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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:November 10, 2020 DOI: 10.3878/j.issn.1006-9895.2010.19237
Abstract:Taking the forecast bias at short lead time and latitude forecast at target lead time (referring to the lead time required to be corrected) of typhoon track numerical forecast as the predictors, the bias prediction equation of typhoon track forecast is established by multiple linear regression method, and then the typhoon track forecast can be corrected in real time. In this paper, taking 12 h as short lead time, and through the application of typhoon track forecast from ECMWF’s (European Center for Medium-Range Weather Forecasts) deterministic prediction system (ECMWF-IFS) and ensemble prediction system (ECMWF-EPS), the following conclusions are obtained. The results of experimental forecasting in 2018 show that the mean track error of corrected typhoon tracks forecasted by ECMWF-IFS at 24, 36, 48, 60, 72 and 84 h is reduced by 7.3, 9.3, 8.9, 6.5, 6.9 and 2.6 km respectively than that of uncorrected typhoon tracks. Generally speaking, the correction effect of strong typhoon track forecast is better, while strong typhoon refers to the typhoon intensity observation at 12 h ≥32.7 m/s. Firstly, the typhoon track forecast of each ensemble forecast member from ECMWF-EPS is corrected, and then the integrated forecast is carried out. The typhoon track forecasts obtained by the following five methods are compared: “corrected deterministic prediction”, “ensemble mean of all ensemble forecast members”, “ensemble mean of selective ensemble forecast members”, “ensemble mean of all corrected ensemble forecast members”, “ensemble mean of corrected selective ensemble forecast members”. The results of experimental forecasting in 2018 show that for the mean track error, “ensemble mean of corrected selective ensemble forecast members” is the smallest at 24 h and 36 h, and “ensemble mean of all corrected ensemble forecast members” is the smallest at 48 h and 60 h, and “ensemble mean of selective ensemble forecast members” is the smallest at 72 h and 84 h. If it is applied in a targeted manner in business, it is expected to obtain an objective comprehensive forecast result of typhoon track with excellent performance in each lead time. At 24, 36, 48, 72 h, the mean track error of “ensemble mean of corrected selective ensemble forecast members” is reduced by 13.3, 11.7, 10.0 and 7.6 km respectively than that of “ensemble mean of all ensemble forecast members”, and reduced by 0.7、2.0、3.9、2.4 km respectively than that of Central Meteorological Office official track forecast (the corresponding lead time is 12 h, 24 h, 36 h, 48 h).
Available online:November 10, 2020 DOI: 10.3878/j.issn.1006-9895.2010.20107
Abstract:A comparison of time domain space and distribution characteristics of different scales for Gaussian function (gauss), second-order auto-regressive function (soar) and superposition of Gaussian components (supergauss) is made in the perspective of time domain and frequency domain space, and correlation function of three types on are applied in GRAPES-3DVar. Researches show that Gaussian correlation function attributes to the insufficiency of small-scale information in the analysis, and larger negative correlation information opposite to the wind field observation. The soar correlation function can increase the middle and small-scale information of the analysis, but with the non-isotropic analysis increment. The application of Supergauss correlation function can not only mitigate the inappropriate negative analysis increment of wind observation, but also increase the middle and small-scale power spectrum in analysis increment. Meanwhile, the analysis increment structure of isotropy with Supergauss correlation function can be obtained through the recursive filter implementation.
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.
Evaluation of assimilation application of yunhai-2 occultation data in Regional Numerical Prediction
Available online:October 23, 2020 DOI: 10.3878/j.issn.1006-9895.2009.20139
Abstract:Based on the WRF model and GSI three-dimensional variational assimilation system, regional assimilation and prediction experiments for China’s autonomic yunhai-2 occultation data were carried out for the first time in May 2019, the experiments’ result demonstrated: After assimilating yunhai-2 occultation data, the improvement for wind field and temperature field is mainly reflected in the middle and later stage of the forecast, while the improvement for humidity field runs through the whole forecast period, and the improvement degree of wind field, temperature field and humidity field tends to be consistent with the extension of forecast time, the improvement for wind field and temperature field is mainly reflected in the middle layer of the model, while the improvement for water vapor mixing ratio is mainly reflected in the middle and lower layer of the model, assimilating yunhai-2 occultation data can reasonably adjust the potential height field, humidity field, temperature field and wind field for the model, and then improve the precipitation forecast results.
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.
Analysis on the source of forecast errors for a heavy precipitation in southwest of Guangdong Province
Available online:October 10, 2020 DOI: 10.3878/j.issn.1006-9895.2008.20130
Abstract:Based on WRF model, this study investigates the source of forecast errors for an extreme precipitation in southwest of Guangdong Province occurred during May 16 to May 17, 2015. First, the forecasts by WRF model using NCEP_FNL as initials (hereafter short for WRF_FNL) has been compared with the deterministic forecasts generated by ECMWF. Results showed that ECMWF has higher forecast skills. Thus the model and the initials that used by ECMWF are deemed accurate. Then we generated a new forecast (hereafter short for WRF_EC) that using WRF model but using the initials of ECMWF, and keeping the physical schemes same as the previous WRF simulations. Results showed that WRF_EC has better forecast skills than WRF_FNL. This indicates that the improvement of initial states has a large impact on the forecasts, so the initial error is a main source of the forecast errors. Furthermore, we analyzed the sensitive area and sensitive variables of the initial error. Results showed that the initial errors mainly come from the North Gulf of the South China Sea and the southwest of Guangxi Province, and both the initial temperature error and the initial humidity error played important role in this heavy precipitation forecast, while the error in the wind and pressure have small impacts. Improving the accuracy of the initial temperature and humidity could largely improve the forecast skill of this heavy precipitation.
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.
Numerical Investigation of the Effects of Boundary Layer Parameterization Schemes on Typhoon Meranti (1614) Landing Process
Available online:September 17, 2020 DOI: 10.3878/j.issn.1006-9895.2004.19135
Abstract:A serious of high-resolution (1.33 km) numerical tests have been done by means of seven boundary layer parameterization schemes in mesoscale numerical model WRF v3.8, namely YSU, MYJ, QNSE, ACM2, UW, GBM, and Boulac, in order to study the effects of different boundary layer schemes on the simulation of landing attenuation stage of Typhoon Meranti (1614) in terms of movement track, intensity, structure, rainfall, and near-surface physical variables. The results indicate that: (1) boundary layer schemes influenced significantly simulation of Typhoon Meranti’s track, intensity, and rainfall during its landing attenuation stage, and the maximum differences in 24-h simulated typhoon track, the lowest atmospheric pressure, the maximum wind velocity, and 24-h cumulative rainfall extremum were 80 km, 11 hPa, 27 m s-1, and 241 mm, respectively. (2) simulation by Boulac scheme resulted in a typhoon track closest to real-time result, followed by GBM, YSU, and MYJ schemes, and then by ACM2 and UW schemes, while QNSE scheme had the worst simulation; lowest atmospheric pressure values simulated by UW and QNSE schemes, and values of the maximum wind velocity simulated by schemes MYJ and QNSE are closest to corresponding observations; all boundary layer schemes simulated features of the typhoon that the lowest atmospheric pressure increased gradually during landing stage and rate of such increase after landing was greater than that before landing, which agreed with real-time results, but increasing rate of the lowest atmospheric pressure before typhoon landing simulated by each scheme was greater than the real-time result, while such increasing rate after typhoon landing simulated by each scheme was less than the real-time result. (3) Boulac scheme is the best in the simulation of 24-h precipitation distribution, heavy precipitation area, structure, intensity, and TS score of precipitation at each level, and MYJ scheme is the second best; as simulated by QNSE, UW and ACM2 schemes, the rain belt advanced so quickly northwestward that TS scores of precipitation at various levels were poor. (4) in overall simulation of track, intensity and precipitation of the typhoon, Boulac and MYJ schemes were optimal, among which, Boulac scheme was superior in simulation of typhoon track and precipitation while MYJ scheme was superior in simulation of typhoon intensity; YSU and GBM scheme had the second best simulation results, whereas QNSE, UW and ACM2 schemes had worse simulation performance. (5) boundary layer schemes differed significantly in calculated latent heat flux and sensible heat flux of near-surface layer, thereby simulation of typhoon track, intensity and rainfall was affected, leading to significantly different simulation results. In comparison, QNSE scheme resulted in an abnormally high latent heat flux, MYJ and Boulac schemes resulted in the most modest values, and other schemes resulted in slightly smaller values; QNSE scheme had a slightly higher sensible heat flux, MYJ scheme had the most modest one, and other schemes resulted in significantly smaller values. (6) boundary layer schemes differed significantly in simulated thermal and dynamical structure of boundary layer and Boulac scheme had the obvious advantages, especially for the structure of boundary layer in daytime.
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.
Available online:August 31, 2020 DOI: 10.3878/j.issn.1006-9895.2008.20118
Abstract:Abstact: The hybrid ensemble-three-dimensional variational data assimilation (En-3DVAR) was constructed based on Rapid-refresh Multi-scale Analysis and Prediction System-Short Term(RMAPS-ST) and RMAPS-EN(-Ensemble) in Beijing Meteorological Bureau.The Hybrid and 3DVAR schemes were conducted in the cold run and cycle run settings based on RMAPS-ST. We get the following conclusions: singleobservation test showed that there is a good correspondence distribution between the hybrid DA(Data analysis)-increment and ensemble spread; the hybrid DA-increment have flow-dependent characteristics in both cold and cycle run but the 3DVAR DA-increment is isotropic; A rainfall case study revealed that Hybrid and 3DVAR have the almost same influence on the precipitation in cold run,but Hybrid scores are significantly improved compared to 3DVAR in the cycle run; the analysis from the ensemble spread and background-error show that they have better correlation in cycle run than the cold run.
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.
A study on the extended-range forecast for the low frequency oscillation of temperature and low temperature weather over the lower reaches of Yangtze river valley in winter
Available online:August 04, 2020 DOI: 10.3878/j.issn.1006-9895.2007.19208
Abstract:Based on the observational data, the variations of intraseasonal oscillation (ISO) of the daily temperatures and its relationships to the low temperature in December－February over the lower reaches of the Yangtze River valley (LYRV) were studied for the period of 1979/1980—2017/2018. It is found that the daily temperatures over the LYRV in December－February is mainly of periodic oscillations of 15－25, 25－40 and 50－70 days, and the interannual variation of the intensity of its 25－40-day oscillation has a strongly positive correlation with the number of low temperature days in December－February. A real-time low frequency components of daily temperature in the LYRV, and the principal components of the Eastern Asian 850 hPa low frequency temperature, over a time period ranging from 2001 to 2018, are used to establish the time-varying extended complex autoregressive model (ECAR) on an extended-range forecast of the 25－40-day low frequency temperature over the LYRV in winter. Using the real-time SSA filtering with the T-EOF extension, it can effectively inhibit the end effects of the traditional SSA and make a better real-time signal of ISO. A 17-year independent real-time extended-range forecast was conducted on the extended-range forecast of low frequency component of the temperature over the LYRV in December－February, for the period ranging from 2001/2002 to 2017/2018. These experimental results show that this ECAR model, which is based on a data-driven model, has a good forecast skill at the lead time of approximately 26 days, with a forecast ability superior to the traditional autoregressive (AR) model. Hence, the development and variation of the leading 25－40-day modes for the Eastern Asian 850 hPa low frequency temperatures and temporal evolutions of their relationships to low frequency components of the temperature over the LYRV in winter supplied the valuable predicting background in determination of extended-range weather process in the persistent low temperature over the LYRV at the 3－4-week leads.
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.
Spatial Observation of the Red Sprites over a Winter Mesoscale Convective System in North America and the Analysis of Its Parent Thunderstorm
Available online:July 28, 2020 DOI: 10.3878/j.issn.1006-9895.2002.19169
Abstract:Red sprites are one type of large-scale transient luminous events (TLEs) that usually occur between about 40 ~ 90 km altitude above thunderstorms, which are caused by cloud-to-ground lightning strokes and subsequent continuous current. Compared with sprites in summer, there are fewer studies on the winter sprites in the world due to fewer comprehensive synchronous observation data. Influenced by the upper trough and warm-moist airflow in low level, a thunderstorm took place in in Arkansas, North America on 27 ~ 28 December 2008, the Imager of Sprites and Upper Atmospheric Lightning (ISUAL) aboard the FORMOSAT-2 satellite was lucky to record two red sprite events . Using red sprites optical observation data obtained by ISUAL, Doppler weather radar data, National Lightning Location data, ultra-low frequency magnetic field data, cloud top brightness temperature data provided by the National Environmental Center/Climate Prediction Center of the United States and the sounding data, this paper makes a detailed study of the characteristics of the winter thunderstorm that produced red sprites and related lightning activity.The results show that ISUAL did not recorded the halo that accompanied two red sprites. The first was columnar sprite, and the second was unable to determine its specific morphology because of its dim light. The parent thunderstorm of red sprites is a mesoscale convective system, which appeared around 1500 UTC on 27th near northern Arkansas and moved from west to east. The thunderstorm developed stronger at about 2359 UTC, and the area of maximum radar reflectivity (55 ~ 60 dBZ) reaches 339 km2, then began to weaken. At 0303 UTC, the intensity of thunderstorm increased, then the cloud gradually spread, and the thunderstorm began to weaken, and completely dissipated at 1100 UTC. Two sprites occurred at 04:46:05 UTC and 04:47:17 UTC, respectively . They tended to be produced in the dissipation stage of the MCS, when the frequency of positive and negative cloud-to-ground lightning is low and POP increases significantly, and they were mostly over the stratiform cloud area with 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 production of the sprite was the decay of the thunderstorm and the area of the stratiform region develops, which is consistent with previous studies on summer sprites. The parent CG of red sprites was positive with a single return stroke, and located in the trailing stratiform region of the MCS, where the radar reflectivity was range 25 from 35 dBZ. The corresponding radar echo top heights were 2.5 km and 5 km, and the peak currents are + 183 kA and + 45 kA, respectively.. Based on ultra-low frequency magnetic field data, the iCMC of two parent lightning is estimated to be + 394 C km and + 117 C km, respectively. Ultra-low frequency magnetic antenna recorded the internal current signal of the first red sprite, indicating that the red sprite discharged strongly.
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.
Available online:July 28, 2020 DOI: 10.3878/j.issn.1006-9895.2005.19214
Abstract:The Weather Research and Forecasting model with a spectral-bin microphysical scheme (WRF-SBM) was used to simulate a hailstorm occurred in summer of Xinjiang. The effects of aerosol concentration on microphysical characteristics and precipitation of the hailstorm as well as formation mechanism of hail are studied by sensitivity tests. The results show that the convection of the hailstorm is stronger with larger aerosol concentration. At the development stage of the hailstorm, the liquid water content increases with the increase of aerosol concentration and the ice water content is the highest under moderate polluted condition. The hail mixing ratio increases first and then decreases with the increase of aerosol concentration. Under moderate polluted condition, there is appropriate cloud droplet size and relatively sufficient supercooled water, which is favor for the transformation of water from liquid phase to ice phase and therefore contributing to hail growth. Hail is initially formed by the riming of supercooled liquid water by ice crystals, but this process is rapidly weakened after the development of hailstorm. Then the freezing of droplets becomes the main source of hail for a short while. However, once the hail is formed, it will grow rapidly by collecting the supercooled water, which becomes the dominant process of the hail growth. The severe polluted condition will postpone the onset of hail formation processes. With enhanced aerosol loading, the surface accumulated liquid precipitation is increased while the ice phase precipitation is increased first and then reduced. The moderate aerosol concentration leads to larger amount of hail mixing ratio and higher percentage of hail in ice-phase precipitation. However, with further increased aerosol concentration, both values are reduced. Therefore, we propose “the optimal aerosol concentration” that is most suitable for hail growth.
Available online:July 28, 2020 DOI: 10.3878/j.issn.1006-9895.2005.19209
Abstract:Stratiform cloud system is important object for the exploitation and utilization of cloud water resources. Precipitation enhancement requires scientific and feasible operational technical indicators to guide the implementation of actual operations, and reasonable and accurate assessment of operational effects is an important issue that need to be solved. It is a necessary way to establish and improve the operation technology to simulate the actual seeding operation process reasonably by numerical model, and then study changes and mechanism of a series of macro and micro characteristics of cloud and precipitation after seeding operation. It is also an effective method to evaluate the effect of practical precipitation enhancement operation. An aircraft seeding operation during the stratiform cloud precipitation in Hebei province on April 15, 2014 was emulationally simulated by a 3D mesoscale cold cloud seeding model. The actual operation process was reasonably simulated by the numerical model. The diffusion and transmission characteristics in atmosphere of AgI particles seeded by aircraft was studied, and seeding influence on the macro and micro characteristics of cloud and precipitation was analyzed, and precipitation enhancement effect of the aircraft seeding operation was evaluated. The results show that the horizontal scale of AgI plume can extend more than tens of kilometer, and most of AgI particles in the vertical direction are concentrated within the range of about 1km above and below the seeding layer. The upward transport of AgI particles is significantly stronger than the downward transport. The outstanding increase of ice crystals and snow particles in the cloud after seeding operation lead to the inhibition of graupel growth in the early simulation stage. However, the enhancement of graupel collection snow process and ice phase particles riming processes near zero layer gradually increase total mass of graupel after some time. After the aircraft seeding operation, the radar reflectivity has obvious enhancement and show different structure characteristics with time. The precipitation decreases first and then increase with time as a result of seeding. Three hours after seeding, the operation influence area extended more than 100km to the downstream of the operation area, and showing the distribution characteristics of rainfall-reducing area first and then rainfall-increasing area in general. The model evaluation indicates that the net rainfall increased by 3.6×107kg in the entire evaluation area, with an average rainfall-increasing rate of 1.1%, and increase of graupel particles concentration in warm layer is the main reason for rainfall increase. Due to the weak seeding operating conditions in the target cloud area, AgI seeding amount of this operation is relatively high, resulting in little effect of precipitation enhancement.
Numerical Simulation of Precipitation Processes during the Opening Ceremony of the Nanjing 2014 Youth Olympic Games
Available online:July 28, 2020 DOI: 10.3878/j.issn.1006-9895.2002.19200
Abstract:In order to evaluate the effect of cloud seeding operation for rain suppression during the opening ceremony of the Nanjing 2014 Youth Olympics Games, the WRF model was used to simulate the precipitation processes and cloud seeding operation. The present work is the first part of the study. Firstly, the effects of precipitation simulation using eight microphysics schemes were evaluated through comparison with the observation. Furthermore, we chose Thompson and Milbrandt-Yau microphysics schemes to analyze the cloud structures and precipitation formation mechanisms. Results show that the cloud structures and precipitation mechanisms simulated by the two microphysics schemes are consistent. The precipitation affecting the Nanjing Olympic Sports Center on the day of the opening ceremony is generated by the weak mixed convective-stratiform cloud system, and the precipitation processes are dominated by ice-phased microphysical processes. The melting of snow is the main source of rainwater, contributing 72% in Thompson microphysics scheme and 60% in Milbrandt-Yau microphysics scheme, and the evaporation is the main sink term, which consumes 94% of the rainwater in Thompson microphysics scheme and 95.6% in Milbrandt-Yau microphysics scheme.
Available online:July 28, 2020 DOI: 10.3878/j.issn.1006-9895.2005.19196
Abstract:In this study, we analyzed the intraseasonal variation of the western Pacific subtropical high (WPSH) in El Ni?o decaying summers and found two distinct modes, i.e., the west mode and the east mode. The west mode exhibits a consistent westward extension from June to August. The east mode shows a westward extension during June and July, but shifts to an eastward retreat in August. The anomaly in the west mode is much larger than that in the east mode. For the west mode, due to the forcing from a positive sea surface temperature (SST) anomaly in the tropical North Atlantic (TNA), there is a teleconnection pattern from the TNA via high latitude of Eurasia to East Asia. As a result, geopotential high in East Asia tends to increase associated with suppressed convection in the warm pool region, inducing the westward extension of the WPSH with strong intensity. The SST anomaly in the tropical Indian Ocean for the east mode is similar to that for the west mode but with a relatively smaller magnitude. Besides, the positive SST anomaly in the TNA reaches a maximum in April and tends to decay afterwards. Therefore, the warm pool convection in August cannot be suppressed sufficiently by the SST forcing over the two oceans. On the other hand, the warm pool convection begins to develop due to the local air-sea interaction associated with the SST rise from June to July. In this case, the WPSH tends to retreat eastward with reduced geopotential height. The result indicates that the SST anomaly in the TNA plays a major role in the WPSH anomaly in August. When predicting the WPSH in El Ni?o decaying summer, we must consider the influence of the SST anomaly over the two oceans.
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.
Construction of a new dataset based on multi-source land surface flux data and its application in the boundary area of EASM
Available online:June 16, 2020 DOI: 10.3878/j.issn.1006-9895.2005.19192
Abstract:The boundary area of the East Asian summer monsoon (EASM) is a hot spot area with intense interaction between land surface process and regional climate. However, the lack of high-quality long-term flux data sets available for this region limits the study of the interaction between land surface water and heat exchange and climate. It is necessary to reconstruct a new dataset based on currently available multiple flux data and then apply it in climate research. In this study, the data sets of land surface energy fluxes in the boundary area of EASM in China were reconstructed by integrating the field observations conducted over Northern China and several gridded datasets. Based on selection of sites with good underlying surface representative and investigation of the scattering distribution of simulations and observations, a set of monthly average sensible heat, latent heat and net radiation data sets are generated by using multiple regression model. The cross validation results show that the accuracy of the constructed data set is improved compared with several original gridded data sets, and the systematic deviation of the original lattice data is eliminated to the greatest extent. Further analysis suggests that among the surface energy balance components, the response of land surface turbulent flux to summer monsoon is more significant, and the interannual variation of land surface turbulent heat flux in the boundary area of EASM shows logarithmic distribution to the duration of summer monsoon. The turbulent heat fluxes present more significant interannual variations as the summer monsoon is in a low persistent state. Weaker summer monsoon system may lead to a stronger impact of land surface processes on climate change. The new data set based on multi-source flux data fusion can provide supports for climate change research, and increase the understanding of the interaction between land surface processes and monsoon climate.
The Variation of Asian Westerly Jet Asymmetry and Its Impacts on East Asian Climate in Boreal Summer
Available online:June 16, 2020 DOI: 10.3878/j.issn.1006-9895.2005.19232
Abstract:Based on the monthly mean rainfall data of CMAP and reanalysis of NCEP/DOE ⅡReanalysis form NOAA and the monthly precipitation and average temperature data from NMC，by defining an index (IAja) that is defined to describe the zonal asymmetric variation of Asian Westerly jet in upper troposphere, we have investigated the characteristics of the intensity difference between the east and west part of the Asian westerly jet and its climate impacts on East Asia from 1979 to 2019. The main conclusions are as follows: there are prominent inter-annual variations of the zonal asymmetry of Asian summer westerly jet, existing significant quasi-period of 6~8 years and 2 years. When the zonal asymmetry of Asian summer westerly jet is typically strong (weak), the wave-like anomalous rainfall pattern signed with the positive (negative)—negative (positive)—positive (negative) signs is observed form lower to higher latitudes in East Asia sector along with negative (positive) anomalies of temperature in the area of Lake Baikal，and simultaneously significant positive (negative) anomalies in regions including the Western area of China and Northern parts of Japan. The divergent and convergent wind components by the anomalous diabatic heating as a potential vorticity source directly induce the anomalies circulation in the mid-latitudes. The anomalous anticyclonic circulation making the intensity of west Asian jet increased and the eastern segment decreased help to the zonal asymmetry of Asian summer westerly jet stronger. The formation and facilitating of zonal asymmetric anomalies of the Asian jet are affected by not only convergence and divergence in the tropics and mid-latitudes, but also by wave energy propagating eastwards in westerlies. The eastward propagation of wave energy may be related to the SST anomaly of the Northern Atlantic. These results are helpful for us to better understand the formation mechanisms of the zonal asymmetry of Asian summer westerly jet.
Evaluation of different Planetary Boundary Layer Schemes in Simulating Precipitation Caused by Southwest China Vortex in Sichuan basin based on the WRF Model
Available online:June 11, 2020 DOI: 10.3878/j.issn.1006-9895.2005.19171
Abstract:Five Planetary Boundary Layer(PBL) parameterization schemes [Yonsei University(YSU), Mellor–Yamada–Janjic (MYJ), Mellor–Yamada–Nakanishi–Niino Level 2.5 (MYNN2), Shin-Hong(SH) and the Asymmetric Convective Model, version 2 (ACM2)] in Weather Research and Forecast model (WRFv4.0) are used to simulate well-developed Southwest China Vortex(SWCV) processes that caused rainstorms in the eastern Sichuan basin in 2016.Each level of precipitation prediction are verified, and the L-band radiosonde data, which has the temporal resolution by 1 second, are used to reveal the fine structure of PBL in the midday, the difference between observation and simulation are assessed, and the reason are discussed based on the characteristics of turbulence algorithm in each scheme. Finally, a parameter of turbulence intensity are adjusted for the ACM2 scheme in order to improve the structure of PBL and precipitation in Sichuan basin. The results show that: The ACM2 and YSU schemes have relatively better Threat Score(TS) performance. Compared with other schemes, ACM2 has less false alarms. The attribute of ACM2, which can switch local or non-local algorithms according to the stability of the surrounding environment, seems to be more suitable for Sichuan basin precipitation simulation. However, all PBL schemes show a high false alarm rate in prediction of the Southwest China Vortex precipitation, especially in heavier precipitation. The fine sounding data further shows that all the PBL schemes predict higher Planetary Boundary Layer Height(PBLH) than the observations, which means the simulation has stronger mixing intensity compare to the real atmosphere. Through the parameter adjustment, the ACM2 scheme with reduced mixing intensity, the potential temperature and humidity structure in PBL is more in line with the observation. Then, the potential temperature of the lower PBL is lower, the humidity is higher, and the false alarm report of heavier precipitation is reduced, so that the precipitation simulation in Sichuan basin get some improvement. The different character of the PBL schemes in simulate the Southwest China vortex is mainly lead to different position of the vortex and the precipitation intensity, but essentially, it is derived from the local or non-local attribute and the intensity of vertical mixing. The selection based on the features of research object is the key to accurate simulation of the PBL structure.
Evaluation of the Simulated Tropical Cyclones over the Western North Pacific with IAP AGCM4.1 Based on K-means Method
Available online:June 05, 2020 DOI: 10.3878/j.issn.1006-9895.2002.19252
Abstract:As the atmospheric component of the CAS-ESM1 (The Chinese Academy of Sciences Earth System Model, version 1), IAP-AGCM4.1 (The Institute of Atmospheric Physics Atmospheric General Circulation Model, version 4.1) is developed independently by the Institute of Atmospheric Physics. The TECA (Toolkit for Extreme Climate Analysis) is used to identify and evaluate the simulated tropical cyclones (TC) over the western north Pacific with IAP AGCM4.1 during 1979 to 2012. The results show that the IAP AGCM4.1 can reproduce the TCs’ spatial distribution, track, and source reasonably compared to observation, while the model underestimates the number of TC, and only 36% of the observed tropical cyclones over the western north Pacific are simulated. Further analysis based on K-means clustering method shows that the underestimation is mostly due to inability to reproduce the northwestward-turning and westward TC. For the TCs with westward-northwestward, westward-turning and eastward-turning tracks, simulated numbers are about 39%, 48% and 85% of the observed ones, respectively. Moreover, correlation coefficients of seasonal variation between simulation and observation can reach up to 0.91 and duration biases are about 1~2 days. IAP AGCM4.1 performs well in simulating the tracks of the westward-northwestward and eastward-turning TC, with the relative biases ranging between 1%-5% for longitude of centroid, 4%-16% for latitude of centroid, and 5%-15% for latitudinal and meridional standard deviations. In addition, IAP AGCM4.1 reproduces the evolution of environmental circulation and subtropical high quite well during the lifetime of eastward-turning TC, as simulated strength and area index of subtropical high are highly correlated with observation (the correlation coefficient is 0.89). The poor simulations of northwestward-turning and westward TC are likely due to simulated biases of subtropical high.
Available online:June 05, 2020 DOI: 10.3878/j.issn.1006-9895.2004.19229
Abstract:Urbanization has an important influence on the frequency and intensity of heat waves, but the mechanism of urbanization affecting the process of high temperature is not fully understood. The Weather Research and Forecasting (WRF) model is used to simulate a summer high-temperature process in Beijing from 2 to 6 July 2010. This paper reports the main results of urbanization effect on surface air temperature of urban areas in the heat wave process. It is found that the optimized WRF model is able to simulate the temporal characteristics of the 5 consecutive days of high temperature and the variation of IUHI (urban heat island intensity) in Beijing. The impermeability of urban underlying surface determines that the 2m relative humidity of urban area is lower than that of rural area, which weakens the ability of urban area to regulate surface air temperature through latent heat. After sunset, the urban sensible heat flux decreases slowly, and cooling rate of urban area is less than that of rural area. At night, the structure of boundary layer is stable and the height of it is low, and the wind speed is small. In this case, the energy transmission between urban and rural areas is restrained, forming a strong urban heat island at night. After sunrise, sensible heat flux and latent heat flux of urban and rural land surface rise rapidly, and the stability of boundary layer decreases. In the afternoon, the urban underlying surface is in favor of the high and low value centers of sensible heat flux and latent heat flux respectively, with the ability to regulate temperature through latent heat weakened. It’s conducive to the energy vertical transfer that the stability of the boundary layer is weak. The IUHI in the afternoon is smaller than that in evening. The extreme high temperature occurred on 5 July. An abnormal continental warm high pressure controls most parts of China, and Beijing is in front of the high-pressure ridge. The Fohn effect of the northwesterly flowing over the mountains aggravated the urban high temperature except for the larger IUHI. In this process of heat wave, therefore, the obvious urban heat island effect created by the urban underlying surface in Beijing has increased the strength and severity of the extreme high temperature event.
Airborne observations of the microphysical characteristics of stratiform cloud over the eastern side Taihang Mountain
Available online:May 14, 2020 DOI: 10.3878/j.issn.1006-9895.2004.19202
Abstract:Fine cloud microphysical characteristic is the research foundation of stratiform cloud precipitation mechanism. Based on the observation data obtained from "Rainfall Enhancement and Hail Suppression project at the eastern side of Taihang Mountains”, the stratiform cloud microphysical characteristics affected by the typical westerly trough process on 21 May, 2018 was analyzed. The result show that the super cold water content at the -5℃ layer is less than 0.05 g?m-3, and the concentration magnitude is 101-102 L-1. The needle-like and columnar ice crystals are often observed in the regions with high number concentration of ice crystal, which may be related to the ice crystal fragments which may be produced by Hallett-Mossop mechanism and other mechanism together were formed by deposition under super saturation respect to ice condition. Most of ice crystal is mainly plane and dendrites in the regions with low number concentration of ice crystal. The ice and snow crystal mainly grow through the process of deposition and coalescence, with weak rimming process. The liquid water content is accounted more than 70% in the regions with the peak value of cloud water content near 0℃ layer. The particle are mainly cloud droplet with diameter between 10 to 50 μm, accompanying with a few polymer. The super cold water content is about 0.05 g?m-3 in other regions near the 0℃ layer, and ice crystal habits are predominantly polymer、rimed and graupel type. Most of the particle are spherical droplet and melting ice crystal in the liquid water layer. The vertical detection show that the ice and snow crystal number concentration increased with height above 0℃ layer. The super cold water content of the mixed layer is much lower in the stable stratiform cloud. Most of particle are mainly grew through the deposition and coalescence, and the degree of ice crystallization is much higher. The existence of a lot of liquid droplets indicates that the transformation between liquid phrase and ice phrase is not sufficient in the strongly developed stratiform cloud regions in where supercooled liquid water is relatively abundant. The particle size distribution at different temperature levels indicate that the average number concentration of ice particle in the regions poor in super cold water content is higher than that of rich in super cold water content, however average diameter is on the contrary.
Possible causes of persistently extreme hot days related circulation anomalies in Yunnan during April to June in 2019
Available online:May 14, 2020 DOI: 10.3878/j.issn.1006-9895.2004.19226
Abstract:Persistently extreme hot days broke the historical length record, which attacked Yunnan province and induced the local serve drought during April to June in 2019. Based on daily in-suit surface air temperature in China, JRA-55 reanalysis datasets and outputs of the Community Earth System Model Large Ensemble (CESM-LE) Project, we investigated the general circulation regime related to the hot weathers in Yunnan, and discuss the possible causes of the persisting hot days in 2019. Our results show the lower- and upper-level strong anticyclonic circulation anomalies directly results in the local hot weather, it induces adiabatic warming by descending motion and enhances the solar radiation heating at surface. The anti-cyclonic anomaly is one node of high and mid-latitude eastward propagating wave trains, originated from North Atlantic. The 2019 April-June mean temperature and associated anticyclonic anomalies was the highest since 1961. The anthropogenic contribution to the magnitude and occurrence probability of the temperature anomaly in 2019 is approximately 37.51% and 56.32%, with an important impact of internal variability. The negative phase of Arctic Oscillation (AO) and warm phase of El Nino and Southern Oscillation (ENSO) are important external forcing factors responsible for the persistent anticyclonic anomalies over Yunnan during April to June in 2019. The negative phase of AO causes the southward shift of the positive geopotential height anomalies over Arctic at the longitudes of Eastern Europe, favors the high-latitude wave train and anti-cyclonic anomaly over Yunnan. The warm phase of ENSO enhances the western North Pacific anticyclone and favors its westward shift, leading to the persistence of the anticyclonic anomalous circulation over Yunnan. The negative phase of AO and warm phase of ENSO jointly reinforce the intensity, duration of the anticyclonic anomalous circulation over Yunnan, and result in frequent and persistent occurrences of the record-breaking extremely high temperatures and serve drought event in 2019 under recent global warming.
Wind Characteristics Analysis of Wind Profiler Radar and Its Quality Control Method for Data Assimilation
Available online:May 09, 2020 DOI: 10.3878/j.issn.1006-9895.2004.19216
Abstract:With 0.5°×0.5° analysis fields of National Centers for Environmental Prediction/Global Forecast System (NCEP/GFS) as numerical forecast background and using surface precipitation data, quality characteristics of wind products from twelve L-band wind profiler radars in Fujian Province, including three CFL-03 radars and nine CFL-06 radars, were analyzed firstly at 00, 06, 12, and 18 UTC in a day from January to December 2017 aiming at data assimilation. Then different quality control (QC) schemes and their different effects were preliminary discussed. The results indicate that: (1) Winds detected by CFL-06 radars are obviously better than those from CFL-03 radars in the aspects of maximum detection height, effective data availability and horizontal wind quality in low levels. (2) Great differences exist in horizontal winds detected by different wind profiler radars with same model regarding its data availability, effective detection height, and vertical distribution of standard deviation, correlation coefficient and bias. These differences have no direct relationship with geographical location of wind profiler radars, i.e. coastal area or inland, and height above sea level. (3) The wind profiler radar products have obviously systematic negative bias relative to GFS u-wind field, that is, the u-winds detected by wind profiler radars are lower than GFS background field. This doesn’t meet the no-bias requirement for data assimilation. So bias corrections are necessary in data assimilation. Whereas v winds are relatively better than u wind. (4) Precipitation impacts greatly wind profiler radar detection. In precipitation days, the data availability reduces in middle-low levels but greatly enhances in middle-high levels. The standard deviations of u and v winds both increase in middle-low levels, whereas the standard deviations of v-winds increase and those of u-winds greatly reduce in middle-high levels. (5) Two QC schemes, i.e. different high-confidence range scheme and different effective detection-height scheme, are advanced up for different wind profiler radars to compare with fixed effective detection-height scheme. The results show that the two QC schemes both have obvious advantages. The QC effect of different high-confidence range scheme is much more obvious, with horizontal wind data of different radars more fully and effectively identified. The scheme does not only reduce unnecessary loss of radar data, but also further eliminates poor quality data. It also achieves good results in precipitation condition.
Available online:May 09, 2020 DOI: 10.3878/j.issn.1006-9895.2003.19242
Abstract:The long-term trend of extreme temperature has been extensively explored by previous studies. In contrast, the interdecadal variation of extreme temperature is rarely documented. Based on the daily maximum temperature, minimum temperature and daily temperature of 839 stations in China from 1961 to 2016, the interdecadal variations of winter extreme temperature index in China were analyzed. The first four wave components of the extreme temperature of each station are extracted by using harmonic decomposition, which is regarded as the interdecadal component. A station is regarded as the station that undergos an evident interdecadal variation if the cumulative variance explained by the interdecadal component is greater than 25%. The results show that the stations with evident interdecadal variation of winter extreme low temperature index are mainly located to the north of the Yangtze River, northern Xinjiang and eastern Qinghai-Tibet Plateau. The inter-decadal variation to the north of the Yangtze River and in northern Xinjiang are basically consistent after 1979. Accordingly, the period after 1979 can be divided into three periods: previous-cold period (1979~1986), warm period (1987~2007) and later-cold period (2008~2016). The inter-decadal variation of the extreme temperature indices of the stations over the above-mentioned two areas might be modulated by the interdecadal variation of East Atlantic/West Russia teleconnection type (EAWR), which corresponds to the interdecadal variation of both the frequency of blocking-like circulation over Ural Mountains and the amplitude of planetary trough over East Asia.
Research on skeleton-based objective quantization and identifying algorithm for quasi-linear convective systems
Available online:May 09, 2020 DOI: 10.3878/j.issn.1006-9895.2004.19210
Abstract:In this paper, the concept of skeleton in Computer Graphics is applied to meteorology, such as echo image preprocessing, skeleton pruning and length-width ratio quantization techniques are developed. The quasi-linear convection systems (QLCSs) in radar echo mosaics conform to the meteorological standards can automatically be identified by this method. Firstly, the detailed identifying algorithm is introduced based on a double QLCSs process in Huang-Huai area in 2016. Then, the QLCSs in Anhui Province in June 2016 are objectively identified by this method, and the moving characteristics of QLCSs are quantitatively. The comparison between disastrous weather observation and subjective identification are carried out. The results show that the shape information of meteorological echo is well preserved by using skeleton image identifying algorithm and the effective identification of QLCSs is realized base on accurately quantifying the long and short axes of convection system. On the one hand, quantitative characteristics such as the moving vectors can be applied to the classification of disastrous QLCSs, providing identifying algorithm and quantitative features for long series statistics and mechanism analysis of disastrous weather, and on the other hand, it can provide new technique for monitoring and warning of QLCSs.
Available online:April 27, 2020 DOI: 10.3878/j.issn.1006-9895.2001.19230
Abstract:In this paper, performances of the median-resolution (about 110km) climate system model （BCC-CSM2-MR）and the atmospheric model （BCC-AGCM3-MR） developed in Beijing Climate Center are evaluated by three reanalysis datasets (ERA5, JRA55 and NCEP/NCAR). Results show that high blocking tendency can be distinguished over “North Atlantic-West Europe” and “Central North Pacific”. Winter and spring blocking frequency is almost twice of that in summer and autumn. Blocking frequency in ERA5 is higher than that in JRA55 and NCEP/NCAR, especially over the North Pacific. Model evaluations show that the atmospheric model can well reproduce the main features of Northern Hemisphere blocking frequency, spatial structures and seasonal variations. Main biases are the overestimation over Europe-Asia in winter and spring especially over Ural Mountains, as well as the underestimations over the North Atlantic. The biases are attributed to the climatological biases of geopotential height at 500hPa. General performances of BCC-CSM2-MR are similar to BCC-AGCM3-MR. But winter and spring blocking over Europe-Asia especially over Ural Mountains are improved. Spring blocking and the double-peak blocking structure in summer over the North Pacific are also better reproduced in the coupled model. Therefore, air-sea coupling may help to improve the blocking frequency reproduction over Europe-Asia and North Pacific. The internal variability of climate system has great impact on the variation of blocking frequency, which will also affect model capability in blocking high prediction.
Available online:April 27, 2020 DOI: 10.3878/j.issn.1006-9895.1912.19219
Abstract:Taking a torrential rainfall occurring at Ili Valley of Xinjiang on June 26, 2015 as an example, the circulation background and unstable conditions of this rainstorm process were analyzed by using observation data and high-resolution numerical simulation results of WRF. Some results are drawn as following: (1) The precipitation occurs under the background of synoptic circulation with the “two-ridge and one-trough” pattern over the middle and high latitudes in middle troposphere and “double highs” pattern of the South Asia high in upper troposphere. Under the effect of the particular terrain of the Ili Valley, which is westward opening trumpet-shaped, the Central Asian vortex located in Kazakhstan causes westerly wind in the low layer of the Ili Valley, and the Central Asian vortex located in the Tarim basin causes easterly wind in the middle layer of the Ili Valley. The vertical shear of horizontal wind in the Ili Valley is enhanced by the interaction of two Central Asian vortexes. In the Ili Valley, affected by the topography and the Central Asia vortexes, the low-layer convergence line is formed and coupled with divergence area caused by the upper jet, which enhances the upward motion. The low-layer westerly wind transports water vapor into Ili Valley and the water vapor accumulates in the valley. The enhancement of the upward motion causes the water vapor to be lift in the Ili Valley. (2) The simulation results of WRF can basically reproduce the location, intensity and evolution process of the precipitation during this weather process, and provide data with high spatial and temporal resolution for analyzing the evolution of rainstorm process. The analysis of the simulation results shows that the divergence distribution, water vapor, vertical shear of horizontal wind and thermal stratification distribution over the precipitation area have important contributions to the generation of precipitation. Through the analysis of vertical and horizontal components of moist potential vorticity, it is concluded that the convective instability affected by the thermal stratification influences the generation of precipitation, and the symmetric instability affected by the vertical shear of horizontal wind influences the enhancement and maintenance of precipitation. The analysis of potential divergence further indicates that the convective instability in the lower layer of the whole precipitation area is mainly caused by the vertical shear part of potential divergence, while the divergence part of the potential divergence can strengthen the convective instability in the leeward slope of the small terrain. It indicates that the dynamic and thermodynamic factors are coupled with each other in the whole precipitation evolution process, which affects the precipitation intensity and area.
Characteristics of subseasonal scale zonal movement of subtropical high in summer and its relationship with precipitation in southwest China
Available online:April 27, 2020 DOI: 10.3878/j.issn.1006-9895.2003.19204
Abstract:The subtropical high of northwestern Pacific (abbreviated as WPSH) is a large-scale circulation system that affects on the climate in China. In this paper, the influence and the physical processes of the sub-seasonal east-west movement of subtropical high on the southwestern China are investigated. First, the east-west displacement index of the WPSH is defined according to the key position of WPSH movement in early summer and late summer. The index has a significant period of 10-30 days and represents well zonal shift of the WPSH. A total of 195 events (1374 days) are chosen based on standardization value of the index. The results show that the subseasonal scale zonal movement of the WPSH is a closed relation to the precipitation in the southwestern China. During the west (east) events, the WPSH move gradually from east→west→east(west→east→west), and the rainfall in most southwestern China changed from less→more →less(more→less→more). The response of southwest precipitation on the sub-seasonal scale to the WPSH change is closed related to the water vapor and vertical movement of the gas flow in the north side of the WPSH and the main area of the WPSH during the movement process of the WPSH. Otherwise, the analysis shows that, in the condition of the WPSH, the consistency of precipitation in Guizhou and Chongqing is good, but the regional difference of precipitation in Yunnan and Sichuan is very significant, especially in Yunnan. In early summer, in the process of the zonal movement of the WPSH, the precipitation in most parts of Yunnan is even contrary to that in most parts of southwest China, especially in Guizhou and Chongqing. Namely, when WPSH move westward, the precipitation is less in most regions of Yunnan, vise verse. In the late summer, the precipitation in north-central Yunnan is opposite to that in other regions.
Available online:April 27, 2020 DOI: 10.3878/j.issn.1006-9895.1911.19193
Abstract:To improve the analysis quality through incorporating the flow-dependent ensemble covariance into variational data assimilation system, the new GRAPES hybrid-3Dvar system is built, based on GRAPES regional 3DVar system which uses the statistic covariance, by augmenting the state vectors with another set of control variables preconditioned upon the ensemble dynamic covariance. The new Hybrid-3Dvar system and localization method has been verified through the single observation assimilation experiment with ensemble samples produced by 3D-Var’s control variable perturbation method. The real observation assimilation and forecast experiment for typhoon Soudelor come to the conclusions: (1) the background covariance which is represented by ensemble samples are flow-dependent and the root mean square (RMS) spread in the ensemble of momentum field and mass field are largest near the center of typhoon; (2) the analysis increments of the new Hybrid-3DVar have more detailed structure and more medium and small-scale information; (3) The analysis and 24h prediction qualities of model variables in the new Hybrid-3DVar are obviously improved in comparison with the 3DVar system, and the precipitation position predictions are more accurate; (4) The 24h forecast track of typhoon Soudelor is closer to observational one and the 48h-predicted intensity approaches the real observation as well.
Influence of Stochastically Perturbed Parameterization method on ensemble forecasting of winter precipitation in China
Available online:April 27, 2020 DOI: 10.3878/j.issn.1006-9895.2001.19157
Abstract:Precipitation ensemble forecasting has great uncertainty, the uncertainty of parameters in the physical process parameterization scheme closely related to precipitation forecast is one of the sources of precipitation numerical prediction error. By introducing Stochastically Perturbed Parameterization (SPP method) on these parameters the uncertainty of representative model precipitation forecast is the frontier research field of international ensemble forecasting. In order to understand whether this method can reflect the uncertainty of numerical prediction of winter precipitation in Chinaand provide a scientific basis for business applications,based on China Meteorological Administration’s GRAPES(Global/Regional Assimilation and Prediction System)mesoscale regional ensemble prediction model,the 16 key parameters are selected from four parameterization schemes such as cumulus convection, cloud microphysics, boundary layer and near-surface layer, which have great influences on the uncertainty of model precipitation forecast,and introducing Stochastically Perturbed Parameterization (SPP),then through the ensemble prediction experiment from December 12, 2018 to January 12, 2019, a total of 31 days, comparing and analyzing the effect of SPP method on winter weather situation and precipitation ensemble prediction.The results show that the test for the random parameter perturbation method is added,the probability prediction techniques for precipitation and isobaric elements are better than control predictions without SPP method,and the improvement effect on low-level and near-surface elements is better than the improvement of the iso-surface elements in the middle or upper floors,The precipitation prediction result is superior to the control prediction test in scoring,but because it did not pass the significance test,there are no statistically significant differences.The above results indicate: under the influence of the East Asian winter monsoon,SPP method has no obvious improvement on the prediction technique of winter precipitation in China. Analyzing the reason,it may be related to the SPP method mainly represents the uncertainty of convective precipitation forecasting,but the winter precipitation process in China is mainly related to the development of baroclinic instability, model precipitation is dominated by large-scale grid precipitation, with less convective precipitation, therefore, the improvement of winter precipitation forecast is not obvious,this provides a scientific basis for applying Stochastically Perturbed Parameterization methods in the operation ensemble forecasting model.
Available online:April 27, 2020 DOI: 10.3878/j.issn.1006-9895.1912.19163
Abstract:To investigate the response of turbulent eddies to the mesoscale heterogeneous land use change over oasis in Northwestern China, two experiments with the cases of before irrigation (BI) and after irrigation (AI) using WRF-LES model are carried out, where heterogeneity strength would be changed by irrigation over oasis. The area-averaged method is used to calculate the area-averaged flux, and the wavelet analysis is used to decompose the pattern of surface turbulent heat fluxes into multi-scale pattern. The results show that the irrigation has large influence on the vertical heat flux, soil moisture, and soil temperature. Irrigation increases the heterogeneity strength, which has large influence on the dispersion of flux pattern. The dispersion height of sensible heat flux decreases with the decrease of sensible heat flux after irrigation. The shape of turbulent eddies in AI is reticular which is similar with that in BI but the energy spectra of sensible heat flux pattern decreases after irrigation. In addition, the dispersion height of latent heat flux depends on the sensible heat flux but the energy spectra of latent heat flux pattern decreases after irrigation. From the spatial lag correlation coefficient, the height of response of vertical heat flux to the surface flux before irrigation is higher than that after irrigation. The shift distance after irrigation is less than that before irrigation. Strong surface heating results in large correlation coefficients and strong entrainment in the top of convective boundary layer.
Available online:April 27, 2020 DOI: 10.3878/j.issn.1006-9895.1912.19176
Abstract:We evaluate the ability of models to simulate aridity index (AI) and project the future dry/wet climate changes in China under the Representative Concentration Pathway (RCP) 4.5 scenario using experimental data of a high-resolution regional climate model (RegCM4) nested within three global climate models. For the reference period of 1986–2005, RegCM4 is capable of simulating the spatial pattern of AI in China reasonably, and the simulated AI slightly differs with the methods to calculate potential evapotranspiration, especially in the western high-altitude and northern regions over the country. Based on the projections of RegCM4, the AI averaged over China would decrease by 2–4% and 2–5% in the middle (2046–2065) and end (2081–2098) of the 21st century relative to the reference period, respectively, with central Northwest China becoming wetter while other regions drier. The predominant factor for the future dry/wet climate changes in China varies with regions. Precipitation is the first leading factor in central Northwest China, while potential evapotranspiration changes due to increasing temperature play the major role in the rest regions.
Observational Analysis of Planetary Boundary Layer Structure and Entrainment Characteristics under Heavy Haze Pollution
Available online:April 27, 2020 DOI: 10.3878/j.issn.1006-9895.1912.19180
Abstract:Abstrac: An unmanned aerial vehicle (UAV) was used to carry on portable instruments to measure vertical profiles of temperature, specific humidity, and particulate matter under different haze-polluted weather conditions in Nanjing from December 22, 2017 to January 18, 2018. The study is aimed to assess the impact of aerosol radiative effect on the atmospheric boundary layer (ABL) and entrainment zone structures as well as their evolution between one day with heavy haze pollution and another with good air quality. Vertical profiles of potential temperature, water vapor, and PM2.5 concentrations are characterized by performing detailed analyses on UAV-measured vertical profiles and surface observations including surface heat flux, 2-m air temperature, specific humidity, winds, and major air pollutants (e.g., O3, PM2.5) under different air pollution conditions. Results indicate that aerosols reduce surface-reaching solar radiation and surface sensible heat flux, postpone the development of the ABL, enhance the atmospheric stability near surface, decrease the ABL height, and exacerbate air pollution. The maximum concentrations of PM2.5 and the largest increase rate were observed at the top of the ABL rather than near surface. Furthermore, aerosol radiative effect imposes an important impact on entrainment and its characteristic parameters. The entrainment-zone depth increases with increasing surface PM2.5 concentrations, and entrainment rate normalized with convective velocity does not follow -1 power function with the convective Richardson number under heavy haze or PM2.5 pollution conditions, which is consistent with the findings by the large-eddy simulation studies. The study indicates that aerosol radiative effect must be included in the ABL and entrainment parameterization schemes to further improve numerical predictions of weather and air quality under heavy pollution conditions.
Evolution Characteristics of Mesoscale Convective System during the Formation of Tropical Cyclone Dujuan(1521)
Available online:April 27, 2020 DOI: 10.3878/j.issn.1006-9895.2001.19184
Abstract:As for tropical cyclone formation, there are few studies focusing on the transition from a tropical disturbance to a tropical depression, which is accompanied by the formation, merger, and development of the mesoscale convective system (MCS). In this paper, FNL data and high-resolution numerical model are used to simulate the No. 1521 typhoon Dujuan three days before its formation, and the development and evolution characteristics of MCS in the process of Dujuan formation are preliminarily explored. During the formation of Dujuan, the tropical upper tropospheric trough (TUTT) was located farther east of it, and Dujuan migrated northwestward from the southeastern edge of the monsoon gyre (MG). The decreased vertical wind shear and MG-related lower-level convergence are conducive to the formation of Dujuan in the early stage of the formation of Dujuan, more MCSs generated favorable for the development of Dujuan from the depression disturbance. In the later stage of the formation, as the convection bursts, the MCSs merge to reduce the number of MCSs become more compact. The largest MCS shrinks towards to the core region of the tropical disturbance. As a result, the formation of Dujuan is accelerated, which is accompanied by a sudden increase in the vorticity at the low level, resulting in the deep convection in the tropical disturbance. The comparison of the characteristics of stratiform and convective precipitation in the MCSs indicates the coverage of stratiform precipitation is the largest, while the precipitation rate of convective precipitation is larger than that of stratiform precipitation and the variation of the coverage of convective precipitation is more significant. The increased percentage of stratiform precipitation is closely related to the intensification of Dujuan, and the increase in the convective precipitation rate is beneficial to the intensification of MCS. Their combined effect can promote the formation of Dujuan.
The properties of convections embedded in the stratiform cloud on basis of airborne Ka radar and DMT system
Available online:April 26, 2020 DOI: 10.3878/j.issn.1006-9895.2004.19185
Abstract:Based on a compact airborne Ka-band Precipitation PMS probe Radar (KPR) and the Droplet Measurement Technologies(DMT), the dynamic and microphysical characteristics of convective bubbles embedded in stratiform clouds initiated by the huanghuai cyclone on April 22, 2018 were analyzed. At first，a total of 36 convective bubbles were observed by KPR in spring in Shandong province. The results based on the echo intensity, horizontal scale and echo-top height of these bubbles show that the average echo intensity of convective bubbles is concentrated at 20 to 30 dBz, accounting for 69%. The horizontal scale of convective bubbles is concentrated at 15 to 30 km, accounting for 61%. The echo-top height of convective bubbles is at 6 to 8 km, which is 2 to 4 km higher than the surrounding stratiform cloud. Afterwards, the microphysical parameters of convective bubbles in the cumulus mixed cloud on April 22 were counted.The results show the inside of convective bubbles is dominated by updraft with maximum wind speed of 1.35m/s, the average updraft is 0.22m/s. There is high supercooled water content in the bubbles with maximum water content of 0.34g/m3, and the average is 0.15g/m3. The ice particle concentration in the convection bubbles is 5.5 times of that outside of bubbles, and their mean diameter is 1.7 times of that outside. The images sampled by cloud image probe show that the ice particles in the front and tail of convective bubbles were mainly columnar and radial, while the ice particles in the core of convective bubbles were polymers. The growth of ice crystals depended on the accretion and collision processes, they showed columnar form when the supercooled water was insufficient, otherwise, they could quickly form graupels. The microphysical formation mechanism of precipitation in the convective bubble is different, and strongly depends on the supercooled water content. When supercooled water content was sufficient in the cloud, graupels were quickly formed, and the surface precipitation was formed after they passed through the melting layer. When less supercooled water existed in the cloud, the formation of precipitation depended on water vapor deposition and aggregation processes.
Available online:April 21, 2020 DOI: 10.3878/j.issn.1006-9895.2004.19251
Abstract:Forest canopy as an active interface between vegetation and environment transmits energy by reflecting, absorbing and transmitting solar radiation by leaves. The radiation above, within and below the forest canopy is very important in the energy balance, water and carbon cycle. Its variation with season and distribution within forest canopies is few studied in Huainan area. Using the data of total radiation measured at Huainan forest observation station from 1 July 2018 to 30 June 2019, the temporal change characteristics of solar radiation above the Sawtooth Oak canopy, and the spatial distribution and transfer of solar radiation through the canopy were analyzed, and also the albedo, transmittance and absorptance of the canopy were given. The results show that: (1) The downward shortwave radiation above the Sawtooth Oak canopy increases from spring to summer, then decreases gradually to winter. But the downward shortwave radiation within and under the canopy demonstrate a different trend with smaller value. It decreases from the early spring and increases from autumn to winter as opposite to that above the canopy. Concerning the upward shortwave radiation, whether above, within or under the canopy, the seasonal variation pattern is the same as the downward one, but the value is much smaller. (2) The downward longwave radiation above, within and under the canopy gradually increases with time from spring to summer, then decreases gradually and reaches the minimum in winter; in terms of spatial change, the radiation value of longwave within and under the canopy is higher than that above the canopy, which enhances longwave radiation and can be as large as 1.3 times under clear skies. (3) The annual average albedo above the canopy in Huainan forest area is 0.14, which is 0.01 lower than that in the temperate monsoon climate area (mainly mixed forest) in northern China (350N), indicating that the forest is denser in Huainan. (4) The shortwave radiation transmittance of the upper part and the whole canopy is mainly affected by the leaves. In summer, the average shortwave transmittance of the whole canopy (τ) is 0.1. But in winter, the leaves wither and fallen, the transmittance increases and tends to a stable fluctuation. The absorptance of shortwave radiation in the canopy was the highest in summer and decreased gradually in autumn, and decreased rapidly in winter as the leaves withered, tending to a stable fluctuation. These results could be useful for validating layered radiative transfer and photosynthesis models and for further study the energy, water and carbon cycle of the forest ecosystem.
Numerical Simulation Study on Microphysical Characteristics of Stratiform Clouds with Embedded convections in Northern China based on Aircraft Measurements
Available online:April 01, 2020 DOI: 10.3878/j.issn.1006-9895.1908.19114
Abstract:To characterize the microphysical characteristics and transformation process of stratiform cloud with embedded convections, a case study was performed using WRFV3 model based on two aircraft measurements on May 1st, 2009. The aircraft observation results showed that the shapes and formation process of ice particles existed significant difference between the regions of stratiform cloud and embedded convection . Compared to the embedded convection region, the shapes of ice crystals in the stratiform cloud was more complicated, including the needle column, capped column and dendrite type. 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 were in good agreement with the aircraft observations, including cloud distribution, LWC and numerical concentration on the flight route, etc. The simulated results showed that in the stratiform cloud, with higher cloud water content and bigger W, could develop into embedded convection because of strong riming process. The hydrometeors of snow, graupel and rain water in the clouds account for 51.9%, 31.0%, 16.0%, respectively, cloud ice and cloud water accounts for very little. In higher level, snow and graupel grew through deposition process. In lower level, they grew through riming process and melted into rain. Stratiform cloud with lower cloud water content and smaller W, would stay to be stratiform cloud. The hydrometeors of snow, rain water and cloud ice account for 90.4%, 6.1% and 3.5%. The ice and snow grew through depositon process, and melted into rain in lower level.
Available online:March 30, 2020 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 such as heavy rain and severe convection occur more frequently in recent years, and its predictability has attracted widely attention. After a brief review of the history of atmospheric predictability research, this paper systematically reviews and summarizes the latest advances on the predictability of heavy rain and strong convection over the last 20 years. The main research methods for meso/micro-scale predictability and their differences with traditional large-scale weather predictability research are first discussed. Then, the primary initial error growth mechanism (error upscaling under deep moist convection) has been elaborated in detail and some arguments (error downscaling, error upscaling and downscaling coexisting) are discussed. The influences of errors in NWP models and convective environment to the practical predictability, as well as some recent mesoscale predictability experiments are also highlighted. Finally, this paper briefly discusses the current problems, challenges and future directions on predictability research of heavy rain and severe convection.
Diagnostic and Numerical Study on Physical Process of Strong Rainfall Associated with Rammasun (1409)
Available online:March 30, 2020 DOI: 10.3878/j.issn.1006-9895.2003.19224
Abstract:The development evolution and landfall process of supertyphoon Rammasun (1409) was investigated using high-resolution simulation data produced by the Weather Research and Forecasting (WRF) model. The diagnostic and numerical study on physical process of strong rainfall was focusing on the period of the rapid enhancement and the landfall on South China Sea of Rammasun, using the three-dimensional surface precipitation equations and the definition of precipitation efficiency. The results showed that the strong average precipitation intensity（PS） had been maintained in the main circulation area of Rammasun, and the relative contribution of PS over the land and ocean was basically reversed. The enhancement of land surface friction was conducive to more water vapor convergence to the land (positive value of moisture advection QWVA), resulting in local atmospheric humidification over the land within a short period before the landfall (negative value the local change rate of water vapor QWVL). Water vapor was rapidly transformed into liquid-phase and ice-phase hydrometeors by means of cloud-related microphysical processes, which promoted the rapid development of clouds and the intensification of precipitation intensity over the land. When the center of the circulation was located in the Beibu Gulf, the relative contribution of QWVA over the ocean was significantly enhanced. The change of the underlying surface during the landfall period led to remarkable changes in the moisture-related microphysical processes, causing significant changes in the cloud system and the strong precipitation center. The surface evaporation (QWVE) over the ocean made more effect and more obvious changes, compared with the one over the land. The average QCLL and QCIL (the local change rate of liquid-phase and ice-phase hydrometeors) in the main circulation were basically “positive – negative - positive” within the period of Rammasun moving to the coastal region of South China. The content of hydrometeors was mainly increased when the center of the circulation was located in the Beibu Gulf, but the reasons for the growth of liquid-phase and ice-phase hydrometeors were different. During the landfall period, the characteristics of the “negative - positive” change of QCLL and QCIL over the land were mainly due to the rapid development of the cloud over the land caused by the enhancement of land surface friction in the early landfall period and the consumption of heavy precipitation in the later period. The high precipitation efficiency had been maintained in the main circulation area of Rammasun. From the contact of the main circulation to the land, the precipitation efficiency over the land rapidly increased, while the precipitation efficiency over the ocean maintained a high value during most of the integration period, but reduced only after the second and third landfall.
Investigation on a severe precipitation event that is caused by the effect of an eastward propagating MCS originated from the Tibetan Plateau and a downstream southwest vortex
Available online:March 24, 2020 DOI: 10.3878/j.issn.1006-9895.1911.19206
Abstract:Based on the automatic station observed precipitation, Himawari-8 satellite temperature of black body and European Centre for Medium-Range Weather Forecasts (ECMWF) ERA5 reanalysis data, we conducted an investigation on a severe precipitation event in which an eastward-propagating mesoscale convective system (MCS) that originated from the Tibetan Plateau (TP) affected a downstream southwest vortex (SWV), and caused heavy precipitation over the Sichuan Basin (the maximum 6 hours precipitation around the Sichuan Basin was up to 88.5 mm). Main results are as follows: this heavy precipitation event was mainly induced by the effect of an eastward-propagating MCS and a downstream SWV, with the strong rainfall appeared in the coupling stage of the MCS and SWV, within the cold cloud area of the MCS. The eastward-propagating MCS lasted for a total of 33 hours, during which it changed obviously in intensity. Overall, compared with the stage before vacated the TP, after moving out from the TP, the eastward-propagating MCS lowered in the height of its centroid of convection, but enhanced remarkably in its convection intensity. During lifespan of the eastward-propagating MCS, the SWV maintained a quasi-stationary behavior around the Sichuan Basin. This vortex lasted for about 21 hours, and persisted in a shallow layer which was mainly located in the lower troposphere. The eastward-propagating MCS affect the SWV significantly. In the coupling stage of the MCS and SWV, superposition of ascending motions associated with the two systems directly induced the heavy precipitation. After that, the MCS moved eastward, whereas the SWV changed little in its location, thus the eastward-propagating MCS decoupled from the SWV. This rendered an intensity reduction of the SWV as well as a precipitation decrease associated with the eastward-propagating MCS. Vorticity budget showed that the convergence effect dominated development and maintenance of the SWV. In addition, the tilting effect was the second most favorable contribution to the positive vorticity production below 800 hPa, while the vertical transport was another dominant factor for the positive vorticity enhancement associated with the SWV above 800 hPa. Overall, these two effects were beneficial to the downward and upward extension of the SWV, respectively. Correlation analysis showed that during development of the SWV, the cold cloud area of the eastward-propagating MCS (using -52℃ as boundary) could reflect variation of the SWV intensity (vorticity) effectively, with the largest correlation (up to 0.83) appeared in the two hours advance correlation.
Available online:March 24, 2020 DOI: 10.3878/j.issn.1006-9895.1911.19132
Abstract:The real-time forecast accuracy of summer precipitation in Northeast China (NEC) is significantly low from 1978～2018. In the last second years, when the prediction of the overall distribution of summer precipitation anomaly for the whole country is relatively accurate, the prediction of summer precipitation anomaly in NEC is contrary to the actual situation. Therefore, it is necessary to analyze the reasons for the low forecasting accuracy. In this paper, the forecasting ability of dynamic models and cognitive defect on forecasting summer rainfall in NEC are discussed. Also, by systematically reviewing the climatic characteristics, influencing factors and prediction methods of summer drought and flood in NEC, and the test of the real-time forecasting, the reasons are as follows: (1) the precipitation in early summer in NEC is mainly affected by the cold eddy 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 can’t 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 global sea surface temperature (SST) is weak and unstable. Especially, the influence of ENSO on summer precipitation in NEC is complicated, the relationship between them varies from decadal to decadal; (3) the summer rainfall in NEC has obvious multi-time scale variability (inter-seasonal, inter-annual and inter-decadal time-scale) influenced by different circulation systems, which makes it more difficult to accurately predict summer precipitation in NEC. Finally, some scientific problems and possible solutions are further discussed for summer rainfall forecasting in NEC, which may be helpful for the future summer rainfall prediction in this area.
Observation and Analysis on Atmospheric Ice nucleating particles with Online Continuous Flow Diffusion Chamber in winter in North China
Available online:March 19, 2020 DOI: 10.3878/j.issn.1006-9895.1911.19194
Abstract:Ice nucleating particle (INP) concentrations are reported for a mountain site on the north-western margin of Beijing, during winter 2017, with the newly purchased continuous flow diffusion chamber(BJ-CFDC). Atmospheric INP concentrations characteristics are analyzed with weather elements, precipitation size distribution measurement and aerosol observation. The relationship among INP concentration, and activation temperature, super saturation with respect to water, aerosol with size larger than 0.5 m are developed. The result shows that atmospheric INP has a large variation at different activation temperature. It ranges between 2.50～76.8 L-1 at -20℃ with an average of 18.347 L-1. Atmospheric INP concentration increases exponentially both with lowering temperature and increasing super saturation respect to water. The result also shows atmospheric INP has a good correlation with the concentration of aerosol particles of sizes larger than 0.5 m. With this relation, the relevance of predicted and measured INP is higher than that the relationship built with only temperature. Atmospheric INP in snow days shows a characteristic of increasing firstly and decreasing later. INP concentration increase in the beginning of snow.The strong wind in the later period of snow scavenge the aerosols and lead to reduced concentration of INP. With the first continuous flow diffusion chamber type instrument used in China, the results of this paper will not only help to study the orographic cloud and precipitation in winter of north China, but also improve the support for development of cloud model and study in weather modification.
Available online:March 17, 2020 DOI: 10.3878/j.issn.1006-9895.1912.19223
Abstract:Based on the daily minimum temperature data from 529 stations in China over the period of 1961-2016, the spatio-temporal features of spring extreme low temperature (ELT) events are investigated. The rotated empirical orthogonal function (REOF) analysis indicates that, according to the variability of the ELT frequency, the whole China can be divided into five regions, which are Northeast China-eastern North China, the Jiangnan region, eastern Northwest China-western North China, Southwest China, and northern Xinjiang. The Morlet wavelet analysis shows that the ELT frequencies over the five regions exhibit a 2-4-year quasi-periodicity. Such a quasi-periodicity with 2-4-year is significant over the whole period for the Jiangnan region, eastern Northwest China-western North China, and northern Xinjiang, over the period before the early 1980s for Northeast China-eastern North China, and over the period between early 1980s and mid-1990s for Southwest China. For the long-term variation, the LET frequencies over all five regions exhibit decreasing trends, however their abrupt change years are different. The analysis using the Mann-Kendall (MK) and moving t-test (MTT) suggests that the abrupt change of the LET frequency occurs in 1987/1988 for Northeast China-eastern North China, 1995/1996 for the Jiangnan region, 1990/1991 for eastern Northwest China-western North China, 1987/1988 for Southwest China, and 1997/1998 for northern Xinjiang. Along with the decreasing of the LET frequency, the LET intensity over the five regions are weakened in the past half century. However, in recent 10 years, the frequency and intensity of the LET over eastern China are increased, which should be concerned.
The Relationship between the Early Spring Low Temperature Enhancement in the North China and Sea Surface Temperature in the North Atlantic since the 1990s
Available online:March 17, 2020 DOI: 10.3878/j.issn.1006-9895.1912.19127
Abstract:Based on the reanalyze data from European Centre for Medium-Range Weather Forecasts (ECMWF) and the sea temperature data from Hadley Center(Hadley) ,this paper analyzed the reason for the reason for the extrem low temperature enhancement in spring in the North China since 1990s and verified by numerical simulation results.The results indicated that the North Atlantic “Horse Hoof ”sea surface temperature mode has a significant correlation with the Eurasian Wave Train(EU) affecting North China. At the same time, the North Atlantic “Horse Hoof ”sea surface temperature mode has a close correlation with the vertical wave energy(TNZ2) in the key areas of the North Atlantic after 1997 and the 500hPa North Atlantic Oscillation(NAO) circulation mode in the North Atlantic after 1997 showed a trend of eastward movement and southward withdrawal. After 1997, the surface temperature on the eastern side of Greenland showed an increasing trend in positive region with anomalies thermal forcing. At the same time, the westerly jet stream increased, which could stimulate the Eurasian Wave Train and formed warm ridges on the downstream Eurasian continent. The Localized Multiscale Energy and Vorticity Analysis(MS-EVA) energy analysis method proves that anomalies thermal focing and pressure gradient force of the surfacee temperature of the key area on the eastern side of Greenland do positive work in the whole troposphere, leading to the increase of high-level kinetic energy and divergence to the outside, making the ridge strengthen to the North.Through EU wave train, the cyclonic anomaly in the downstream North China was strengthened, and the Asian polar vortex was strengthened and maintained. The temperature in the North China decreases sharply and the extreme low temperature events increase. Finally, the CAM5.1 model simulation is used to study the effects of the North Atlantic “Horse Hoof ”sea surface temperature mode on atmospheric circulation anomalies and the extreme low temperature in the North China. The simulation results verify the observation results well. It further shows that the North Atlantic “Horse Hoof ”sea surface temperature mode can affect the atmospheric circulation anomalies in the Eurasia by stimulating the Eurasian Wave Train, thus leading to the intensification of cyclone and meridional circulation and the increase of extrem low temperature events in the North China.
Contrasting Salinity Interannual Variability in the Tropical Pacific and Its Effects on Recent El Ni?o Events: 1997/1998, 2014/2015, 2015/2016
Available online:January 15, 2020 DOI: 10.3878/j.issn.1006-9895.1912.19172
Abstract:Ocean salinity variation provides a new insight into related ENSO expressed by climate variability. In this study, the comparisons of salinity variability and its related dynamical processes responsible for SSTA have been extensively described and analyzed among two strong ENSO, 1997/1998, 2015/2016 and one special ENSO, 2014/2015. The study shows that the development of ENSO is significantly associated with the occurrence and eastward diffusion of large-scale surface salinity anomaly (SSSA) in the western-tropical Pacific. In April of 1997 and 2015 corresponding to two strong ENSO events, there was a significant negative SSSA in the western-central Pacific, and then it moved eastward to the west of the dateline which induced the mixing layer depth (MLD) shallow, and the barrier layer thickness (BLT) thickened, which enhanced the surface warming in the tropical central Pacific and the early warming in the equatorial eastern-central Pacific. Although negative SSSA occurred in April of 2014/2015 weak event in the equatorial western-central Pacific, it did not develop eastward, resulting in weakened thickening process of BLT and weak modulation effect on surface temperature. For salinity change process corresponding to three ENSO events, surface advection and surface forcing caused by fresh water flux (FWF) were the major contributors to salinity budget. Surface advection influenced the former variability of salinity tendency, inducing the occurrence of ENSO signal. The precipitation in the tropical western Pacific played the most significant negative influence on FWF, which made a decisive role in the occurrence of SSSA and in the development of ENSO. Compared with the two strong ENSO events, the early FWF negative anomaly in 2014/2015 did not develop and moved eastward and weakened rapidly, resulting in a slowing of the negative salinity tendency in the western-central Pacific, deepening of MLD, thinning of BLT, and rapid cooling of the surface layer, which inhibited the early warming in the equatorial-eastern Pacific. The results of study demonstrate that the salinity change is closely related to ENSO, and early SSS in the tropical western-central Pacific could be used as the index of SSTA. In particular, SSSA not only affects the strength of ocean SSTA, but also could be used as a precursor to judge the development and strength of ENSO.
Correction for the cloud top height of cirrus with MODIS and CALIPSO dataset in Beijing-Tianjin-Hebei region
Available online:December 23, 2019 DOI: 10.3878/j.issn.1006-9895.1911.19181
Abstract:Cirrus plays an important role in atmospheric radiation. It affects weather system and climate change. Satellite remote sensing has great advantage in cirrus detection, relative to traditional observation. As a passive remote sensing instrument, large deviations are found at thin cirrus cloud top height from MODIS (Moderate Resolution Imaging Spectroradiometer). Comparatively, CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization) aboard CALIPSO (Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observation) which is an active remote sensing instrument, can acquire more accurate characteristics of thin cirrus cloud. In this study, MODIS cloud products in Beijing-Tianjin-Hebei region from 2013 to 2017 are selected. Using CALIPSO cirrus cloud top height data, a linear fitting method is obtained based on cross-validation method, and MODIS cirrus cloud top height is corrected. The difference between MODIS and CALIPSO changes from -3~2 km to -2.0~2.5 km, and the maximum difference changes from about -0.8 km to about 0.2 km. In the context of different vertical levels and cloud optical depth, MODIS cirrus cloud top height is improved after correcting, which is more obvious at lower cloud top height and optical thinner cirrus.
Contribution of Tropical and Subtropical Circulation Anomalies to the Intensity of East Asian Winter Monsoon over Lower-latitude Region
Available online:December 09, 2019 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 from 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. 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 ascending over the tropical western Pacific and South China Sea is strengthened. This anomalous ascending 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 quasi-geostrophic northerly near the entrance of the jet. Associated with the forced positive secondary circulation anomaly with anomalous descending (ascending) to the north (south) of the jet, the low-level anomalous northerly appears under the jet, 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 ITCZ seems more important. When the two circulation anomalies simultaneously enhance (i.e., more active convective of 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 above-mentioned 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 second-order vertical difference scheme for non-uniform distributed layers and its application in GRAPES model
Available online:November 20, 2019 DOI: 10.3878/j.issn.1006-9895.1906.19145
Abstract:Under non-uniform distributed layers, the vertical difference used in GRAPES (Global/Regional Assimilation and Prediction System) model can only achieved one order accuracy. A second order scheme was designed and introduced into GRAPES model for the process of vertical discretization. Ideal test with 1-D profile showed the new scheme could improve the accuracy of difference computation, while the improvement was not so markedly compared to the error caused by difference process itself. Density flow ideal test was conducted to verify the correctness and stability of new scheme in GRAPES model. A statistical evaluation of medium-range with second order scheme showed an improvement of forecast skill in large-scale fields, especially for the forecast after 120 hours. In addition, the second order scheme was tested with a real-case experiment for the extreme rainfall at South China, which again indicated improvement for the forecast of precipitation after 48 hours.
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
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
2002,26(6):721-730, DOI: 10.3878/j.issn.1006-9895.2002.06.01
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.
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
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
2004,28(6):979-992, DOI: 10.3878/j.issn.1006-9895.2004.06.15
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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