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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
Available online:April 26, 2021 DOI: 10.3878/j.issn.1006-9895.2104.20159
Abstract:Regional development of air pollution in North China is having a significant impact on agroecological regions, to understand the seasonal distribution characteristics of atmospheric fine particulate matter PM2.5 pollution in agricultural areas in North China, seasonal PM2.5 was conducted at the Yucheng Comprehensive Experimental Station of the Chinese Academy of Sciences in Yucheng, Shandong Province in July, September, September 2017, and April 2018 Sample collection and determination of 30 chemical components in the sample. The results showed that the overall concentration of carbonaceous aerosols was 13.11 ± 8.37μg/m3. The Organic Carbon (OC) concentration was higher in winter and spring. The Elemental Carbon (EC) concentration was higher in autumn and winter. At the same time, the OC/EC ratio was significantly lower in autumn, indicating that the contribution of secondary carbonaceous aerosols to PM2.5 was small in autumn. The overall concentration of water-soluble ions is highest in winter and lowest in spring. The NO3-/SO42-ratio, which characterizes the contribution of traffic sources and fixed sources to atmospheric pollution, is significantly lower in summer at 0.69, indicating that the contribution of fixed point sources to air pollution is relatively higher in the summer. The metal elements in atmospheric particles are mainly crust elements such as Na, Mg, Al, Ca, K, Fe, etc. The annual average concentration level of metal elements such as Co, Cr, Ni, Pb, As and other carcinogenic effects is (0.32±0.24), (5.40±5.42), (10.23±7.46), (42.23±27.75), (5.66±3.79) ng/m3.The calculation results of the PMF receptor model show that the main sources of PM2.5 are secondary pollution sources, biomass combustion sources, coal-fired fuel sources, diesel vehicle exhaust and soil sources, with contribution rates of 37.1%, 18.2%, 14.2%, 9.4,% and 7.9%. The pollution of fine particles is affected by the multiple effects of industrial and agricultural in North China agricultural region.
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.
Characteristics of MCC from Convective Initiation to Mature Stage over Sichuan Basin based on FY-4A data
Available online:April 25, 2021 DOI: 10.3878/j.issn.1006-9895.2104.20143
Abstract:The characteristics of MCC(mesoscale convective complex) from convective initiation to mature stage over Sichuan Basin in 2018, were analyzed by the high-frequency FY-4A data. The results indicated that the area of MCC convective cloud increased at a rate of 0～50 pixels per 15min at the convective-initiation stage and 150～200 pixels per 15min at the mature stage respectively, and finally reached about 7,000 ～ 10,000 pixels. The maximum area of temperature gradient is located on the side of the low-level inflow zone at the convective-initiation stage, which concentrated near the 240K contour with the most abundant texture in the cloud top, with the maximum value of 30～40, and it basically disappears at the mature stage. The variation of the minimum IR1(infrared radiation 1 channel) and IR3 (water vapor channel) at the convective-initiation and mature stage are consistent, both of which rapidly decrease to 190K at the convective-initiation stage, while the minimum value remains basically unchanged at the mature stage. The distribution patterns of CTC (IR1) (cloud top cooling rate) and CTC (IR3) are also similar. The significant CTC , which is closed the 240K on the side of the low-level inflow zone, reaches -40K/15min at the convective-initiation stage, but it remain stable from -25K to -10K per 15min at the mature stage. The maximum BTD(brightness temperature difference) is 6～10 K at the convective-initiation stage and 0～6 K at the mature stage. Additional, CTC(BTD) in the low-level inflow zone is the most drastic at the convective-initiation stage, while it is remain steady at the mature stage.
Simulation of the daily precipitation characteristics using CAS-ESM in the Eurasia: sensitivity of physical schemes and horizontal resolutions
Available online:April 25, 2021 DOI: 10.3878/j.issn.1006-9895.2010.20171
Abstract:In this paper, the two Earth System Models, i.e., CAS-ESM (Earth System Model, Chinese Academy of Sciences) and NCAR CESM (Community Earth System Model, NCAR), were used to carry out a series of simulations with different physical schemes and horizontal resolutions, and the performance of daily precipitation characteristics was analyzed in the Eurasia. Four 19-year (1998-2016) Atmospheric Model Intercomparison Project simulations have been conducted in this study. They included CESM with the CAM5 physical scheme combination at resolution of 1.9°×2.5°, CAS-ESM with the CAM4 (Lcam4) and CAM5 (Lcam5) combinations at resolution of 1.4°×1.4°, and CAS-ESM with the CAM5 combination at resolution of 0.5°×0.5° (Hcam5). Compared to GPCC and CMORPH, both models can well reproduce the climatological means of precipitation and extreme precipitation. However, the two models overestimated precipitation frequency and underestimated precipitation intensity. CESM produced the smallest biases of the number of heavy raining day, and Hcam5 outperformed the other three experiments in reproducing the maximum daily precipitation. For CAS-ESM, both physical schemes and horizontal resolutions had impact on the daily precipitation characteristics. Noticeable improvement of precipitation characteristics was demonstrated by Hcam5. The precipitation frequency of Lcam4 was higher than that of Lcam5 in the middle and high latitudes of the Eurasia. Over eastern China, the biases of precipitation frequency for Hcam5 is smaller than that of Lcam5, and the high-resolution experiment distinctly improve the simulation of extreme precipitation. Further, compared to Lcam5, Lcam4 produced more large-scale precipitation and stronger water vapor flux over the Europe. Over eastern China, the improvement of daily precipitation frequency and extreme precipitation was related to the reduction of the convective precipitation frequency, the more large-scale rainfall and stronger water vapor flux.
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.
The interdecadal variation of Warm Arctic - Cold Eurasia mode and its association with North Atlantic Sea Surface Temperature
Available online:March 30, 2021 DOI: 10.3878/j.issn.1006-9895.2103.20205
Abstract:Based on the NASA"s Goddard Institute for Space Studies surface air temperature datasets and National Oceanic and Atmospheric Administration-Cooperative Institute for Research in Environmental Sciences reanalysis data of the 20th century, and Historical experiment of the Coupled Model Intercomparison Project Phase 6, this study analyzes the interdecadal variation characteristics of "Warm Arctic - Cold Eurasia" (WACE) mode in the Eurasia and Arctic region for the period of 1910/1911~2019/2020 during boreal winter after removed the external forcing. The results show that the WACE has the remarkable interdecadal variability. When WACE is in the interdecadal positive phase, The high frequency of Ural blockage favors the heat transport to the polar region, which leads to warm advection in the polar region, and water vapor transport to the polar region leads to water vapor convergence in the polar region, which leads to the increase of downward long-wave radiation., and the increase of convective activity leads to the release of latent heat, which result in the increase of temperature in the polar region. At the same time, the weakening of polar vortex and westerly wind in Eurasia and the high frequency of Ural blockage favor cold air invading Eurasia and causing cold advection, and the divergence of water vapor in Eurasia reduces downward long-wave radiation, and the decrease of convective activity reduces the latent heat release, which lead to the decrease of temperature in Eurasia. Finally we use CAM3.0 atmospheric circulation model to simulate the north Atlantic SST influence on WACE. Model results are consistent with the statistical results, further illustrates the north Atlantic SST positive anomaly can force the lower and upper atmosphere circulation anomalies, leads to water vapor convergence in polar and divergence in Eurasia, thus affecting the decadal variability of WACE.
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 03, 2021 DOI: 10.3878/j.issn.1006-9895.2011.20164
Abstract:ABSTRACT: The Singular Vectors (SVs) that include the linearized moist physical process in the calculation are called Moist SVs. Studying the sensitivity of moist SVs to the horizontal resolution and the optimization time interval (OTI) is important for the ensemble forecasting system. Based on the operational version of GRAPES-GEPS, which was independently developed by the China meteorological administration numerical forecast center, this paper analyzes the characteristics of subtropical Moist SVs and their ensemble forecasts at four groups of different horizontal and temporal resolutions. The characteristics of moist SVs in terms of energy norm, energy spectrum and spatial profile were analyzed, and the evaluation of the ensemble forecast of four groups of experiments will be made in terms of isopressure variables scores, precipitation scores, and precipitation probability prediction. The results show that increased the horizontal resolution of Moist SVs leads to a larger growth rate of their perturbation. The upward propagation of Moist SVs energy is greater than the downward propagation after shorting the OTI, and can produce larger SVs perturbation in the mesoscale ranges. The initial Moist SVs under different OTIs are less similar and their structures are different to each other. From the perspective of ensemble forecast, the average ensemble perturbed energy of the shorter OTI (24h) has increased greatly, and the ensemble spread has been improved during the prediction of 0-96h, especially for the 2-m temperature, and the outlier score has also been improved in the near-surface variables. It was further found that increasing the horizontal and temporal resolution can improve the precipitation probability prediction. The precipitation scores show that at the same spatial resolution, the shorter the OTI, the better the scores, while increasing the spatial resolution of the moist SVs failed to improve the precipitation scores in the light rain to moderate rain.
Research on the Relationship between Different Western North Pacific Subtropical High Indexes and Summer Precipitation in Eastern China
Available online:March 02, 2021 DOI: 10.3878/j.issn.1006-9895.2008.20160
Abstract:Based on the reanalysis data and monthly precipitation data, in this study, we investigate variabilities of western North Pacific subtropical high indexes (WPSHI), and discuss the relationship between WPSHI and summer precipitation in eastern China and relevant circulation changes. Results show that WPSHI can be divided into three categories, namely, absolute intensity index (ASI), relative intensity index (RLI) and north-south index (NSI). The upward trend in ASI is associated with the expansion of area over 5870gpm. The downward trend in RLI accords well with the shrink of 0gpm line in eddy geopotential height (He). The variation of NSI indicates the ridge of WPSH oscillates around 25°N. Depicted by circulation anomalies and precipitation patterns, ASI can not reflect the variation of local relative vorticity of WPSH, but it has close relationship with EAP pattern. In this case, ASI has the best correlation with precipitation in the Yangtze River Basin. In high ASI years, north wind anomalies is favorable for less rainfall in northern China, while the convergence strengthens the precipitation along the Yangtze River and vice versa. RLI can better describe the variation of local relative vorticity of WPSH, but it has a weaker relationship with the EAP pattern and precipitation in eastern China. In high RLI years, precipitation center appears in the middle reaches of the Yangtze River. Conversely, drought occurs in north of Yangtze river with anomalous north winds over China. NSI can describe both the variation of local relative vorticity of WPSH and the EAP pattern, and has a high correlation with precipitation in southern and northern China. In high NSI years, enhanced vapor flux lead to flood in northern China and drought along Yangtze River. Otherwise, the precipitation is trapped in southern China, resulting from a reversed circulation pattern.
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.
Analysis of a dust weather over the East Asia in May 2019 based on CALIPSO satellite and ground-based lidar data
Available online:March 02, 2021 DOI: 10.3878/j.issn.1006-9895.2008.19249
Abstract:In May 2019, a strong sandstorm appeared in northern China, lasting for a long time and influcing large-scale areas. The optical properties, vertical distribution and the transmission of dust are analyzed using satellite-borne lidar CALIPSO (the Cloud-Aerosol and Infrared Pathfinder Satellite Observation) and ground-based lidar. At the same time, the track of this dust process is validated by the hourly data from the China Meteorological Data Service Center, the influence on air quality by the air-quality data from the PRC State Environmental Protection Administration. In addition, era-interim reanalysis data are used to analyze the causes of the dust weather. The results show that: (1) The sand-storm process can chiefly be divided into two stages, the firstSstageSisSfrom May 10th to 12th and from 13th to 16th UTC is the second. (2) The dust aerosols exist at a high altitude of 5-10km in the Pacific region, and the depolarization ratio and the color ratio of dust are 0.14 and 1.29 individually. (3) After the comprehensive analysis of the data from the spaceborne and ground-based lidar, it was found that the dust was transimitted to Jeju in South Korea and Fukuoka in Japan, and the dust contributions are 39.25%, 35.43% respectively. (4) The comparison of the attenuation backscattering coefficients and the apparent ratio from the ground-based from ADNET (the Asian Dust and aerosol lidar observation NETwork ) data and the corresponding CALIPSO spaceborne lidar data show similar trends. (5) During the dust process, the concentration of particulate matter increased significantly. On May 12th, the mean values of PM2.5 and PM10 were quite high in Gansu province, which reached 7 times and 12 times respectively of the national first-level standard, and on May 14th . And the ratio of PM10 and PM2.5 reach above 6 in many places in Gansu and Xinjiang province. (6) Due to the low trough in the west of Inner Mongolia deepens and extends to east, and the South Asia High move steadily southward, bring the cold air southword. On May 14th(UTC), the instability stratification strengthened, leading to the outbreak of dust weather.
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.
A Return Radiosonde Trajectory Forecast Method Based on High Resolution Numerical Weather Prediction Model and Preliminary Evaluations
Abstract:A return radiosonde trajectory forecast method based on high resolution numerical weather prediction model is proposed in this paper. The trajectory forecast method will be used for simulating the return radiosonde network observations and supporting evaluations of the observation impacts on numerical weather prediction. Meanwhile, the trajectory forecast system will also be used for supporting the return radiosonde application in target observation experiments in the future. A return radiosonde trajectory forecast system is established by embedding return radiosonde’s trajectory equations of ascent stage, drift stage and descend stage into numerical weather prediction model GRAPES (Global/Regional Analysis and PrEdiction System) regional model with 3km horizontal resolution. Dynamic equation of parachute in descend stage is also considered in this system. By using the trajectory prediction system, the preliminary test of the return sounding trajectory prediction is carried out for 63 successful real return radiosonde observations. The results show that the forecast trajectories using this trajectory forecast system are reasonable and reliable, and the average error of track prediction is less than 40 km in 6 hours.
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.
The Relation between Atmospheric intraseasonal Oscillation and Continual Rainstorms in Guangdong during the First Rainy Season in 2019
Available online:February 05, 2021 DOI: 10.3878/j.issn.1006-9895.2101.20136
Abstract:In order to make better medium-term and extended-range forecast of continuous backflow rainstorms, wavelet analysis and Lanczos time filter were used to analyze the relation between daily precipitation and atmospheric intraseasonal oscillation during the first rainy season of Guangdong in 2019. The mean circulation of two backflow continuous rainstorms over southwest of Guangdong during April and May and its low-frequency propagation characteristics were analyzed and compared with the frontal type continuous rainstorms over north of Guangdong during June. The results show that the two continuous backflow rainstorms near Yangjiang from April to May and the precipitation during the first rainy season exhibit quasi-23-day periodic oscillation. They are continuous backflow rainstorms with or without the influence of cold air, continuous rainstorms and their corresponding atmospheric circulation fields and low-frequency propagation are significantly different. During the continuous backflow rainstorms on April 12-14, there were stable "west blocking" and "east blocking" at middle and high latitudes at 500hPa, which made cold air continuously move southeast from the south of high pressure transformed over the East China Sea, and the convergence asymptote of relatively dry and cold strong southeast wind, and the warm humid southerly wind from the south-central of South China Sea by Indochina Peninsula was formed near Yangjiang at 925hPa; During the continuous rainstorms on May 23-26, there was a stable high-pressure ridge over Eastern China～East China Sea～Yellow Sea at 500hPa, which kept Guangdong in the southwestern airflow in front of the plateau trough for a long time, and Guangdong was in the southwestern of degenerative high pressure ridge from the East China Sea and in front of trough from Beibu Bay to southwest of China on the ground, and the convergence asymptote of strong southerly winds which came from Bay of Bengal centered in Yangjiang and southeast winds which came from east of the Pearl River Estuary, was formed at 925hPa. The low-frequency northeast wind (Southeast wind) from the south(southwest) of low frequency anticyclone in Central China(east of the East China Sea) was gradually strengthened southward, merged the enhanced low-frequency northerly (southerly) winds which came westward from western Pacific near 140oE (eastward from Bay of Bengal) in the southwest of Guangdong. And they encountered with (without) low-frequency eastern wind from the north of low frequency cyclone propagating northward from the central and north part of the South China Sea, and finally resulted in the continuous backflow rainstorms with (without) obvious influence of cold air. But the frontal type continuous rainstorm during June was affected by convergence between cold air induced by deep East Asian trough and southwesterly from Bengal Bay. The enhanced low-frequency southwesterly propagated southwards and eastwards to Guangdong from mid China and Bengal Bay, and low-frequency cold dry northeasterly winds from the periphery of the mid-latitude anticyclone, converged at the south of Yangtze River or the north of South China Sea and leaded to the happening of the continuous rainstorm during June 9-13.
Evaluation of assimilation effect of Yunhai-2 occultation data in global numerical weather prediction model
Available online:February 05, 2021 DOI: 10.3878/j.issn.1006-9895.2010.20187
Abstract:Based on T799L91 global numerical weather forecast model and its four-dimensional variational assimilation system, the assimilation process of Yunhai-2 occultation data is constructed, and one month assimilation-forecast experiment for Yunhai-2 occultation data was carried out in July 2019, and the evaluation of the assimilation forecast results for East Asia, Northern Hemisphere and southern hemisphere shows that : In the global numerical weather forecast model, assimilating Yunhai-2 occultation data can effectively improve the accuracy of the forecast field. With the extension of the forecast time and the increase of the forecast days, the improvement range gradually increases. In the 72 hour, 120 hour and 168 hour forecast with longer time effect, the assimilation effect of Yunhai-2 occultation data and GPS occultation data is similar in the middle and early days, and the assimilation of Yunhai-2 occultation data is gradually better than GPS occultation data in the middle and late days. With the increase of forecast days, the advantage of joint assimilation of Yunhai-2 occultation data and GPS occultation data gradually appears.
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.
An in-situ case study on micro physical properties of aerosol and shallow cumulus clouds in North China
Available online:February 02, 2021 DOI: 10.3878/j.issn.1006-9895.2012.20162
Abstract:An aircraft observation of aerosol and shallow cumulus in Xinzhou was carried out by Weather Modification Office of Shanxi Province on August 15, 2014. In this paper, the microphysical properties of aerosols, CCN and shallow cumulus clouds and their interactions in North China is analyzed in detail, based on airborne cloud physical data. Main results are as follows.(1) In this case, the height of the boundary layer is about 3600m, and Na near surface can reach 2500cm-3. The vertical profiles of aerosol particle concentration (Na), aerosol effective diameter (Eda) and CCN number concentration are obviously different under different stratification conditions. (2)The main source of CCN is aerosol particles in accumulation mode, Aegean mode or nuclear mode. At 0.2% supersaturation, the results of the aerosol activation rate (AR) at each height layer do not change much, while AR decreases with height at 0.4% supersaturation condition. (3HYSPLIT4 model analysis shows that, aerosols below 2km mainly came from local urban emissions, which were composed of fine particulate pollutants. Above 2km, aerosols mainly came from deserts in northwestern China and Mongolia, which were composed of submicron sand and dust. They can be potential IN due to their low solubility. (4) The cloud physical characteristics of two adjacent shallow cumulus clouds (Cu-1 and Cu-2) are also analyzed. Cu-1 is loose with much entrainment. The cloud base height and cloud thickness is about 4500m and 600m. LWC in Cu-1 is basically maintained at 0.5 g m-3, the average Nc is 278.3 cm-3, and EDc is overall within 15 μm. The max Nd is 0.002 cm-3, with almost no precipitation particles in Cu-1. The particle spectrum width increases with height, and ED mainly concentrated within 30μm. Cu-2 is much denser, with cloud base height at 3900m and 1200m cloud thickness. There are plenty of supercooled water in Cu-2, and LWC has multiple areas over 1 g m-3. Ice crystals appear near the cloud top, and particles growth process changes from condensation to mixed phase state directly. The horizontal distribution of particles in Cu-2 is uneven, and Nc at the same height differs greatly, with max value up to 1240 cm-3. EDc increases with height. The particles spectral width expands with height, up to 1100 μm, and the spectral pattern changes from single peak to multi-peak. The images of precipitation particles and ice crystals are mostly graupel, needle-shaped and plate-shaped.
Available online:January 27, 2021 DOI: 10.3878/j.issn.1006-9895.2101.20216
Abstract:The Northwest China adjacent to the Qinghai-Tibet Plateau is the largest Eurasian arid region. Its precipitation response to global changes and the arid environment and its climate change in the Qinghai-Tibet Plateau are of special significance. Based on the observation data of daily precipitation and monthly temperature in 144 stations in northwest China from 1961 to 2018, this paper analyzes the characteristics and trends of precipitation change in northwest China. The results showed that since nearly 60a, 92% of the stations in Northwest China have shown an increasing trend of annual precipitation, while less than 10% of the stations in Southeastern Gansu have shown a decreasing trend. On the seasonal scale, precipitation at more than 75% stations in spring, summer and autumn showed an increasing trend. But the most significant is that almost all stations have an increasing trend of precipitation in winter. The increase of precipitation in autumn and winter is relatively small, which reflects the influence of winter wind on precipitation in Northwest China. Another feature is that since the beginning of the 21st century, the summer and annual precipitation in northwest China still maintain the quasi-3a cycle. The spring and autumn cycles have stages, and the winter precipitation has a relatively stable cycle of nearly 3a. Therefore, natural cycle changes do not contribute much to the increase of precipitation. The results of this paper also show that the precipitation in northwest China has shown an increasing trend in the past 60 years. Especially since the 21st century, the precipitation has been continuously increasing. However, the increased precipitation is limited, which is not enough to change its arid and semi-arid climate characteristics.
Abstract:Affected by stable weather, there had been ten fog events in Jinan from 19 December 2016 to 9 January 2017, during which the minimum value of visibility was below 50 m. Low visibility caused by continuous fog events brought serious harm to the activity of industry and agriculture and people"s lives. In this paper, microphysical characteristics were analyzed, microphysical processes and intensity were deduced and their effect on visibility was discussed based on the measurements from fog drop spectrometer, automatic weather station and conventional meteorological instrument during 10 fog events. The results were as follows：The droplet spectrum distribution was different in winter fog with different thickness, which changed from mono-modal to multi-modal as fog became thick. Number concentration(NC) were the most inversely associated with visibility(V), while it was not stable that liquid water content(LWC) and relative dispersion of the droplet size distribution(S) were inversely associated with V. The temperature of air had an impact on the microphysical processes. The activation and condensational growth (or droplet evaporation) processes played a leading role in the whole winter fog life time. The collection processes arose in the development and mature stages, but not or very infrequently in the formation and weaken stages. The results from autoconversion rate show that collection occurred rarely in fog with V＞200 m, while it was very weak, with uncollected or uncontinuous collection in heavy fogs with 100 m≤V＜200m. The collection mainly occurred in extremely dense fog with V＜50 m and heavy fogs with 50 m≤V＜100m, but it was more frequently and stronger in heavy fogs with 50 m≤V＜100m than in extremely dense fog with V＜50 m. There were a larger number of small fog droplets which lead to poorer visibility in extremely dense fog with V＜50 m, but the maximum values such as NC, LWC appeared in heavy fogs with 50 m≤V＜100m, in which these quantities showed the biggest change also. It may be related to the more frequently and stronger collection in which collision–coalescence and collision-fragmentation played a role on the biggest change of microphysical quantities in heavy fogs with 50 m≤V＜100m. The trend of calculated values of visibility by observation data from fog drop spectrometer agreed well with the actual values, but the calculated values were much greater than the actual ones, it was mainly caused by a large number of aerosol particles in fog. In polluted fog, it is not enough to estimate visibility in fog only based on observation data from fog drop spectrometer, and the influence of aerosol particles on visibility must be considered at the same time.
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.
Influence of the High-altitude Trough and Ridge on the Asymmetric Precipitation of Typhoon Usagi before Extratropical Transition
Abstract:Based on the data of reanalysis data from NCEP/NCAR, rainfall data from TRMM (Tropical Rainfall Measuring Mission) and RSMC Best-track database, the evaluation of precipitation distribution and large-scale circulation of tropical cyclone（TC）Usagi before extratropical transition（ET）has been analyzed. On this basis, the numerical experiment of modifying the high-latitude trough and ridge were carried out using the piecewise potential vorticity inversion, to discuss the influence of the tough and ridge on the asymmetric precipitation of TC before ET process. The results are as follows: （1）The precipitation of Usagi before ET is concentrated on the left side of the path, accompanied by meridional to zonal circulation adjustment, at the mean time the subtropical high retreats southward while weakening and propelling westward；（2）The asymmetric distribution of TC precipitation is directly related to the relative strength of the cold and warm front, the water vapor transport and the falling area of the upper-level cold air.（3）After the circulation adjustment was strengthened, the baroclinity of system was enhanced, which means the increased westerly showed by increased circulation index is beneficial to increase the LOT (left of track) rainfall.
Available online:December 23, 2020 DOI: 10.3878/j.issn.1006-9895.2011.20102
Abstract:In this paper, the characteristics and formation of "train effect" in the spiral cloud belt of Typhoon "Fitow" (No 1323) are analyzed. The results show that there is a “train effect” during persistent rainfall over the south bank of the Bay of Qiantangjiang in Zhejiang. According to the stable position of the rain belt, it can be divided into two "train effect", they last about 3-4 hours, and the space span is 1-2 longitude distance;The rainstorm area shows the characteristics of band, with high precipitation efficiency. The precipitation exceeds 25mm per hour and propagates forward linearly. The average reflectivity of radar echo in the spiral cloud belt of typhoon with over 35dBZ also shows a linear band structure. The trend of precipitation belt and radar echo deviated from the typhoon center by more than 25 degree to the right. From the formation of the "train effect", the direction of the high-altitude guided air flow and the direction of the surface mesoscale disturbance convergence zone are basically consistent with the movement direction of the convective cell in the "train effect", which provides the basis for its nearly linear arrangement. The mesoscale disturbance convergence or vortex formed along the coast provides important conditions for the occurrence or development of convection in the spiral cloud belt in coastal areas, and forms the beginning and maintenance mechanism for the rapid growth and subsequent development of convective clouds, which plays an important role in the formation, development and maintenance of "train effect".
Observational Study on the Evolution Characteristics of Hail Embryos in Weining, Guizhou Based on X-band Dual Linear Polarization Radar
Available online:December 23, 2020 DOI: 10.3878/j.issn.1006-9895.2005.20105
Abstract:In order to explore the evolution characteristics of hail embryo particles during the hailstorm in Weining area, this paper uses the Barnes interpolation method to interpolate polarization parameters after unfolding, filtering, and adaptive attenuation correction of X-band dual polarization radar data. The fuzzy logic algorithm was used to identify the particles of the aquatic objects, and then, two typical hailstorm processes in Weining County were systematically analyzed, and the following conclusions were obtained: (1) The sources of high-density graupel were aggregates and frozen drops during the two processes, and the sources of low-density graupel particles were ice crystal; high-density graupel particles contributed the most to hail production during the single-cell hail storm, the low-density graupel particles contributed the most to hail during the multi-cell hailstorm. (2) In a multi-cell hailstorm, the high-altitude convergence and low-level divergence of the attenuating cell can promote the formation of hail embryos in developing cell. (3) Before the hailfall for single-cell hailstorm: the number of low-density graupel particles is almost unchanged; due to the increase of the aggregates, the consumption of supercooled water particles is accelerated, resulting in a decrease in supercooled water particles, and some high-density graupel particles fall to the ground, which eventually leads to a decrease in the number of high-density graupel particles, with a rate of change of -10 pools per minute(The pool is the number of pools of aqueous particles in the particle recognition result of the cell); during hailfall: high-density graupel particles attach to supercooled water particles and then grow to form hail, resulting in a decrease in high-density graupel particles with a rate of change of -12.1 pools per minute; and under the“Bergeron process”and the adhesion of the aggregation, the low-density graupel particles increased rapidly, with a rate of change of 36 cells per minute; after hailfall: low-density graupel particles fall, and the number decreases rapidly, with a rate of change of -36.6 pools per minute; during the descent process, low-density graupel particles that are attached to supercooled water, aggregates, etc. are converted into high-density graupel particles, resulting in the total amount of high-density graupel particles almost unchanged, with a rate of change of 2 pools per minute. (4) Multi-cell hailstorm are similar to single-cell hailstorms before the haifall, except that the development speed, number of hail embryos, and echo intensity of the single-cell hailstorm are higher than that of the single-cell hailstorm under the promotion of dissipating cells; during hailfall: low-density graupel particles attach to supercooled water droplets to form hail during the falling process, resulting in a rapid decrease in the number of -62.6 pools per minute; high-density graupel particles increase in the rate of 16.5 pools; the time from mature phase to hailfall of the muti-cell hailstorm is about 15 minute longer than the single-cell hailstorm; it is similar to a single-cell hailstorm after the haifall. (5) Based on the analysis of the hailstorm mechanism in Weining area, the conceptual models of single-cell hailstorm and multi-cell hailstorm were established respectively. This paper studies the evolution characteristics of hail embryos in each process of two typical hailstorms and obtains preliminary results, which has high application value for hailstorm early warning, prediction, and artificial hail suppression.
Large eddy simulation study of the turbulence structure characteristics of the convective boundary layer in the Source Region of the Yellow River
Available online:December 23, 2020 DOI: 10.3878/j.issn.1006-9895.2009.20111
Abstract:In order to study the turbulence characteristics of boundary layer and its effects on the transport of momentum, heat and water vapour in the Source Region of the Yellow River (SRYR), northeast of the Tibetan Plateau，large eddy simulations are performed for the first time to investigate the fine structure of turbulence in the convective boundary layer (CBL) over two different underlying surfaces (grassland and lake) in the SRYR. The GPS soundings and eddy covariance data observed during a field experiment in the Ngoring Lake Basin in summer 2012 are used. It shows that the averaged structure of CBLs over the grassland and lake is in good agreement with the observations, but the characteristics of the turbulence structure over both surfaces are in large differences. The budget of turbulence energy, spatial-temporal distribution of turbulent properties and the structural feature of turbulent eddies above the grassland are consistent with the thermal-driven CBL over the land. The organized convective rolls presented over the lake boundary layer driven by shear. There is larger turbulence intensity at the top of CBL over the lake due to the strong entrainment, while the same thing occurs in the surface layer over the grassland. It is found that the simulated results are sensitive to the horizontal resolution over both surfaces. The higher horizontal resolution applied over the lake helps to improve the accuracy in the simulation of turbulent kinetic energy and turbulent fluxes of the surface layer and the entrainment layer, and the contributions of various scale’s waves to turbulent fluxes are fully simulated as well. The grid spacings of 100 m-200 m are recommended to simulate the fine turbulence structure over the grassland if the simulated time is considered.
The Comparative Analysis of Rain Band Position Forecasted by the Model in an Extreme Meiyu Rainfall Event
Available online:December 23, 2020 DOI: 10.3878/j.issn.1006-9895.2010.20116
Abstract:From June 30th to July 4th, 2016, an extreme precipitation event occurred which is the strongest rainfall process since the flood season. Obvious errors appeared in the prediction of the location of the rain band in operational forecast. Based on the deterministic and ensemble model data from European Centre for Medium Range Weather Forecasting (ECMWF) and National Centers for Environmental Prediction / Global Forecast System (NCEP), the difference of the model performance between them is compared and the causes of forecast error are analyzed. The results show that the forecast of NCEP is more accurate and there are obvious northward errors in the rain band location forecasted by the ECMWF deterministic model. Further analysis from ensemble data points out that although the forecast error of NCEP is smaller, which is caused by the obvious instability of its errors, it appears alternately northerly and southerly error from day to day. Though ECMWF model predicts the rain band with further north location, this northerly error is stable. In addition, by dividing the ensemble members into the accurate and bias groups, the composite analysis between them reveals that the member with further north rain band location will have stronger precipitation, and vice versa. Finally, the connection between the westerly trough in 500hPa level and the position of rain band is discussed. The results reveal that when the strength of the westerly trough is stronger, the rain belt is located further north and the intensity of rainfall is more obvious, and vice visa. The westerly trough forecasted by the ECMWF model continued to be strong. That’s the reason why this model gives a stable northerly position of the rain belt forecast. Because the NCEP model predicts stronger and weaker westerly troughs alternately, its forecast of the rain belt location shows corresponding northerly and southerly errors. This conclusion could serve as an important reference for the forecast errors correction of the rain belt location in the forecast operation, and also help to improve the forecast accuracy of rain band location in Meiyu season.
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.
Mechanism analysis of a sudden rainstorm triggered by the coupling of gravity wave and convection in mountainous area
Available online:December 23, 2020 DOI: 10.3878/j.issn.1006-9895.2008.20137
Abstract:Based on the ERA5 reanalysis data ,the China Merged Precipitation Analysis data and the FY-2G satellite cloud image data provided by NSMC (National Satellite Meteorological Centre), we conducted a synoptic dynamic analysis on the characteristics of gravity wave during an abrupt rainstorm on 21-22 May 2018 in the southwest of Sichuan Basin. The results indicate that the sudden rainstorm in the mountain area was affected by the gravitational wave activity with a wavelength of 150km and a period of 5h. The gravity wave was triggered by the combination of terrain and vertical shear instability. The shear instability appears in the downstream before the propagation of the gravity wave. The Richardson number is a good indication of the direction of gravity wave propagation and precipitation area. Before the rainstorm, the updraft in the gravity wave transports the water vapor to facilitate the development of the convection, while the downdraft causes the unstable energy in the lower layer to accumulate continuously. With the development of the Northeast low-level jet, a critical layer is formed in the transition zone of the East-West wind shear in 700-800hpa. The critical layer continuously absorbs the wave energy from the upper air and makes the gravity wave energy to be transmitted down, triggering the release of the unstable energy in the lower layer, and promotes the convection to be strengthened continuously, which finally causes the sudden rainstorm.
Relationship between Tropical Intraseasonal Oscillation and Intraseasonal Variation of the South Asian High
Available online:December 23, 2020 DOI: 10.3878/j.issn.1006-9895.2007.20144
Abstract:The relationship between the tropical intraseasonal oscillation (ISO) and the intraseasonal variation of the South Asian high (SAH) in boreal summer and associated physical mechanisms are investigated by using both the observational analysis and numerical model experiments. The results indicate that the intraseasonal variation of SAH is closely related to tropical ISO activity. The eastern edge of SAH shifts westward when the active phase of the tropical ISO appears in the Indian Ocean (including Phase 1, 2 and 3), and shifts eastward when the ISO appears in the Pacific (including Phase 5, 6 and 7). The most sensitive region in response to the tropical ISO forcing is over the eastern part of SAH, which is the region connecting East Asia and western Pacific (110o-140oE,15o-25oN). The vertical profile of tropospheric atmosphere in the region exhibits a baroclinic structure. An eastward extension (westward retreat) of the eastern edge of SAH corresponds well to an eastward retreat (westward extension) of the western edge of the western Pacific subtropical high (WPSH) in the middle and lower troposphere. The intraseasonal variability of SAH is primarily affected by the tropical ISO. About 40% of the variability in the key region is attributed to the tropical ISO, whereas the influence of the SAH on the tropical ISO is weak. The physical processes by which the tropical ISO affects SAH are summarized as follows. As the tropical ISO moves from the Indian Ocean to the western Pacific, it also propagates northward under the favorable summer monsoon background state. To the northwest of the ISO heat source, cyclonic circulation and negative height anomalies (anticyclonic circulation and positive height anomalies) appear in the lower (upper) troposphere, which leads to the eastward extension of SAH. However, when the tropical ISO appears in the Indian Ocean, the atmospheric circulation response is in general opposite to the above, leading to the westward retreat of SAH.
Response of Winter Extremely Low Temperature Events in Mid-latitude Asia to Abnormal Warming over Barents and Kara Seas and Possible Mechanisms
Available online:December 23, 2020 DOI: 10.3878/j.issn.1006-9895.2011.20168
Abstract:Based on the NCEP-DOE reanalysis data during 1979-2019, this paper investigated that response of frequency of winter extremely low temperature events in mid-latitude Asia to abnormal warming over Barents and Kara Seas and possible mechanisms. The results show that the abnormal warming over Barents and Kara Seas usually corresponds to the enhancement of Siberian high and the frequent occurrence of extreme low temperature in mid-latitude Asia. It is found that positive geopotential height anomaly appears over Novaya Zemlya and Urals with the abnormal warming over the Barents and Kara Seas. Significant cold advection and sinking motion anomalies appears on the east and south sides of the positive geopotential height anomaly and abnormal divergent flow appears on the ground, which makes Siberian high strengthen and extend to Southeast. The diagnosis of thermodynamic equation further indicates that cold advection anomaly caused by northeast wind on the south side of the Siberian high pressure anomaly, ascending motion anomaly caused by Easterly wind anomalies passing the terrain and abnormal non-adiabatic cooling caused by radiation, sensible heat, and latent heat result in an increase in the amplitude of the near-surface temperature’s seasonal cycle in mid-latitude Asia, which conducive to frequent occurrence of extreme low temperature.
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).
Available online:December 17, 2020 DOI: 10.3878/j.issn.1006-9895.2009.20112
Abstract:In this paper, the mesoscale model WRF is used to simulated mountain-valley wind circulation in the region of south Gaoligong mountain in the dry season and wet season, in 2016, respectively. Applicability of five layer parameterization schemes under complex underlying surface of the Gaoligong mountain are compared. The results show that YSU scheme performs best in temperature simulation. The mean absolute error of wind speed in simulation is the smallest using ACM2. The absolute error of wind direction simulated by MYNN3 scheme is the smallest. The diurnal variation of wind direction simulated by the YSU and MYJ schemes is more consistent with the observation. In the southern Gaoligong mountain, valley wind circulation appears at 9:00 am and turns to mountain wind circulation at 7:00 pm. It is mostly southerly wind in the daytime, and northerly and westerly wind at night. During the day, the air flow converges at the top of the mountain while it diverges in the valley. At night, it is opposite. The wind speed in daytime is greater than that at night. In the dry season, the west wind is weak, which is conducive to the development of local circulation in the lower troposphere. In the wet season, the eastward background wind is strong, which suppresses the development of local circulation, thus, the height of the boundary layer is lower than that in the dry season. In the dry season, the westerly wind encounters the Gaoligong Mountain, sinks on the west slope, forms vortices, and then the turbulence on the west side is well mixed, leading to a deep boundary layer. In the wet season, the easterly wind weakens the valley wind on the west side of Gaoligong Mountain, and the height of the boundary layer between Tengchong and Baoshan is similar.
Available online:December 11, 2020 DOI: 10.3878/j.issn.1006-9895.2011.20169
Abstract:Based on the monthly mean NOAA outing longwave radiation (OLR) data and surface heat fluxes derived from ERA-5 reanalysis data during 1980 to 2019, the spatial and temporal distributions of OLR and convection over the Tibetan Plateau (hereinafter referred to as TP) and the relationship between surface heat fluxes and convection were investigated. The results showed that the minimum value of OLR was located in the middle of the TP, and increased outward. The OLR (convection) was lower (stronger) in the eastern plateau than that in the western region. In the past 40 years, the OLR (convection) increased (decreased) gradually with a period of 6 a and 2-3 a over the TP. However, the tendency of convective activity had significant differences among different regions and seasons. A growth trend of convective activity occurred in boreal summer, while it weakened in the other seasons. The convection had a weakening trend in the vicinity of the Three Rivers’ Headstream region, but increased in the north of the Himalayas on the TP. In boreal summer, the surface latent heat flux was generally stronger than the surface sensible heat flux. The evolution of convection was significantly related to both surface sensible and latent heat fluxes. The surface sensible heat flux was highly negatively correlated to the evolution of convective activity, while there was an east-west dipole pattern of correlation between the surface latent heat flux and convection. There was a positive correlation between the surface latent heat flux and the evolution of the convection in the western plateau, whereas a negative correlation in the eastern region.
Available online:November 30, 2020 DOI: 10.3878/j.issn.1006-9895.2009.20126
Abstract:Tropical cyclone Remote Precipitation (TRP) often becomes high-impact weather, which is a difficult point for operational forecast. In this paper, ground observation data, sounding observation data, radar remote sensing data and NCEP re-analysis data of 0.5°×0.5° four times a day are used to analyze the process of long-distance rainstorm in the Yangtze river delta region caused by typhoon “Mangosteen” during its landfall in guangdong province in 2018. The results show that: 1) these long-distance heavy rains occured within the control range of the of the high pressure at the edge of subtropical high, while the lower layer is affected by typhoon trough. 2) The heavy rain process in the first stage is mainly under the condition of strong convective instability, which is triggered by convergence line of the typhoon “Mangosteen” inverted trough in the lower troposphere, and the high-altitude "Mangosteen" poloidal outflow merges into the mid-latitude westerly wind and increases its speed, producing divergence over the Yangtze river delta region and strengthening the upward movement in the rainstorm area. In the second stage, the convective instability conditions have weakened, but a low vortex is formed at the north end of typhoon inverted trough at 850 hPa, and a short-wave trough develops at the edge of subtropical high at 500hPa, so the dynamic conditions of rainstorm are more favorable. 3)The three heavy rainfall centers in the Yangtze River Delta are located at the mouth of the Yangtze River, along the coast of Jiaxing on the north coast of Hangzhou Bay and along the coast of Ningbo, all of which are near the land and water boundary. 4)the diagnosis of vorticity input and output in the remote rainstorm area suggest that: the initial disturbance of the rainstorm is mainly provided by the horizontal convergence and divergence term near the surface, and the horizontal convergence and divergence term of 850 hPa combined with the twist term strengthen the development of the low vortex, and the vertical upward movement increases the vorticity of the middle layer near 700-500 hPa, and the 500hPa short-wave trough is thus developed. The 850hPa vorticity comes from the southerly jet at the edge of the typhoon inverted trough and the subtropical high. 5) During this long-distance rainstorm, a southerly low-altitude jet formed between typhoon “Mangosteen” and the western Pacific subtropical high (subtropical high) transports water vapor to the Yangtze river delta, which is similar to the typical TRP event. The difference is that the initial disturbance of the rainstorm in a typical TRP is generally provided by the westerly trough, while in this rainstorm it is mainly provided by the low-altitude typhoon inverted trough and the southerly jet stream. The formation of the upper-air short-wave trough after the vorticity is uploaded is a physical process different from that of a typical TRP event.
Available online:November 04, 2020 DOI: 10.3878/j.issn.1006-9895.2010.20121
Abstract:Atmospheric ice-nucleating particles (INPs) trigger heterogeneous ice nucleation, thereby significantly affecting microphysical properties of clouds and the radiative balance. As a result, in order to better understand aerosol-cloud interactions and their climatic effects, it is crucial to elucidate the abundance, sources and properties of INPs in the atmosphere. In this article, via reviewing relevant laboratory and field studies, we discuss INP measurement techniques, ice nucleation activities of representative aerosol particles and the corresponding ice nucleation parameters, and summarize recent progress and remaining challenges in the atmospheric INP research. In addition, we point out the necessity to enhance ice nucleation research in China, and also discuss in brief the main open questions in this field.
Available online:October 23, 2020 DOI: 10.3878/j.issn.1006-9895.2010.20110
Abstract:As global warming intensifies, the growing season has been changed accordingly over China. We would like to stress that earlier pertinent studies only cover short-term periods, use low horizontal resolution data, and focus on limited regions. Based on these premises, we investigated the climatology and trends of the start of the growing season (GSS), the end of the growing season (GSE), and the growing season length (GSL) during 1961–2018 over China, by using the daily mean temperature data from high horizontal resolution gridded dataset CN05.1 (0.25o × 0.25o). The relationship between the changes in the growing season and the seasonal average temperature is also analyzed. The results indicate that the national average GSS and GSE are March 31 and October 29, respectively, and GSL lasts 212 days over China during 1961–2018. Spatially, annual average GSS delays from southeast to northwest, while GSE has the opposite trend; GSL decreases from southeast to northwest. In general, the trends of national average growing season display the earlier start (–1.3 days/10years), later end (0.9 days/10years), and longer length (2.2 days/10years) over China from 1961 to 2018. Finally, both the advance of the national average GSS and the extension of GSL are mainly related to increased spring temperature, while the delay of GSE originates from increased autumn temperature.
Available online:September 23, 2020 DOI: 10.3878/j.issn.1006-9895.2009.19240
Abstract:A new convective-stratiform separation technique, which is based on the CMPAS (CMA multisource precipitation analysis system) hourly precipitation and radar reflectivity mosaics data, by using the fuzzy logic method, is developed to improve the cloud analysis scheme in GSI assimilation system (referred to as CUST scheme). The improved scheme was tested in a severe Mei-yu rain occurred on 19 June 2019 in Zhejiang Province. Several hourly-cycle assimilation experiments were carried out using with the WRF model and GSI assimilation system, to analyze the impact of the new scheme on the precipitation simulation by comparing the new scheme to other schemes. The results show that: (1) The new convective-stratiform separation technique divided the convective / stratiform cloud accurately, and can be used as the discriminating factor to improve the cloud analysis scheme in the GSI assimilation system. (2) The CUST scheme adopt the convective cloud analysis scheme in the convective region, and the stratiform cloud analysis scheme in the non-convective region, which reduced the false alarm rate in the simple convective cloud scheme and the underestimate in the simple stratiform cloud scheme, effectively improved the simulation ability of short-term precipitation. (3) The CUST scheme shown obvious improvement on the initial stage of the model (within 6 hours or even 3 hours), and the improvement of small rain level is greater than that of the heavy rain. (4) Compared with the hybrid cloud analysis scheme (referred to as CSW scheme) based on the convective scale velocity determined by the surface sensible heat and latent heat flux, the CUST scheme shown more reasonable result on the convective / stratiform cloud partition and precipitation simulation, which indicated a good application prospect.
Available online:September 11, 2020 DOI: 10.3878/j.issn.1006-9895.2009.20106
Abstract:As a critical system of the East Asian winter monsoon, the Siberian High has an important impact on the winter weather and climate anomalies in Eurasia. Based on the National Center for Environment Prediction-Climate Forecast System, version 2 (NCEP-CFSv2), the seasonal and monthly prediction performance of the Siberian High is comprehensively evaluated during the winter time (November to February). It is found that the NCEP-CFSv2 model can skillfully predict the Siberian High intensity only in November, the reasons for which are that local thermal process, dynamic process and Siberian snow cover extent mainly affect the intensity of Siberian High in November. In terms of thermal process, NCEP-CFSv2 can better reproduce the intensity of Siberian High in November and its related surface soil temperature, upward long wave radiation and other thermal factors in Siberia. In aspect of dynamic processes, the NCEP-CFSv2 can better reproduce the strength of the Siberian High in November and its associated with low-level tropospheric divergent circulation, the sinking movement of the upper and middle layers respectively in Siberian area. Meanwhile, the model also reproduces the relationship between the snow cover extent over Siberia and the intensity of the Siberian High in November. The thermodynamic process of the Siberian High and the snow cover in the area are predictability sources of the Siberian High intensity in November, and the NCEP-CFSv2 can reasonably reproduce these predictability sources in November.
Available online:September 11, 2020 DOI: 10.3878/j.issn.1006-9895.2008.20161
Abstract:Based on regional model simulations of one typical cases of the eastward moving southwest vortex with the subtropical high east retreat (case 20150721),this study discusses the effect of subtropical high strength for the eastward southwest vortex through the numerical sensitivity test of enhancement subtropical high. The main results are as follows: (1) The circulation field firstly changes when the subtropical high strength increases. the invading north wind is weaker,southwest guided airflow is stronger,and it eventually led to the faster and more weak southwest vortex.(2)The change of circulation field directly affects transport, convergence and divergence of the water vapor,and further affects the whole evolution process of southwest vortex.(3) after the increase of the subtropical high,the faster southwest vortex result in the deviation between the center of southeast vortex and the thermal center in the lower layer, and result in the mismatch between the dynamic and thermal centers, thus strength of the southwest vortex weaken.
Stratosphere-troposphere interactions during nationwide extensive and persistent extreme cold events in boreal winter
Available online:September 08, 2020 DOI: 10.3878/j.issn.1006-9895.2007.20125
Abstract:Based on 1959–2017 ERA40/ERA-interim daily-mean reanalysis data and National Climate Center daily station data, this study diagnoses the extratropical circulation anomalies and stratosphere-troposphere interaction processes during nationwide extensive and persistent extreme cold events (EPECEs). The results show that nationwide EPECEs experience three stages from the cold-accumulation in Urals-Siberia, the cold-outbreak to the cold-decay, which corresponds to the formation, development and collapse of the Urals-Siberia tilted ridge in the middle troposphere, and the recovering, persisting and re-weakening of the stratospheric polar vortex. Meanwhile, prominent stratosphere-troposphere interactions are seen in the mid- and high latitudes. Firstly, the wavenumber-2-type anomaly appears in extratropical troposphere because of dominant disturbance in the Atlantic, and propagates upwards to affect stratospheric circulation. Subsequently, anomalous planetary waves propagate downwards and the stratospheric polar vortex recovers when the wavenumber-2-type anomaly is adjusted to the wavenumber-1-type anomaly. The wavenumber-1-type anomaly appears in the troposphere afterwards and the positive center is located in eastern Europe, which leads the Ural blocking to form and cold air from the Arctic Ocean to accumulate in front of the blocking. Thereafter, the anomalous upward tropospheric planetary waves continue to be restrained and reflected downwards mainly over Europe due to the persisting of stratospheric polar vortex. The reflected planetary waves favor the positive center of the wavenumber-1-type anomaly gradually shifting eastwards to central Siberia while the blocking develops downstream into a large-scale tilted ridge arching across the Siberia. The accumulation area of cold wave also moves eastwards to central Siberia. For the outbreak of cold wave, the large-scale transversal trough in front of the ridge, involving an upstream short-wave trough, leads to a trench structure. As the transversal trough turns into be vertical, extensive areas of China are in the grip of cold outbreak. Concurrently, the wavenumber-1-type anomaly propagates upwards again owing to the maintenance and development of the large-scale tilted ridge, which weakens the stratospheric polar vortex again. Finally, the blocking and trough collapse as cold wave bursts out. Tropospheric planetary wave and meridional transport of lower atmosphere are decreased, which suggests decay of the EPECEs..
Research on the multi-factor descending dimension method of objective forecast for summer precipitation in Southwest China
Available online:September 08, 2020 DOI: 10.3878/j.issn.1006-9895.2005.20120
Abstract:In this paper, the existing criteria of EOF modes temporal stability are improved, a scheme for electing stable high-correlated prediction signals is proposed. Moreover, temporal stability of EOF modes, the temporal and spatial characteristics, and the key signals to predictable modes of summer precipitation anomaly percentage in Southwest China are analyzed. On that basis, multi-factor descending dimension prediction model is established. The results show that the first nine modes are stable within 3 years lead and in nearly 10 years for climate prediction. They account for nearly 70% of the variance contribution rate and primary to summer precipitation anomaly percentage in Southwest China. The optimal prediction signals and correspongding equations for primary PCs are selected and built with the stable high correlation concept and the Optimal Subset Regression method. These equations have good skills for PC fitting. Their complex correlation coefficient all pass the significance test of 99.99%, and sign coincidence rates are greater than 69%. The prediction model built on that basis has a good hintcast skill for the spatial distribution, variation trend and abnormal level of summer precipitation in Southwest China. Its mean ACC score and PS score are 0.58 and 84, respectively. While the TCCs pass the 90% significance test in the whole region except for sporadic stations, and pass the 99.9% significance test in most areas. According to forecast test in 13 years, the mean PS score of this model is 72. And the mean PS score is 77, which is higher than that of published forecasts in recent 3 years.
Twenty-first Century Climate Change Projection over Xinjiang based on an Ensemble of Regional Climate Model Simulations
Available online:September 08, 2020 DOI: 10.3878/j.issn.1006-9895.2006.20108
Abstract:Based on a set of 21st century climate change projections by a regional climate model (RegCM4) at a 25 km grid spacing driven by five global models, we investigate future climate change over Xinjiang in Northwest China under the middle and high representative concentration pathways of 4.5 and 8.5 (RCP4.5 and RCP8.5). Results show the multi-RegCM4 ensemble (MME_R) captures well both the spatial distributions and amounts of mean temperature and precipitation. The temperature and precipitation over Xinjiang are projected to rise or increase continuously in the future, most significant under RCP8.5 compared to RCP4.5. The regional mean increases in annual temperature and precipitation are as 4.9oC and 28% (102 mm), respectively, by the end of the 21st century under RCP8.5. Increases of temperature and precipitation extremes are also reported as measured by different indices, indicating more heat waves, less cold spells, and more extreme precipitation in the future. To be more specific, for temperature, greater increase of TNn (annual minimum of daily minimum temperature) is found compared to TXx (annual maximum of daily maximum temperature). In the end of century under RCP8.5, the increase of regional mean TXx and TNn over Xinjiang are 4.9oC and 5.8oC (TNn), respectively. As for extreme precipitation indices, an increase of RX1day by 29% (5 mm) and decrease of CDD by 10 days are found. The change of snow cover show spatial differences, with a general decrease except a large percentage increase in the western Tarim Basin. A 13% reduce is reported for the regional mean snow cover at the end of century under RCP8.5. The total runoff and soil moisture are projected to be an increase, but with more hydrological droughts in northern Xinjiang. The MME_R show consistencies among the ensemble members. Overall with the consideration of the variables analyzed, a “warmer and more humid” tendency of climate as observed in the late decades is expected in Xinjiang in the future. However, this may not change the fact of the dominance of arid and semi-arid climate over the region. In addition, hydrological droughts is also projected to be increase in the future. Thus high attention still needs to be paid for the availability and risks on water resources over the region.
Preliminary study on the effect of intraseasonal evolution of the tropical Atlantic SST anomalies on summer persistent heatwave events over the area south of the Yangtze River
Available online:September 08, 2020 DOI: 10.3878/j.issn.1006-9895.2005.19235
Abstract:The area south of the Yangtze River is an area highly impacted by heatwave disasters. Previous studies have revealed the effects of sea surface temperature anomalies (SSTAs) in several crucial sea regions (e.g., the tropical Atlantic, the North Indian Ocean, and the tropical central eastern Pacific) on summer mean air temperature anomalies over southern China on interannual or interdecadal time scales. However, there has little research on the impacts of the SSTAs in these key sea regions on the occurrences and maintenances of heatwave events over the area south of the Yangtze River on the intraseasonal time scale. For this reason, using observational data in gauge stations in China, NCEP/NCAR reanalysis, and NOAA SSTs, the present study focuses on the potential contribution of SSTAs in the tropical Atlantic to heatwave events over the area south of the Yangtze River by analyzing two summer heatwave events in 2016. In addition, based on the composites of multiple heatwave events during 1981–2016, a possible physical link that explains the effect of the intraseasonal evolution of SSTAs in the tropical Atlantic on heatwave events over the area south of the Yangtze River is further explored. The results indicate that, the development and maintenance of warmer SSTAs in the tropical western Atlantic is conducive to stimulate a relatively stable Rossby wave train over Eurasia on the intraseasonal time scale. As a result, a deep high-pressure anomaly governs East Asia and its coastal areas, resulting in long-term persistent (≥8 days) heatwave events over the area south of the Yangtze River. This phased increase and maintenance of the SSTAs in the tropical Atlantic and associated steady Rossby wave train are prior to the long-term persistent heatwave events, and the latter might occur in a month since the SSTs in the tropical Atlantic significantly enhanced. On the intraseasonal time scale, there is a clearly phased increase in the SSTs in the North Indian Ocean, around 10 days after the significant increase in the SSTs in the tropical Atlantic Ocean. This implies that in addition to directly exciting the Rossby wave train across Eurasia, it is probably that the SSTAs in the tropical Atlantic may affect the occurrence and maintenance of the heatwave events through modulating the phased variation of the SSTAs in the North Indian Ocean on the intraseasonal time scale. Furthermore, during the period of El Ni?o decay and transition to La Ni?a, the synergistic intraseasonal variations accompanied with cooler SSTAs in the tropical central eastern Pacific and warmer SSTAs in the North Indian Ocean may also contribute to the long-term persistent heatwave events. The intraseasonal variations in the SSTAs in the three key sea regions seem to be able to be used as precursory signals for the heatwave events. Nevertheless, the specific process that explains the intraseasonal effects of the SSTAs in the three key regions, especially the joint effect of the SSTAs in these regions and related physical process, requires further investigation in the future.
Available online:September 03, 2020 DOI: 10.3878/j.issn.1006-9895.2007.19236
Abstract:Although the nonstaggered grid has better physical consistency than staggered grid, it is still not widely used mainly because its poor accuracy for simulating the geostrophic adjustment process under 2nd order accuracy difference scheme. However, weather this conclusion remains valid under higher order finite difference scheme is still unknown. In this paper we conduct theoretical analysis and numerical test for shallow water equation under high order difference schemes, it is found that: (1) for low wave number, the dispersion of staggered grid does not change with the accuracy of difference scheme, while the dispersion of unstaggered grid is improved obviously, and the dispersion of both grid get very close under 4th order scheme. (2) the maximum frequency of unstaggered grid still exist under high order difference scheme, and it move towards higher wave number as the accuracy of difference scheme get higher. The frequency of staggered grid is monotonically increasing with wavenumber and gets more close to real solution under high order difference scheme. (3) when the high frequency noise was removed with explicitly adding a diffusion term, the pros and cops to grid staggering choices diminish with high-order schemes. In general, the unstaggered grid is an attractive choose for the discretization of the dynamical frame in numerical model under high-order difference scheme.
Impacts of decay of different El Ni?o types on boreal summer rainfall and surface air temperature in the South Asian Monsoon region and Tibetan Plateau
Available online:September 02, 2020 DOI: 10.3878/j.issn.1006-9895.2005.20141
Abstract:Impacts of El Ni?o events on boreal summer rainfall over the East Asian Monsoon and South Asian Monsoon (SAM) regions and the associated mechanisms have been extensively examined. Nevertheless, the varying impacts of an El Ni?o on the Tibetan Plateau (TP) regional seasonal and monthly rainfall and circulation have not been systematically examined. Based on the timing of El Ni?o decay with respect to the boreal summer season, El Ni?o decay phases are classified into two types in this study using 1950-2018 sea surface temperature (SST) data, which are as follows: (1) early decay and (2) late decay. If El Ni?o decays below the threshold before spring, a La Ni?a sea surface temperature anomaly (SSTA) pattern usually develops during summer with increasing anomaly amplitudes from June to September, which causes an enhanced westward shift of the Walker circulation with a strong ascending branch over the tropical Indian Ocean (TIO) and the SAM, and induces concurrent heavy rainfall over the SAM and southwestern TP area from July to September. Meanwhile, the developing La Ni?a SSTA forces a response of the anomalous North Western Pacific anticyclonic (NWPAC), an anomalous cyclonic circulation over the Arabian Sea and an anticyclonic circulation over the Western Asian region, which induce a strengthening southerly wind anomaly, enhance water vapor transport to the Indian and TP regions from tropical Indian Ocean, and thus increase summer precipitation over northern India and southwestern TP. In contrast, if El Ni?o decays below the threshold after September, the eastern Pacific El Ni?o SSTA pattern and strong SST warming over the tropical Indian Ocean persists in June to July, then gradually weakens from August to September. This causes the anomalous ascending branch of the Walker circulation over the eastern TIO with a weak ascending branch over the western TIO and SAM, an anomalous eastwardly extended NWPAC, an anomalous anticyclonic circulation over the Arabian Sea and an cyclonic circulation over the Western and Central Asian region, which induce a strengthening westerly wind anomaly and reduces water vapor transport over the TP. The above responses result in deficient rainfall and warm surface temperatures in the central and northern SAM regions in June, but relatively increased rainfall and cool SAT in most of the SAM region during August and September. This coincides with dryness over northeastern India and the southwestern TP in June, and then with increasing precipitation over northwestern India and the western TP in September. Our results demonstrate that the decaying El Ni?o has a significant impact on summer seasonal and monthly precipitation and temperature over the TP, which may explain the positive correlation of precipitation between Indian and southwestern TP recently discussed in some studies. Our results also suggest that the differences in an El Ni?o decay phase have strong impacts on the seasonal and intraseasonal rainfall over the SAM region and over the southwestern TP.
Available online:September 01, 2020 DOI: 10.3878/j.issn.1006-9895.2008.20134
Abstract:Ground-based microwave radiometers (MWR) have been widely used in recent years due to providing high-temporal resolution atmospheric temperature and humidity profiles. The quality of multi-channel brightness temperature (TB) is the basic guarantee for retrieving atmospheric profile products. In general, periodic absolute calibration of liquid nitrogen can better maintain the quality of TB observations, but the actual operation is more complex and difficult. As an auxiliary tool, radiative transfer model can be used to detect TB quality of MWR. Combined with the observations from Beijing radiosonde, and two RPG MWRs located at Beijing Observatory (GXT) in Beijing and Xianghe (XH) site in Hebei, respectively, three radiative transfer models, including MonoRTM, ARTSs and MWRT, are evaluated by comparing their simulations with corresponding observed TBs at both sites. The results show that for MWR 14 channels the simulations of the three models are very close to the observed TBs at most of channels of MWR, with high consistency (i.e. correlation coefficient up to 0.99), while for temperature channel ch8 (51.26 GHz) and ch9 (52.28 GHz), there was also a significant absolute deviation (about 4 ~ 5K) between the simulated TB and the observed TB, and the correlation coefficient decreased significantly (< 0.80), which indicated that model simulation at the two channels needs to be improved. Among the three models, MonoRTM shows obvious systematic deviation at temperature channel ch8, ch9 and ch10 (53.86 GHz), especially at ch8 with bias up to 5K; ARTS displays worse simulation at the water vapor channel ch1 (22.24 GHz); relatively, MWRT simulations are more stable and closer to the corresponding TB observations at 14 channels, especially the systematic deviation is the smallest. In addition, location of the radiosonde measurements is different from that of the MWR site, which has a significant impact on the simulation for water vapor channels of MWR. The comparisons of observed TB and simulation at both sites indicates that the observation quality of water vapor channels for MWR at GXT needs to be improved.
Interannual variation of summer rainfall at the Tianchi station in Changbai Mountains and its associated circulation anomalies
Available online:August 31, 2020 DOI: 10.3878/j.issn.1006-9895.2008.20117
Abstract:Changbai Mountains are located in the southeast of Jilin Province. Rainfall has a crucial impact on the forest ecosystem and water resource in Changbai Mountains, which is a national key ecological function area. Based on the monthly precipitation data from 47 stations in Jilin Province and ERA-Interim monthly reanalysis data from 1979 to 2016, we investigated the climate characteristics of the summer precipitation at Tianchi station in Changbai Mountains and circulation anomalies associated with the interannual variation of rainfall at this station, and compared with the other stations in Jilin Province. The results show that the summer precipitation at Tianchi and its interannual variability are approximately two times larger than other stations in Jilin Province. On the one hand, the circulation anomalies associated with interannual variability of precipitation at Tianchi are consistent with those for Jilin Province. The circulation anomalies that favor more rainfall at Tianchi and Jilin Province are characterized by a cyclonic anomaly over Northeast Asia and enhanced East Asian jet in June, and enhanced western Pacific subtropical high and northward displaced East Asian jet in July and August. On the other hand, the precipitation in Tianchi is associated with unique circulation anomalies, which are quite different or almost opposite to those related to the precipitation in Jilin Province rainfall, and may be attributed to by the terrain of Changbai Mountains. The present results suggest that the climate variability in mountains may be related to complicated and various patterns of large-scale atmospheric circulation anomalies.
Verification for GRAPES-REPS model precipitation forecasts over China during the flood season in 2019
Available online:August 31, 2020 DOI: 10.3878/j.issn.1006-9895.2008.20146
Abstract:A regional EPS (Global and Regional Assimilation and Prediction Enhanced System-Regional Ensemble Prediction System (GRAPES-REPS V3.0) ) with a horizontal resolution of 10km has been developed and put into operation by the Numerical Weather Prediction Center of China Meteorological Administration in 2019, The background field of which comes from GRAPES global model, and initial perturbation and model perturbation method are Ensemble Transform Kalman Filter and Stochastic Perturbed Parameterization Tendencies respectively. The system was run in real-time for the 2019 summer season (July through September) and compared with GRAPES-REPS V2.0 and ECMWF global ensemble prediction system using statistical verification and case analysis methods, for an objective and comprehensive evaluation of precipitation forecast skill and forecast uncertainties for GRAPES-REPS V3.0 model, And We further analyze the physical mechanism leading to the forecast uncertainties of meso-scale intense precipitation, The results could provide basis for diagnosing regional ensemble prediction systems and developing ensemble forecast methods. The results are as follows: (1) GRAPES-REPS V3.0 model has a better precipitation ETS scores than GRAPES-REPS V2.0 model in terms of all the forecast lead times and rainfall classes with more equal rainfall members, And the probability forecast FSS scores are also better, So the precipitation forecast skills of GRAPES-REPS V3.0 model are totally better than those of GRAPES-REPS V2.0 model. (2) The ensemble mean precipitation BIAS and ETS scores of GRAPES-REPS V3.0 for light rain and rainstorm are better than ECMWF global ensemble forecast system, and the probability forecast skill of above two models is comparable. (3) The case studies show that, Different ensemble prediction systems capture precipitation forecast uncertainties by describing meso-scale physical quantities uncertainties, At initial lead time, The circulation patterns of GRAPES-REPS V3.0 regional ensemble prediction system and ECMWF global ensemble prediction system are similar, However, With the evolution of forecast lead time, GRAPES-REPS V3.0 ensemble prediction model could better describe meso-scale dynamic and thermal fields, with a more accurate location and magnitudes of rainfall and a better probabilistic forecast result. (4) Compared with ECMWF model, The ensemble members of GRAPES-REPS V3.0 model can well forecast the occurrence, development and extinction of rainfall processes, So GRAPES-REPS V3.0 model has shown a higher skill in forecasting the precipitation of Chinese flood season.
Research on cloud particle shattering and its impact on the cloud microphysical parameters’ measurement in the stratiform clouds with embedded convection
Available online:August 21, 2020 DOI: 10.3878/j.issn.1006-9895.2006.19255
Abstract:The stratiform clouds with embedded convective cells is an important precipitation system. Precise knowledge on the cloud’s microphysical structure can be useful for the development of the weather numerical prediction model and the cloud water resource. In order to study the different influence of the shattered particles on the microphysical measurement in the stratiform cloud region and the convection cloud region, a time-variant threshold method to identify the shattered fragments is presented. Then the shattered impact on the PSD, particle number concentration and ice water content measurement is analysed. It is found that the influence of the shattered artifacts on the particle spectrum distribution (PSD) can be in the two ends(<500μm and >1000μm). In the stratiform area the influence on the little end is less than 300μm while in the convective area it is less than 500μm. On average, the shattered particles has 20% more impact on the PSD in the convective region than that in the stratiform region. The shattered fragments can increase the particle number concentration by 4.56 times in the stratiform area while in the convective area the value is 8.47 time, nearly twice times that of the stratus. The shattered artifacts can also overestimate the ice water content in the convective region more than 30% than that in the stratus.
Available online:August 14, 2020 DOI: 10.3878/j.issn.1006-9895.2007.20149
Abstract:This study first uses the volumetric soil water content data from 732 stations in China provided by the National Meteorological Information Center (NMIC) at the China Meteorological Administration (CMA) to evaluate the simulations of the Community Land Model version 4.5 (CLM4.5) driven by CFSR near surface meteorological data (referred as to CLM4.5-CFSR). Then we use CLM4.5-CFSR to investigate the spatialtemporal characteristics of soil moisture memory during 1980-2009 in China. The soil moisture memory is calculated by both Pearson correlation method and autocorrelation method. The effects of precipitation frequency, precipitation intensity and near-surface temperature on soil moisture memory are then explored. The results show that CLM4.5-CFSR can reflect the soil moisture changes on the monthly time scale in China. The spatial distributions of soil moisture memory from two methods are simliar, but their seasonal characteristics are different. The soil moisture memory duration does not vary much with soil depth, but it ranges from 0.85-2.2 months across China with relatively large magnitude in the northeast of the Inner Mongolia, but small in the southwest of Xinjiang Province. In spring, the wetter soil is, the longer soil moisture memory is. When the precipitation frequency is low, it has little effect on soil moisture memory in areas where the evaporation rate is high. When the precipitation intensity is high, soil moisture memory will be shortened by it because it can replenish the soil water rapidly and destroy the initially dry or wet conditions of soil. Changes in the near-surface temperature will shorten the soil moisture memory through its effect on the soil evap-oration. In the future, it is necessary to carry out climate model sensitivity experiments to take insight into the physical mechanism associated with the conclusions obtained in current study, and then to provide a basis for further improving the seasonal and intraseasonal precipitation prediction.
Analysis of the Characteristics and Causes of the Interdecadal Changes of Summer Extreme Precipitation over Eastern China
Available online:August 03, 2020 DOI: 10.3878/j.issn.1006-9895.2007.19247
Abstract:Using daily precipitation data of eastern China, the interdecadal shift of summer extreme precipitation (SEP) are tested, and the characteristics before and after the climate shift are analyzed. Furthermore, the cause analysis is discussed from the perspective of different response to global warming between land and ocean, which leading to circulation adjustment. The result shows that the SEP in eastern China presents obvious interdecadal shift around 1990, which become positive anomaly after the shift. Compared with the SEP before the shift, the distributions of major modes shift to south, the intensity of SEP enhances and the contribution rate to summer precipitation increases in both South China and East China, while in North China all of the above factors are with opposite change. The change of temperature difference between land and ocean caused by strongly positive anomaly of sea surface temperature (SST) in western Pacific warm pool is one of the important driven factors to this shift, leading to interdecadal adjustment of East Asia summer monsoon system. In low-middle latitude, the intensity of summer monsoon weakens, West Pacific subtropical high (WPSH) strengthens and moves to south, and South China Sea high also enhances. While in middle-high latitude, the cyclonic anomaly is broken and the East Asia Trough strengthens. Under the influence of circulation adjustment, the water vapor decreases (increases) and vertical motion weakens (enhances) in North China (South China and East China). Thus, the SEP in North China decreases, and increases in South China and East China.
Available online:July 31, 2020 DOI: 10.3878/j.issn.1006-9895.2005.19198
Abstract:A broken line convective system, appeared in the northwest of North China, moved to southeast and collided with multi-cells in the plain of Beijing, and eventually formed an organizational strong squall line. It caused local flash rain, gust and big hail over Beijing on Aug. 7, 2015. Based on multiple sources data, the analyzes indicated that, (1) the squall line formation process had three stages: The developing and moving of broken line convective system in the upstream, the rebirth and consolidation of multiple cells in the plain and the organization of the squall line after the upstream broken line convective system crossed over mountains and merged into multi-cells in plain. During the second stage, the local convection was triggered near the urban city by the inhomogeneous temperature distribution together with local convergence；Corresponding with the cold pool, the reborn convection propagated toward south because of the enlarging scale of the inhomogeneous temperature and the southward intensified temperature gradient; (2) At the squall-line development stage, dynamic structure is characterized by two strong inflows. The mid-tropospheric rear inflow was at 4500 to 5000m height, another was the low-level strong inflow perpendicular to squall-line orientation. The two inflows induced two separated vertical clockwise circles at the former and behind the squall line. The vertical circulation in the front of squall line was intensified continuously since the rear and front inflows enhanced together, corresponding with strengthening vertical wind shear. This dynamical process induced the advantageous ambient mesoscale vertical shear for the squall-line organization, which also was a significant factor for the squall-line rapid movement and development. After the rear inflow disappeared, the frontal vertical circulation weakened and squall line dispersed gradually; (3) for the thermodynamic structure, a stronger cold pool appeared with the disturbance temperature -8℃, depth 1.5km, when the upstream convection system merged into the multi-cells system in the plain. As a result, upward motion was strengthened in the leading edge of the β-mesoscale temperature gradient. It was favor to squall line intensifying and developing and induce gust front.
A Numerical Study on Cloud Structure and Precipitation Mechanism of Low-Trough Low-Vortex Weather Process over the Liupan Mountain Area
Available online:July 28, 2020 DOI: 10.3878/j.issn.1006-9895.2005.19225
Abstract:Liupan Mountain area is water conservation forest base in the northwestern of China, but the drought and lack of rain restricted the agricultural and economic development of this region. Using the Weather Research and Forecasting (WRF) model, a precipitation process occurred in Liupan Mountain area in southern Ningxia on August 21, 2018 was simulated as the basis of further research on artificial precipitation enhancement technology in this area. Based on observational data, the favorable circulation situation was analyzed, the microphysical structure and precipitation formation mechanism in the precipitation cloud system were discussed. The results show that the weather system of this precipitation process developed in the dynamic field of aloft trough associated with low vortex, and the low vortex was at the slower moving compared with the aloft trough because of the blocking effect of the Liupan Mountain terrain. The vertical structure of cloud showed a remarkable “seeding-feeding” stratified structure，but the vertical microstructure was different in different parts of the cloud system，resulting in the difference of contribution of cold and warm cloud processes to precipitation. Besides, the precipitation on the east windward side of Liupan Mountain was stronger than that in the west. Rain water was produced mainly by melting of graupel and collection of cloud water by rain. The accretion of supercooled rain was the main process of graupel growth. Layer of cloud water on the windward slope was deep with high water content. It promoted the process of accretion of rain by graupel in the supercooled layer, and provided abundant cloud water for the process of coalescence growth of raindrops, which enhanced both cold and warm cloud precipitation process. The terrain had an impact on the development of clouds and the formation of precipitation, where the cloud water decreased the warm cloud process weakened with the terrain height lowering, which also affected the growth process of the graupel.
Study on the stable components of atmospheric circulation during the continuous heavy rainfall of Meiyu in 2016
Available online:July 09, 2020 DOI: 10.3878/j.issn.1006-9895.2006.19167
Abstract:Meiyu precipitation has significant stage changes. It is important to study the key circulation stable components that lead to continuous heavy rainfall for the analysis and prediction of Meiyu precipitation. Using NCEP/DOE Reanalysis II data from 1979 to 2016, the potential height field, wind field and relative humidity field during the Meiyu period in 2016 were analyzed, the key stable components of the circulation system were extracted, the spatial structure, evolution characteristics and the background situation of longer time scale were analyzed, which provided the basis for the extended period forecast of the periodic heavy rainfall in Meiyu area Reference resources. The results suggest that: (1) The distribution of "three poles" in the stable components of the geopotential height field is the key system to maintain the continuous heavy rainfall in Meiyu area in 2016. The "three poles" correspond to the Ural Mountains blocking high, the Okhotsk Sea blocking high, the Southeast Asia high and the West subtropical high. This configuration structure is conducive to the convergence of warm and cold air in Meiyu area to form persistent heavy rainfall. (2) In the middle and high latitudes, there is a relatively deep negative anomaly between the two deep positive anomaly areas. On the one hand, it is conducive to the South transportation of cold air. On the other hand, the stable component of relative humidity field indicates that the water vapor transportation and convergence of the northern branch of the air flow play an important role in the continuous precipitation of Meiyu area. (3) The positive anomaly area in the middle and low latitudes is zonal in the east-west direction, and mainly exists in the middle and high troposphere, but there is a positive area on the East-West side of the Meiyu area, which continues down to the lower level, and they together strengthen the convergence and transportation of warm and humid air flow in the south of the Meiyu area. (4) Through the analysis of the evolution of the stable component, it is shown that the establishment and evolution of the "three pole" system lead to the precipitation in Meiyu area showing different stage characteristics.(5) The longer time scale (60 days) circulation stable components provides an important circulation background for the key stable components of the "three pole" in the period of continuous heavy rainfall.
The comparative analysis of the moisture transport anomalies in two interdecadal changes of summer precipitation in South China
Available online:June 18, 2020 DOI: 10.3878/j.issn.1006-9895.2005.19231
Abstract:Based on the daily rainfall data at 2374 stations from the China Meteorological Administration (CMA) and reanalysis data provided by the National Centers for Environmental Prediction/ National Center for Atmospheric Research (NCEP/NCAR) during 1979-2017, the characteristics of moisture transport of precipitation increasing during 1993-2002 and decreasing during 2003-2012 over South China are compared by using the modern statistics method and the physical diagnosis method. Then, the impacts of the East Asian summer monsoon circulation systems and the sea surface temperature anomalies in the key areas on these characteristics are discussed. Finally, the reason of the two decadal changes of summer precipitation in South China is explained by using moisture transport budget equation. Results show that the increase of summer precipitation during 1993-2002 in south China was mainly influenced by the eastward movement of South Asia high and westward extension of West Pacific subtropical high (WPSH), which resulted in a strengthening of the moisture transport from the Philippines and the southwest side of the WPSH and a strong convergence of moisture in the lower layer of south China .The decrease of summer precipitation during 2003-2012 in south China was mainly influenced by the westward movement of South Asia high and eastward extension of WPSH, resulting in a weakening of the moisture transport from the Philippines and the southwest side of the WPSH and a divergence of moisture in the lower layer of south China. The interdecadal variation of summer precipitation in south China is mainly related to the anomaly of dynamic divergence of moisture transport caused by the change of wind speed, it is also affected by the anomaly of thermodynamic divergence of moisture transport caused by the change of specific humidity and the anomaly of subseasonal-scale eddies caused by typhoon and other synoptic scale eddies. In addition, we found that the moisture transport anomalies were closely related to the circulation and SST anomalies.
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.
2008,32(4):653-690, DOI: 10.3878/j.issn.1006-9895.2008.04.01
2002,26(6):721-730, DOI: 10.3878/j.issn.1006-9895.2002.06.01
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
2004,28(6):979-992, DOI: 10.3878/j.issn.1006-9895.2004.06.15
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.
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
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
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.
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.
Interdecadal Variation of Summer Precipitation Pattern over Eastern China and Its Relationship with the North Pacific SST
2009,33(4):835-846, DOI: 10.3878/j.issn.1006-9895.2009.04.16
Abstract:By use of monthly precipitation at 160 stations over China, ERA-40 reanalysis data and NOAA ERSST extend-reconstructed global sea surface temperature data, the authors find that the precipitation pattern over eastern China experiences two interdecadal variation processes in recent 50 years, and has close relation to interdecadal variation of the North Pacific SST. In the interdecadal variation process of the mid- and late-1970s, the precipitation pattern form north to south over eastern China changes from “＋－＋” to “－＋－” tripole distributions, and has close relation to the PDO (Pacific decadal oscillation) phase, with SST anomaly from positive to negative in the mid-latitude area of the North Pacific. East Asian summer monsoon circulation is a main bridge of PDO influencing the precipitation pattern. In the interdecadal variation process of the late 1980s and early 1990s, the precipitation pattern changes from “－＋－” tripole to “－＋” dipole distributions, and has close relation to higher SST from the northwestern Pacific to the south of Japan and round the Philippines. The first interdecadal variation shows that the same mode changes from one phase to another, and the second one exhibits the changes between different modes.
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