ISSN 1000-0526
CN 11-2282/P

Volume 46,Issue 8,2020 Table of Contents

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  • 1  Analysis of Two Forecast Failure Cases of Warm-Sector Rainstorms on Hunan-Guangxi Border Area in Severe Southwest Jet
    FU Wei TANG Minghui YE Chengzhi
    2020, 46(8):1001-1014. DOI: 10.7519/j.issn.1000-0526.2020.08.001
    [Abstract](734) [HTML](468) [PDF 5.91 M](1572)
    Based on the conventional upper-air and surface observations, temperature of brightness blackbody (TBB) from FY-2G Satellite, Doppler weather radar data and reanalysis data from ERA-Interim, two forecast failure cases of warm-sector rainstorm forecasts in severe southwest jet on 5 May 2016 and 23 April 2018 are analyzed. The results are as follows. The water vapor sources of the two warm-sector rainstorms came from strong southwest jet at 925 hPa and southwest wind respectively. The water vapor convergence in the former rainstorm had higher intensity and wider range than the latter, resulting in a broader range of torrential rain. The superposition between the strong convergence and the special topography of Nanling Mountain on the Hunan-Guangxi border could result in a stronger dynamic uplift and a longer lifetime of rainstorm. Strong atmospheric instability in the former process led to more severe rainstorms in warm sectors. The wet layers of the two rainstorms were more shallow than that in typical frontal rainstorms. And the significant wet sectors in the middle and lower layers led to the occurrence of rainstorms. The energy maintained a long time in the former process which was caused by the warm moist air mass transport of the low-level jet that led to repetitive formation of low-level convective instability with high temperature, high humidity and high energy, finally resulting in longer rainfall duration and more precipitation. The weaker moisture convergence zone ahead of the front was an important predictor of the warm-sector rainstorms in the April 2018 process. So attention must be paid to the relationship between the trough and warm-sector rainstorms. The length of meridional trough was an important factor for the precipitation zone forecast in the warm-sector rainstorms. The topography contributed to the precipitation in these two rainstorms. The rainstorm centers mainly appeared on the windward slope of valleys or basins. The rainstorms were closely related to the dynamic convergence of the boundary layer and the water vapor supply. Therefore, in forecasting operation, we should pay attention to the convergence area of the boundary layer, the special topographies and locations, and make appropriate empirical revisions to numerical forecasts so as to improve its accuracy. The radar wind profile data analysis showed that the enhancement of vertical wind shear can indicate the occurrence of rainstorms. The differences of the southwest wind thickness can reflect the difference of precipitation intensity, providing an important reference for the ascertaining of the alert level.
    2  Analysis of Low-Level Temperature Cooling Mechanism of a Local Snowstorm Process
    PENG Xiayun LIU Hanhua LI Wenjuan ZHAO Fang KONG Zhaolin
    2020, 46(8):1015-1025. DOI: 10.7519/j.issn.1000-0526.2020.08.002
    [Abstract](599) [HTML](775) [PDF 4.33 M](1598)
    On 5 December 2015, with strong southwest warm and humid air and weak cold air, a heavy snowstorm occurred in the area from Hangzhou of Zhejiang Province to Huangshan of Anhui Province. The forecast for this process was quite inaccurate. The analyses of water vapor, power and temperature conditions in this paper show that the strong southwest warm and humid airflow, the convergence of cold and warm air provided abundant water vapor and dynamic conditions for the generation of a large amount of surface precipitation. The deep wet layer, suitable intermediate temperature and humidity conditions were favorable for the generation of a large number of snow and ice crystals for landing. The abnormal temperature drop at the lower level of Hangzhou Station before the snowfall was the key cause for snowfall. The analysis of the cooling mechanism shows that the cause of the low-level temperature drop at Hangzhou Station was mainly non-adiabatic heating related to water vapor phase transition, and the effect of cold advection was weak. The low-level cooling before 08:00 BT on 5 December was mainly caused by the evaporation of precipitation particles; the low-level cooling during the day was mainly caused by a large amount of ice (snow) crystal melting to absorb the latent heat of the environment, and the cold advection and vertical transport items also contributed. The main reason for the failure of the forecast was the dependence on temperature index and the insufficient analysis of the cooling mechanism. In this case, the cooling mechanism such as advection was weak, and the melting and heat absorption of ice crystals caused the middle and lower layers to form a uniform temperature layer of 0℃, so that the ice crystals reached the ground. The middle and lower temperature was quite different from the temperature index commonly used in discerning precipitation type, which indicates that the temperature index can not be applied mechanically, and the physical mechanism of snowfall formation should be comprehensively analyzed. When precipitation amount is large, melting heat absorption can become the main mechanism of cooling, which should be fully considered in the forecasting operation.
    3  Analysis of the Source of Model Precipitation Prediction Bias for a Heavy Rainfall Event in the Pre-Flood Season in South China
    HU Ning FU Jiaolan WANG Hui
    2020, 46(8):1026-1038. DOI: 10.7519/j.issn.1000-0526.2020.08.003
    [Abstract](610) [HTML](386) [PDF 13.53 M](2272)
    For the heavy rainfall event in South China in 19-20 May 2015, ECMWF-IFS model (EC mo-del) overestimated its rainfall intensity near the large-scale shear line, but underestimated the heavy rainfall induced by the mesoscale convective systems (MCSs) in warm sector, resulting in a northward displacement bias of the forecasted rainfall compared with the observation in Guangdong Province on 20 May 2015. In this paper, a high resolution numerical simulation of WRF model is performed to explore the source of forecasting error of EC model. The results indicate that the well-organized MCSs in the warm sector induced a significant cold pool outflow, which converged with the strong warm and moist southwest flow. New MCSs were triggered along the convergence line continually and produced heavy rainfall in the warm sector. The WRF model successfully depicts the whole process, while EC model failed to present the above mechanism and caused the underestimated rainfall in the warm sector. The feedback of convective rainfall to synoptic scale flow in South China can be described by WRF model, which can simulate the well-orga-nized MCSs. Most of the rainfall in EC model is the stratiform rainfall caused by the shear line, which may further strengthen the circulation in the middle and low level and increase the precipitation along the shear line in turn. The underestimate of convective rainfall in warm sector and a strong stratiform rainfall feedback work together to cause a northward displacement bias of the forecasted rainfall in EC model.
    4  Study on Forecast Errors and Predictability of a Warm-Sector Rainstorm in South China
    QIN Wu LIU Guozhong LAI Zhenquan QIN Yuefeng LIANG Yiling
    2020, 46(8):1039-1052. DOI: 10.7519/j.issn.1000-0526.2020.08.004
    [Abstract](651) [HTML](459) [PDF 7.19 M](1826)
    For warm-sector rainstorms, forecast errors often appear in the short-term forecast due to the complexity of environmental conditions, the difficulty in capturing the trigger mechanisms and the poor prediction ability of the numerical models. From 19 to 20 April 2016, a warm-sector rainstorm event occurred in Guangxi, China, but forecasters and numerical models both failed to forecast the rainfall intensity. Based on numerical forecast products, observation data of automatic weather stations, conventional surface and upper-air observation data, Doppler radar and FY-2G data, the forecast errors of this heavy rainfall event were analyzed. The results show that the development of the low- and mid-level jet and the southwest warm low pressure provided the environment with high temperature, high humidity and high energy in the area between the northern part Vietnam and the central and southern part of Guangxi, the topographic convergence and vortex triggered the convection and the mesoscale convergence lines effectively organized the convection. In addition, radar echoes were characterized by low centroid and high rainfall efficiency. The reasons for the failure are that forecasters and numerical models lacked the forecasting ability of the short-term warm-sector rainstorms, and could not accurately grasp the trigger mechanism of the convection. By analyzing the convective cloud clusters in the upstream zone, the evolution of the surface mesoscale convergence lines and the topographic effects and other trigger conditions, forecasters could have made qualitative nowcasting of warm-sector rainstorms and issued early warnings to make up for the ability shortage of short-term forecasting. Therefore, strengthening the understanding of the mechanism of warm-sector rainstorms and making fine analysis are effective ways to improve the forecast ability.
    5  Synoptic Characteristics of Linear Mesoscale Convective System in Shandong Province
    HOU Shumei SUN Jing ZHENG Yi HAN Yongqing
    2020, 46(8):1053-1064. DOI: 10.7519/j.issn.1000-0526.2020.08.005
    [Abstract](539) [HTML](545) [PDF 1.67 M](1878)
    The synoptic characteristics of LMCS (linear mesoscale convective system) were analyzed based on 27 selected LMCSs affecting Shandong from the 2012-2016 radar data. The filter conditions are as below: the contiguous or quasi-contiguous echo band larger than 40 dBz is over 100 km and lasts for at least 1 h, the 35 dBz echo embedded with the 40 dBz echo is strictly contiguous, the linear or quasi-linear convection area shares a common leading edge, and the maximum echo intensity is over 50 dBz. The characteristics of the study are concluded as follows. The LMCS affected Shandong has a high frequency in August, and the formation time is concentrated in the dusk to the first half of the night, lifespan is generally 1-2 h, and most of them have characteristics of backward propagation. Most of the initial convective cells are generated in Hebei and generally move eastward by southward. Most of the LMCSs are in a northeast-southwest trend, and the scale is between 100 and 200 km. Three types of synoptic models of LMCS formation are constructed, including forward-tilting trough, backward-tilting trough and cold vortex. The warm temperature ridge or warm center at 850 hPa is the important feature of LMCS formation. The mid-level jet stream at 500 hPa in cold vortex and forward-tilting trough category, and the low-level jet stream below 700 hPa in backward-tilting trough category all play very important roles in the LMCS formation. LMCS constantly occurs when 850 hPa specific humidity is greater than 8 g·kg-1 and the Lift Index (LI) and Showalter Index (SI) are both negative. The probability of LMCS occurrence is up to 80% when T850-500 >25℃ and CAPE >1 〖KG-*5〗000 J·kg-1 with small CIN. When LMCS appears, it is usually accompanied by short-duration heavy precipitation, 70.4% LMCS contributes to disasters of thunderstorm gale, hail and severe precipitation. The hail and gale require higher stratification instability than short-duration severe precipitation. When only the short-duration heavy precipitation occurs, the heights of 0℃ and -20℃ layers are significantly higher than those when hail and gale occur.
    6  Research on High-Resolution Intensity Field Fusion Method of Array Weather Radar
    YE Kai YANG Ling MA Shuqing ZHEN Xiaoqiong SUN Jingyi
    2020, 46(8):1065-1073. DOI: 10.7519/j.issn.1000-0526.2020.08.006
    [Abstract](386) [HTML](465) [PDF 15.84 M](1483)
    Array weather radar (AWR) is a novel weather radar with high spatio-temporal resolution, which can detect the fine flow field and intensity field of severe convection weather by employing a distri-buted phased array technology and provide a new technology and instrument for researching the small-scale weather systems. A high-resolution intensity field fusion method is proposed in this paper. The resolution expansion factors for each azimuth and elevation direction are calculated, and the intensity is filled successively. The polar intensity field is converted to Cartesian coordinates, and the intensity field of different transmit-receive subarrays are fused. The objective evaluation of simulated small-scale severe convection detection is used to verify the performance of a proposed high-resolution fusion method. Through a case of real precipitation process, it is proved that 100 m intensity field fusion data obtained in this paper has a more detailed and complete echo structure.
    7  Case Study on Generation Mechanism of Extreme Ocean-Effect Snowstorm in the East of Shandong Peninsula in January
    LI Gang LIU Chang CAO Yueyao MENG Xiangui
    2020, 46(8):1074-1088. DOI: 10.7519/j.issn.1000-0526.2020.08.007
    [Abstract](485) [HTML](2003) [PDF 5.03 M](1588)
    Analysis is conducted on the generation mechanism of ocean-effect snowstorm in Bohai which occurred from 9 to 11 January 2018, based on multiple observation data of buoy station, automatic weather station, Doppler weather radar, L-band radar, NCEP/NCAR 6 h reanalysis and precipitation, combined with horizontal wind data retrieved from EVAP technology. The results are as follows: This ocean-effect snowstorm process was an extreme snowfall event, characterized by long duration of heavy snowfall, large snowfall amount and near meso-γ scale distribution of snowstorm. The temperature of Bohai Sea and Shandong Peninsula decreased continuously caused by two strong cold air effect before and after the snowstorm. The 850 hPa temperature dropped to -18 to -16℃, which was helpful to the strong ocean-effect snowstorm generation. The cold air strength of this process was obviously stronger than December ocean-effect snowstorm. It was found that the median cold air strength of January ocean-effect snowstorm was about 5℃ lower than in December. Obvious difference of air and SST led to prominent sense heat flux with maximum 226.8 W·m-2 when strong cold air was active, and convective instability occurred over Bohai Sea and the coastal areas of northern Shandong Peninsula. It was a thin convection limited below 850 hPa. There was an obvious thermal ridge of pseudo-equivalent potential temperature during the snowstorm. The radar reflectivity factor graphs show up the obvious “train effect”. NE and WSW winds appeared over a small coastal area of northern Shandong Peninsula, causing wind direction shear lines of NW and NE, WSW and NE. The lower shear line led to the narrow-band echo which triggered the snowstorm. And the NE wind reached the height of less than 1.2 km, mostly under 0.6 km. Characteristics of the January ocean-effect snowstorm that less appeared are revealed though the study of this extreme snowstorm process. Its characteristics such as the circulation pattern, thermal instability and dynamical condition are similar to those of the December ocean-effect snowstorm. The major difference is that the cold air strength is stronger than in December, which could be the primary forecasting focus of January ocean-effect snowstorm. Buoy data and EVAP radar retrieved wind can quantitatively reflect the mesoscale feature of ocean-effect snowstorm.
    8  Study on Threshold of Lower Limit Temperature Index for Maize Sowing in Heilongjiang Province
    YAN Ping JIANG Lixia WANG Ping JI Shengtai ZHU Haixia
    2020, 46(8):1089-1097. DOI: 10.7519/j.issn.1000-0526.2020.08.008
    [Abstract](426) [HTML](608) [PDF 1.08 M](1350)
    By using the observation data from maize field observation stations during 1981-2005 in Heilongjiang Province, and cluster analysis and typical year analysis methods, this paper studied the lowest temperature limit index of maize field sowing. At the same time, the index was verified with the 2006-2018 observation data and the experiment data from the field sowing by stage. The results showed that the ground temperature is the main factor for the maize sowing, and the daily average ground temperature (10 cm) can be used as an indicator of the minimum temperature limit. The lowest threshold temperature index of the daily ground temperature (10 cm) is 6.0℃, at which the maize cannot be sown. The lowest threshold temperature index of the daily average ground temperature (10 cm) is from 6.0℃ to 8.0℃, at which the maize can be sown, but there is a small number of maize suffering from low temperature disasters. The lowest threshold temperature index of the daily average ground temperature (10 cm) is 8.0℃, at which the maize can be sown safely.
    9  Assessment of Precipitation in the Three Gorges Reservoir Area with TRMM and CMORPH Satellite Data
    ZHANG Tianyu GUI Shu YANG Ruowen WANG Yong LI Yonghua
    2020, 46(8):1098-1112. DOI: 10.7519/j.issn.1000-0526.2020.08.009
    [Abstract](474) [HTML](460) [PDF 22.66 M](1512)
    Based on the TRMM and CMORPH remote sensing satellite precipitation data from 1998 to 2016 and the observation data in the Three Gorges Reservoir Area in the same period, this paper evaluates the local precipitation changes in the Three Gorges Reservoir Area by comparing the precipitation changes of the main stream and the branch stream as well as the far and the near meteorological stations, and by comparing the characteristics of precipitation, precipitation days and precipitation intensity before and after impoundment. The results show that the interannual variation characteristics of TRMM and CMORPH satellite precipitation data in the Three Gorges Reservoir Area are generally consistent with those of meteorological observation stations. The inversion effect of TRMM on the daily scale is slightly lower than that of CMORPH, and that of CMORPH on the seasonal scale is slightly lower than that of TRMM. The inversion effects of both satellite data on winter precipitation are weak.The precipitation variations of the main stream and the branch stream stations in the Three Gorges Reservoir Area are generally the same, having strong interannual variation characteristics. Compared with CMORPH, TRMM can roughly reproduce the interannual variation characteristics of precipitation at the main stream and branch stream stations. The interannual fluctuation amplitude of CMORPH precipitation is generally larger than that of meteorological stations. Compared with the time period before and after water storage (1998-2003 and 2004-2016), the CMORPH distribution is closer to the variation trend of the stations than TRMM in terms of the inversion effect of the precipitation intensity, precipitation days, seasonal precipitation frequency and total amount of precipitation at different grades, and the inversion effect is slightly better than TRMM. However, the precipitation frequency and distribution of the two satellites are similar to observation. The errors of the stations do not change significantly before and after impoundment. In addition, the meteorological stations, TRMM and CMORPH data show that the ratio of annual precipitation in the far and the near reservoir areas of the Three Gorges fluctuates stably after impoundment, which indicates that there is no obvious change in precipitation in the vicinity of the Three Gorges Reservoir after impoundment.
    10  Preliminary Analysis of the Ability of C-Band Mountaintop Radars to Detecting Forest Fires
    XU Balin YANG Weijie XU Shuyang XIE Liyan LU Peng WANG Xin
    2020, 46(8):1113-1121. DOI: 10.7519/j.issn.1000-0526.2020.08.010
    [Abstract](518) [HTML](1049) [PDF 11.11 M](1702)
    Based on the methods and results of previous researches on detecting forest fires and urban fire smoke by S-band Doppler weather radar as well as the classification and comparison of 34 forest fire cases from 10 C-band radars in Yunnan Province, some unique indices of forest fire echoes by C-band mountain-top radars were obtained in this paper. The paper also put forward setting forest identification threshold according to radar altitude. Some new indices and methods, such as velocity field features, precipitation echo filtering, are introduced to detect forest fires. Then conducting radar detection to forest fires in such kinds of areas and operational practices proved to have achieved certain results by practical tests and field surveys. The main technical indices are as follows. Due to the effect of altitude difference of mountain radars, correspondly indices and parameters of different radars should be set respectively. According to the rising and drifting characteristics of forest fire echoes, the radar echo velocity is different from the surrounding echoes sometimes. By filtering clear air echo, clutter, ground object ehco, secondary echo, precipitation echo and identifying special echoes of velocity field, the suspected echoes of fire smoke can be obtained. These indices and methods could be used as reference for radar detection in the western mountains.
    11  Analysis of the May 2020 Atmospheric Circulation and Weather
    MAI Zi ZHANG Tao
    2020, 46(8):1122-1128. DOI: 10.7519/j.issn.1000-0526.2020.08.011
    [Abstract](550) [HTML](584) [PDF 6.27 M](1648)
    The main characteristics of the general atmospheric circulation in May 2020 is that the circulation finished the transition from a threewave pattern to a fourwave pattern in middlehigh latitudes of the Northern Hemisphere, and the South China Sea summer monsoon erupted in the fourth pentad. In addition, there was one polar vortex center in the Northern Hemisphere, slightly weaker than usual. The strength of Western Pacific subtropical high was a little stronger than that in normal years, and the south branch though was a little weaker. The monthly mean temperature was 17.2℃, 1.0℃ higher than normal, which ranks the fourth highest since 1961. The monthly mean precipitation amount was 68.5 mm, closed to normal (69.5 mm). Six regional torrential rain processes occurred in China this month. The earliest high temperature process since 1961 occurred in the central and eastern China in May, and the highest temperature in many places exceeded the historical extreme values of the month. Severe droughts occurred in Jianghuai, Huanghuai and Yunnan, and two sanddust weather events happened in the northern part of China.

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