.20174
Abstract:Abstract The similarities and differences of monsoon circulation during Meiyu in the Middle and Lower Reaches of the Yangtze River between 2016 and 1998 following strong El Nino and the Physical Mechanisms are analyzed here. The results show that: (1) The monsoon circulation during Meiyu in 2016 and two Meiyu periods in 1998 has several similarities: WPSH ( Western Pacific Subtropical High ) is stronger and westward than usual, SAH(South Asia High) is stronger and Eastward, and Southwest Monsoon is weaker from the bay of Bengal to South China Sea. WPSH continuously guides Southwest Monsoon to the middle and lower reaches of the Yangtze River, forming a strong warm and humid southwest air flow, which converges with the dry and cold air from the cold trough, and forms heavy rainfall under the condition of high-level divergence.（2） During the three periods of Meiyu, there existed a warm ridge above Qinghai Tibet Plateau and nearby. The obviously positive temperature anomalies at middle-upper levels over most of Qinghai Tibet Plateau to Jiangnan and South China due to the warm advection ,heat sources over the Plateau and latent heat of condensation of Meiyu, Positive SSTA with strong convection and heat sources over Indonesian archipelago, both factors led to the similar characteristics of monsoon circulation. (3) In 2016, during the Meiyu period, the strongest warm ridge over Qinghai Tibet Plateau, the shallowest eastern cold trough, the greatest and the most northerly positive SSTA lead to the most northerly SAH and WPSH, and the most northerly Meiyu rain belt. After Meiyu, there was no continuous cold air flowing southward, and the positive SSTA over South China Sea, the eastern ocean of the Philippines and even the East China Sea increased rapidly with active convection, all these led to strong and northerly WPSH, so there is no longer a second Meiyu. (4) In 1998, from middle July to early August, the ridge over Qinghai Tibet Plateau was weak with negative geo-potential height anomalies,so the cold air over northern Qinghai Tibet Plateau was strong, the cold trough in the East was deep, and there was no significant change of positive SSTA in equatorial Northwest Pacific, so SAH and WPSH continued to be strong and abnormally southward, so that the second section of Meiyu appeared.
Variability of the Phase reversal of the East Asia Temperature from Early to Late Winter and Possible Influencing Factors
Available online:February 04, 2021 DOI: 10.3878/j.issn.1006-9895.2011.20181
Abstract:In addition to winter mean temperature, the conspicuous intraseasonal oscillation of East Asia winter air temperature (EAT) exerts a large influence on human activities and the economy. Based on the station data and the reanalysis data during the period of 1959-2018, we analyzed the intraseasonal spatiotemporal variability for East Asian winter temperature by using the method of season-reliant Empirical Orthogonal Function (S-EOF). The results show that the spatiotemporal of EAT is mainly characterized by the intraseasonal phase inversion variation pattern, which means warmer (colder) than normal in early winter and colder (warmer) in late winter. This mode accounts for the variance contribution by 31.1% and also represents the intraseasonal winter temperature over the entire Asian continent. The circulation evolutionary of this mode mainly characterized by the phase inversion of Eurasian teleconnection pattern (EU) from a positive (negative) phase in early winter (December) to a negative (positive) phase in late winter (February), accompanied by the of turning of lower Siberian high and Aleutian low intensity, and the intraseasonal evolution of the subtropical jet. The possible influencing factors which affecting the EU phase reversal are as follows: first, the circulation over the North Atlantic reverse between early winter and later winter and which influences the EU through the North Atlantic heat flux. Second, wide and narrow SST anomalous of ENSO events. In El Ni?o years, the events of wider (narrower) SST anomalies are more likely to inverse from the phase of warmer (colder) in the early winter to colder (warmer) in late. When La Nina events occur, cases on the opposite.
Observation and Analysis of Snowbands Structure in a Cyclone Frontal Snowfall at Beijing with a Ka-band and an X-band Polarized Radars
Available online:December 30, 2020 DOI: 10.3878/j.issn.1006-9895.2009.20103
Abstract:In this paper, a simultaneous observation method of a Ka-band polarized radar and an X-band polarized radar in the same station of Institute of Atmospheric Physics, Chinese Academy of Sciences are designed for the first time. The formation and development of snowbands during the snowfall process of a frontal cyclone system inBeijing on February 14, 2019 are observed by this method, and the lifecycle and vertical structure of snowbands are analyzed. Results show that structure of snowbands is similar but different from the four-layer structure composed of condensation layer, aggregation layer, riming layer and melting layer of rainbands. Snowbands only contains condensation layer, aggregation layer and riming layer formed from seeding of upper layer cloud to lower layer cloud. Multiple snowbands continuously generate and develop to maintain the snowfall. Horizontal wind speeds vary from layer to layer, so the three layers of a snowband may not be arranged vertically. Snowflakes are formed continuously in snowbands until condensation layer becomes empty, and the cloud splits from this layer into multi-layer clouds and then dissipates respectively. It is proved that the dual frequency polarized radar simultaneous observation is necessary and efficient. It complements the observation of snowfall by Ka-band and X-band radar, and enriches the understanding of snowbands of frontal cyclone system.
Comparative Analysis of Precipitation Characteristics of two Westward Landfall Typhoon under Different Monsoon Background
Abstract:In this paper, two westward landfall typhoon cases “Bilis” and “Sepat” are taken as research objects, which both caused heavy rainfall in southern Hunan province, however, under different monsoon background. Using CMORPH precipitation product, JRA_55 reanalysis data and TBB data from FY-2E satellite, instable energy source of convection, uplift mechanism and vertical shear of environmental wind were compared between cases “Bilis” and “Sepat”. The comparative analysis shows that under basically same underlying surface and terrain, the main causes of rainstorm by two cases are different, which is mainly reflected in the factors that initiate and maintain convection. The strong convergent updraft and convective instability under the strong monsoon background are the main factor causing the heavy rainstorm of “Bilis”, while the rainstorm of “Sepat” was mainly caused by local orographic lift and baroclinic instability under the weak monsoon background. Moreover, monsoon flow could lead to obviously asymmetric precipitation by changing vertical shear of environmental winds indirectly.
Impacts of cloud microphysical process on warm-sector precipitation over Jilin province, Northeast China
Available online:December 23, 2020 DOI: 10.3878/j.issn.1006-9895.2007.20128
Abstract:This study simulated a warm-sector rainstorm event that occurred in Yongji, Jilin province on July 13 2017, which reproduced the development process that include the initiation of convective cells and linear system, organized and the stagesthat form bow echo; based on these data, the cloud microphysical characteristics of the mesoscale convective systems were analyzed, and then discussed the possible cloud microphysical mechanisms causing the warm-sector precipitation. The simulated results show that this precipitation process in the Yongji occurred in a favorable multi-scale environmental configuration dominated by the northeast cold vortex. The mesoscale systems was mainly the cold cloud systems. The warm zone had a large range, so that the location of supercooled water was high, and ice and supercooled water coexisted, the "seeding" effect of the coexisting area caused a large amount of graupel. Diagnoses of the mass- and heat-hydrometeor budgets showed that the main source of rainwater was the accretion growth of cloud droplets, and the main sink was the collection of raindrop by ice during the triggering and organization of the precipitation system; while in the bow-shaped echo stage, the melting of graupel add to the main source terms, the main sink terms were the evaporation of rainwater in the lower layer and the collection of rainwater by the graupel in the upper layer. The main heat source of warm-sector precipitation was the latent heat release from condensation of water vapor, and the main cooling term was the evaporation of rain and cloud water. In the bow-shaped echo stage, confluence of the inflow at the front and the backward inflow above the cold pad on the ground bring water vapor into the upper layer, and the "seeding" effect significantly increased the content of the graupel particles near the height of 8km from the ground, which coincided with the high temperature area formed by the condensation of water vapor to release a large amount of latent heat. Therefore, a large amount of graupel melted into rainwater, resulting in a strong precipitation process.
Atmospheric circulation characteristics of heavy precipitation events over the southern China in the autumn of 2016 and impact of SST
Available online:December 23, 2020 DOI: 10.3878/j.issn.1006-9895.2011.20180
Abstract:Based on the precipitation data from weather stations in China, UK Hadley Centre SST (Sea Surface Temperature) and NCEP/NCAR reanalysis datasets, this paper studies the atmospheric circulation characteristics of the abnormally heavy precipitation events over the southern China in the fall of 2016 and impact of SST. Results show that the subtropical westerly jet in East Asia was much stronger in the fall of 2016, and the southern China was just located to the right of the jet stream entrance, which was conducive to an ascending motion. The western Pacific subtropical high also was much stronger than its normal, with a larger area and more northwestward shifted location. The anomalous southeasterly winds on the southwest side of the western Pacific subtropical high transported warm and moist air from the tropical Pacific to the southern China, leading to heavy precipitation there. In addition, more landing typhoons along the coast of the Southeast China also contributed to the heavy precipitation. Further analysis shows that the heavy rainfall event was mainly related to the abnormally higher SST over the equatorial western and southeastern Pacific simultaneously on inter-annual time scales, and anomalous warming over the North Atlantic on inter-decadal time scales. These aforementioned SST anomalies could affect the East Asian atmospheric circulation through exciting downstream-propagating teleconnection wave trains or Gill-type atmospheric responses. The above results are further confirmed by a series of numerical model simulations using CAM5.3 (Community Atmosphere Model Version 5.3).
1992,16(4):482-493, DOI: 10.3878/j.issn.1006-9895.1992.04.11
Analyses of the Causes of Severe Drought Occurring in Southwest China from the Fall of 2009 to the Spring of 2010
2012,36(3):443-457, DOI: 10.3878/j.issn.1006-9895.2011.11101
Abstract:A severe drought occurred in Southwest China from the fall of 2009 to the spring of 2010. Either its persistent time and area or decreased amount of rainfall were less observed during the last 50 years. Thus, in this paper, the occurring causes of this drought are analyzed by using the NCEP/NCAR reanalysis data and SST data from the impacts of thermal anomalies in the tropical western Pacific (TWP) and the tropical Indian Ocean (TIO) on the atmospheric circulation over the TWP and South Asia. The results show that during the period from the fall of 2009 to the spring of 2010, both the TWP and the TIO were in a warming state. Under the common thermal effect of both oceans, a strong anticyclonic anomalous circulation appeared in the lower troposphere over the TWP and the South China Sea, which caused not only the strengthening of the southwest flow anomaly, but also the appearance of a low trough anomaly over South China and Central China. In this case, the northwest flow anomaly and descending flow anomaly behind the trough controlled the eastern part of the Tibetan Plateau, and water vapor was difficultly transported from the Bay of Bengal into the Yunnan-Guizhou Plateau (Yun-Gui Plateau in short). Thus, less rainfall for a long time was caused in this region. Moreover, the analysis results also show that the circulation anomaly over the mid- and high latitudes had an important impact on the severe drought. Since the polar wave guide of quasi-stationary planetary wave propagations over the high latitudes was stronger, but the low-latitude wave guide was weaker from the fall of 2009 to the spring of 2010, which led to convergence and divergence of the wave E-P fluxes for quasi-stationary planetary waves in the upper troposphere and the stratosphere over the region about 60°N and in the middle and upper troposphere over the region about 35°N, respectively. Thus, the zonal mean wind was weakened in the upper troposphere and the stratosphere over the region about 60°N, but it was strengthened in the upper troposphere over the region about 35°N. This caused an obviously negative phase of the AO (Arctic Oscillation) and brought strong winter monsoon and eastward tracks of cold waves into East Asia, which led to a weakening of cold air arriving in Southwest China. Thereby, the persistent severe drought occurred in Southwest China.
Assessing the Quality of APHRODITE High-Resolution Daily Precipitation Dataset over Contiguous China
2012,36(2):361-373, DOI: 10.3878/j.issn.1006-9895.2011.11043
Abstract:A daily gridded precipitation dataset (APHRO) for Asia was created by the Asian Precipitation-Highly Resolved Observational Data Integration Towards the Evaluation of Water Resources (APHRODITE) project in Japan. The resolution of this dataset is 0.25°×0.25°, and the length is from 1951 to 2007. This study aims to assess the quality of the APHRO rainfall in contiguous China from the perspective of climatological mean, rainfall classes, and long-term trend. Daily rainfall records observed by 559 rain gauges are used for the comparison. The results are as follows: (1) For the mean states, the APHRO shows similar distribution of precipitation amount to station data, and can accurately characterize the seasonal migration of rain-belt. However, compared with station data, this dataset underestimates precipitation intensity, but overestimates precipitation frequency.(2) For the distribution of different classes of precipitation, annual mean precipitation amount for heavy rainfall derived from the APHRO data is lower, while the amounts for light and moderate rainfall are higher than the station data.(3) The trends of precipitation amount in China during 1956－2005 derived from two dataset are identical, the trends of precipitation frequency are also highly consistent, and both show an “increase in the west but decrease in the east” pattern. A large difference between the two datasets is found in the spatial pattern of precipitation intensity trends. The precipitation intensity derived from station data shows an increasing trend throughout the Chinese mainland in the past 50 years; the APHRO data exhibit a similar trend in the southeastern coastal region and northwestern China, but a different pattern in northern China, northeastern China, and Jianghuai region. In addition, analysis on the seasonality of interdecadal variability indicates that the characteristics of “southern China food and northern China drought” and “Jiangnan late spring drought” derived from the APHRO data are different from those derived from station data. The APHRO data tend to underestimate the trends.
An Overview of Recent Studies on Atmospheric Boundary Layer Physics and Atmospheric Environment in LAPC
2008,32(4):923-934, DOI: 10.3878/j.issn.1006-9895.2008.04.18
Interdecadal Variation of the Leading Modes of Summertime Precipitation Anomalies over Eastern China and Its Association with Water Vapor Transport over East Asia
2011,35(4):589-606, DOI: 10.3878/j.issn.1006-9895.2011.04.01
Abstract:Interdecadal variation of the leading modes of summertime precipitation anomalies in the monsoon regions of eastern China and its association with the spatio-temporal variations of summertime water vapor transport fluxes over East Asia are analyzed by using the daily data of the ERA-40 reanalysis and precipitation data at 516 observational stations of China for 1958－2000 and the EOF analysis method. The analysis results show that there are two leading modes in the spatio-temporal variations of summertime precipitation anomalies over the monsoon region of eastern China: The first leading mode exhibits not only a characteristic of obvious interannual variation with a quasi-biennial oscillation, but also a feature of interdecadal variability, and its spatial distribution is of a meridional tripole pattern. And the second leading mode exhibits a characteristic of obvious interdecadal variability, and its spatial distribution is of a meridional dipole pattern. This shows that these two leading modes have a significant interdecadal variability. During the period of 1958－1977, the distribution of summertime precipitation anomalies in eastern China exhibited a “＋－＋” meridional tripole pattern from the south to the north, and the distribution of precipitation anomalies for 1978－1992 showed a “－＋－” meridional tripole pattern in the region, which was opposite to that for 1958－1977, but during the period of 1993－1998, since the role of the second leading mode in summertime precipitation anomalies in eastern China was intensified, the distribution of summertime precipitation in this region showed a combination of “＋－＋” meridional tripole pattern and “＋－” meridional pattern, which caused the increase of summertime precipitation in South China. Moreover, the analysis results also show that the interdecadal variation of these two leading modes is closely associated with the spatio-temporal variations of summertime water vapor transport fluxes over East Asia, which is associated not only with the interdecadal variation of the EAP pattern teleconnection-like wave-train distribution of summertime water vapor transport flux anomalies over East Asia and the western North Pacific, but also with the interdecadal variation of the EU pattern teleconnection-like wave-train distribution of summertime waver vapor transport flux anomalies in the westerly zone over middle and high latitudes of Eurasia.
2003,27(4):712-728, DOI: 10.3878/j.issn.1006-9895.2003.04.18
Projection and Evaluation of the Precipitation Extremes Indices over China Based on Seven IPCC AR4 Coupled Climate Models
2009,33(1):109-120, DOI: 10.3878/j.issn.1006-9895.2009.01.10
Abstract:Climatology of the observed daily precipitation extreme indices (SDII, simple daily intensity index; CDD, the maximum number of consecutive dry days; R10, number of days with precipitation greater than 10 mm; R5d, maximum 5-day precipitation total; R95t, fraction of total precipitation due to events exceeding the 95th percentile of the climatological distribution for wet day amounts) at 550 stations in China during 1961－2000 is used to evaluate the simulation ability of 7 IPCC AR4 Coupled Climate Models, the projected change of the precipitation extreme indices over China under IPCC SRES A2、A1B and B1 is also studied. The results show that the state-of-the-art IPCC AR4 models can simulate the spatial distributions and the linear trends of precipitation extremes well. The multi-model ensemble (MME) shows the best skill, but both the MME and single model fail to simulate the interannual variability and have large biases, such as there are excessive extreme precipitation over the eastern side of the Tibetan Plateau while in the monsoon regions the modeled intensity of precipitation extremes is lower than the observation. In the 21st century the precipitation will become more “extreme”， there would be an overall increasing trend in extreme precipitation events over most of China under a warming environment, and the change scope is scaled to the emissions scenarios.
2002,26(6):721-730, DOI: 10.3878/j.issn.1006-9895.2002.06.01
2008,32(4):653-690, DOI: 10.3878/j.issn.1006-9895.2008.04.01
2004,28(6):979-992, DOI: 10.3878/j.issn.1006-9895.2004.06.15
Characteristics of the Water Vapor Transport in East Asian Monsoon Region and Its Difference from that in South Asian Monsoon Region in Summer
1998,22(4):460-469, DOI: 10.3878/j.issn.1006-9895.1998.04.08
The Seasonal March of the North Pacific Oscillation and Its Association with the Interannual Variations of China's Climate in Boreal Winter and Spring
2011,35(3):393-402, DOI: 10.3878/j.issn.1006-9895.2011.03.01
Abstract:Based on the monthly mean NCEP/NCAR reanalysis dataset and the observed surface air temperature and precipitation from 160 China stations, the seasonal march of the North Pacific Oscillation (NPO) and its association with the interannual variations of China's climate in boreal winter and spring are investigated in this paper. By employing the Empirical Orthogonal Function (EOF) analysis method, the NPO is identified as the second EOF mode of the monthly mean Sea Level Pressure (SLP) field over the North Pacific. The seasonal mean NPO indices are then defined as the average of the monthly mean indices for each season. Wavelet analysis reveals that the significant periods of the NPO indices are below 8 years for all the four seasons, indicating strong interannual variability and weak interdecadal variability of the NPO. Besides, the winter mean NPO index experiences significant linear trend towards its positive polarity. For all the seasons, NPO is featured with a large-scale meridional dipole in the SLP field over the North Pacific region and resembles the western Pacific pattern in the middle troposphere. The two surface centers are located around Aleutian Islands and the northwest of Hawaii, respectively. Their positions vary a little with season. Comparatively the south center experiences more zonal movement and the north center bears more meridional movement. Vertically the NPO is featured with an equivalent barotropic structure in summer and tilts a little westward with height for the rest three seasons. To put the NPO jet fluctuation in perspective, the positive phase of NPO is characterized by a northward shift and downstream extension of the East Asian jet stream especially in the jet exit region. The NPO variability is influential for the China's climate. Regression analysis indicate that during boreal winter the positive phase of NPO favors significant southerly anomalies along the coasts of East Asia, which may bring warm and moisture air from the south. Consequently, significant warming is observed over most areas of eastern China as well as the eastern part of the Tibetan Plateau. Meanwhile, more precipitation is observed over southeastern China, the Huaihe River valley, and several stations of northwestern China. During boreal spring, the influence of NPO is mainly confined to North China. More precipitation and weak cooling can be observed over this region in the positive phase of NPO. Such changes may be accounted for by the NPO-associated anomalous low-level wind convergence and the secondary circulation around the entrance of East Asian jet stream. In addition to the simultaneous influences, it is further found that the wintertime NPO is closely related to the temperature and precipitation of the following spring. If the NPO phase is positive in the preceding winter, significant warming will be observed over northern China and southwestern China in the following spring. Meanwhile, about 20% more-than-normal precipitation will be observed over southwestern China. Therefore, the wintertime NPO may act as a potential predictor for the climate of the following spring in China.
2011,35(3):403-410, DOI: 10.3878/j.issn.1006-9895.2011.03.02
Abstract:The authors propose a method to separate the truncation error and the round-off error from the numerical solution. The analytical truncation error formulas of a partial differential equation are given for the upstream scheme and the centered difference scheme, respectively. The reference solution method is then introduced to separate these two types of errors for more general equations. A scheme based on the reference solution is used to obtain the approximate truncation error. Comparing the results for the upstream scheme and the centered difference scheme, the authors find that：1) the approximate truncation error is highly consistent with the analytical one. 2) The truncation errors of 1-D wave equations for the two schemes both show wavy periodicities with amplitudes being related to the parameters of computation. 3) The analytical error is suitable for the analysis of any slice of t, while the approximate one is only suitable for the analysis of a certain time range. However, the approximate error can be more easily obtained for general differential equations without a complex theoretical deduction.
A Study on the Relationships between ENSO Cycle and Rainfalls during Summer and Winter in Eastern China
1999,23(6):663-672, DOI: 10.3878/j.issn.1006-9895.1999.06.03
Recent Progresses in Studies of the Temporal-Spatial Variations of the East Asian Monsoon System and Their Impacts on Climate Anomalies in China
2008,32(4):691-719, DOI: 10.3878/j.issn.1006-9895.2008.04.02
The Impact of the Tibetan Plateau Warming on the East Asian Summer Monsoon—A Study of Numerical Simulation
2008,32(5):997-1005, DOI: 10.3878/j.issn.1006-9895.2008.05.01
Abstract:East Asian monsoon precipitation and circulation have experienced an interdecadal change in the last 50 years that was concurrent with the changes in global SST and recent global warming. What gives rise to this long term change has been an outstanding issue. One possible cause is the change over the Tibetan Plateau. To understand the possible impact of the Tibetan Plateau surface conditions on the downstream East Asian monsoon, a suite of sensitivity experiments were performed with Hamburgs atmospheric general circulation model (ECHAM). The land surface albedo was changed in two sets of sensitivity tests so that the Tibetan Plateau land surface temperature was changed accordingly. The results show that a warmer condition over the Tibetan Plateau tends to enhance the upper tropospheric South Asia high and the westerly jet stream to its north and the Indian monsoon to its south, meanwhile the moisture transport toward East Asia increases. The changes in the precipitation pattern are featured by increasing rainfall over northwestern India and Meiyu and decreasing rainfall in the regions under the control of Pacific subtropical high and the Bay of Bengal. Preliminary diagnostic analysis suggests that the plateau warming has initially increased sensible heating and convective heating locally, which then enhanced the low-level southwest monsoon transport toward down stream of the plateau, resulting in increased precipitation and latent heat release in eastern China. The changes in the overall diabatic heating strengthen the upper-tropospheric South Asia high and downstream subtropical trough. This result may have meaningful implications for understanding the interdecadal change and future change of the East Asian summer monsoon.
Atmospheric Water Vapor Transport and Corresponding Typical Anomalous Spring Rainfall Patterns in China
2009,33(1):121-134, DOI: 10.3878/j.issn.1006-9895.2009.01.11
Abstract:The atmospheric water vapor transports associated with typical anomalous spring rainfall patterns have been investigated using NCEP/NCAR, ERA40 monthly mean reanalysis data and precipitation data of 160 stations in China during 1951－1999. Results show that origins of water vapor supply related to anomalous rainfall patterns are different from the climate mean situation. In anomalous pattern 1, with a heavier rainbelt along the South China coast, the main moisture comes from the Philippine Sea and the adjacent South China Sea. The background large-scale circulation changes include the intensification of the western Pacific subtropical high (WPSH) and the southwest shift of the East Asian jet stream (EAJS). In anomalous pattern 2, with a main rainbelt along the middle and lower reaches of the Yangtze River, the origins of water vapor supply contain the western tropical Pacific and the tropical Indian Ocean. Both the WPSH and the EAJS move to the north of their normal position. In anomalous pattern 3, a rainy region is located in the Huaihe River valley. The moisture originates from the northwestern Pacific. The 500-hPa anticyclone anomaly moves to northeastern China and the EAJS is weaker than its normal condition. Although water vapor transport along the southern edge of the Tibetan Plateau is one of the main branches in the climate mean pattern, none of the typical water vapor transport related to typical anomalous rainfall reflects this water vapor flow.
2010,34(1):184-192, DOI: 10.3878/j.issn.1006-9895.2010.01.17
Abstract:Based on the monthly rainfall of 160 stations over China and the sea surface temperature data in the Niño 3 region, the instability of interannual relationship between summer rainfall in China (SRC) and pre-winter sea surface temperature (WSST) in the Niño 3 region is studied by using sliding correlation, composition etc. It shows that the long-term variation of interannual relationship between SRC and WSST in the Niño3 region has obvious regionality and is more instable in northeastern and northwestern China than in eastern China. At the same time, it has a significant stage characteristic. The period from 1951－2007 can be separated into 1962－1977, 1978－1992, 1993 to present, according to the sliding correlation coefficients. Each stage has about sixteen years in length. Every turning from one stage to another is very short and is characterized by abrupt change. There are three obvious abrupt changes in the last 50 years, such as in the early 1960s, in the late 1970s, and in the late 1980s and early 1990s. Before and after the abrupt change, the correlation sign and intensity have obvious differences in some regions. It also shows that the influences of El Niño events in different stages have different features. There are two rainbelts during 1962－1977: one was located in North China, Northeast China Plain, and eastern Inner Mongolia, and the other was located in the middle reaches of the Yangtze River; there was less rainfall in the Qingling-Daba Mountains and the Yangtze River and Huaihe River valley. During 1978－1992, there was only one rainbelt located in the Qinling-Daba Mountains area and the middle and lower reaches of the Yangtze River; there was less rainfall in the Yellow River and Huaihe River valley, and North China to the south of Northeast China. And in the present stage (1993 to present), the spatial distribution of rainbelts is similar to that during 1962－1977, there are also two rainbelts located in southern China and northern China respectively. But the north rainbelt moves southward to the Sichuan Province, Chongqing, the Yellow River and Huaihe River valley, the lower reaches of the Yellow River, and Northeast China Plain; there is less rainfall in northern Hubei Province and the lower reaches of the Yangtze River; the feature of rainfall distribution is that there is more rainfall in northern China while less rainfall in southern China. So, not only the mean interannual relations between SRC and SST in the Niño 3 region but also their instabilities must be considered in the forecasting of flood season precipitation by using ENSO events.
Transient Response of the Atlantic Meridional Overturning Circulation to the Enhanced Freshwater Forcing and Its Mechanism
2009,33(1):179-197, DOI: 10.3878/j.issn.1006-9895.2009.01.16
Abstract:The mechanism for the transient response of the Atlantic Meridional Overturning Circulation (AMOC) to the enhanced and continuous freshwater input into the Arctic Ocean and the Nordic Seas for 150 years is investigated using a fully coupled climate model (Bergen Climate Model, BCM for short). The responses of the Sea Surface Temperature (SST), Salinity (SSS), Potential Density (SPD), the North Atlantic Deep Water (NADW) formation, Diapycnal Mixing (DM) and the wind stress are analyzed. The transient response of AMOC follows a quick dropping down during the first 50 years, with a gradual recovery for the later 100 years in the freshwater perturbation experiment (FW1). The authors find that the initial weakening of AMOC in the FW1 is mainly caused by decreasing of SSS and SPD which leads to a stable vertical stratification and then to the weakening of the NADW formation; however, AMOC recovers though the enhanced freshwater input is continuous and constant during the following 100-year integration by means of a series of feedbacks, which can be summarized as follows: 1) With the reduction in the NADW formation, the vertical density stratification in the mid-to-deep ocean at the mid-low latitudes of the North Atlantic is getting weaker, as a result, the strength of DM, parameterized by stratification-dependent method, increases and then leads to the increased upwelling; 2) the strengthened westerly and weakened easterly over the Southern Ocean and the strengthened easterly over the North Atlantic Ocean contribute together to the recovery of the northward Ekman transport; 3) the increased northward salt transport and the reduced precipitation over the North Atlantic subpolar region cause the recoveries of the SSS and SPD at northern high latitudes, and then the recovery of NADW formation, mainly in the Irminger Sea.
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