线状MCSs强降水特征及形成条件

Formation conditions and characteristics of heavy precipitation with quasi-linear MCSs

基于2010—2019年国家气象站观测资料和多普勒雷达资料,以及美国国家环境预报中心(National Centers for Environmental Prediction,NCEP)再分析资料和欧洲中期天气预报中心(European Centre for Medium-Range Weather Forecasts,ECMWF)第5代全球大气再分析产品——ERA5,对冀中廊坊线状MCSs强降水雷达回波、气候特征,以及降水过程中物理量变化进行定性和定量分析。结果表明:(1)线状MCSs强降水的雷达反射率因子回波形态有3类,即层状云后置(trailing stratiform,TS)型,层状云前置(leading stratiform,LS)型和层状云平行(parallel stratiform,PS)型,其中TS型出现频率最高,LS型和PS型出现频率相对较少,线状MCSs强降水发生具有明显的月际变化和日变化特征,高发于一年中的7月和一日中的前半夜;(2)线状MCSs强降水形成于4种天气尺度环流形势下,即低槽型、横槽型、低涡型和西风环流型,以低槽型最为普遍;(3)700 hPa偏西方向来的相对干冷空气与低空西南气流共同作用,加剧了大气的层结不稳定性,提高了降水效率,850 hPa偏南水汽分量越大,越有利于形成雨区相对较小、但雨强较大的强降雨天气,925 hPa东南风的配合明显扩大了强降雨落区;(4)线状MCSs生成于强的热力环境背景下,对流有效位能(convective available potential energy,CAPE)在316.7~1545.7 J·kg^(-1),垂直能量螺旋度(vertical energy helicity,VEH)为正值且明显大于2×10^(-4)J·m·kg^(-1)·s^(-2)是其形成的有利能量条件。PS型MCSs强降水过程中,高空水平辐散加强了抽吸作用,使大的上升速率得以维持,优越的动力条件是强降雨持续时间更长的重要原因之一。

Based on the observation data of national weather station,Doppler radar data,National Centers for Environmental Prediction(NCEP)reanalysis data and European Centre for Medium-Range Weather Forecasts(ECMWF)fifth-generation global atmospheric re⁃analysis(ERA5)from 2010 to 2019,the echo pattern,climatic characteristics as well as the change of physical quantity of heavy precipi⁃tation with quasi-linear MCSs were analyzed qualitatively and quantitatively.The results are as follows:(1)There were three types of echo patterns,namely trailing stratiform(TS)type,leading stratiform(LS)type,and parallel stratiform(PS)type.TS type had the highest frequency,while LS and PS types were relatively rare.Heavy precipitation with quasi-linear MCSs had obvious characteristics of monthly and diurnal variation,and it occurred frequently in July and first half of the night.(2)The heavy precipitation with quasi-linear MCSs formed under four synoptic-scale circulation patterns,namely low trough,transverse trough,low vortex and westerly circulation types,and the low trough type was most.(3)The relatively dry and cold air coming from the west direction at 700 hPa and the low level southwest airflow acted together,and it intensified the stratification instability of the atmosphere and improved the precipitation effi⁃ciency.The larger southerly wind component of water vapor at 850 hPa was more conducive to the formation of rainfall weather with relatively small rain area but large rain intensity.The coordination of the southeasterly wind at 925 hPa significantly enlarged the heavy rainfall area.(4)The heavy precipitation with quasi-linear MCSs generated under strong thermal environment.The convective available potential energy(CAPE)varied from 316.7 to 1545.7 J·kg^(-1),vertical energy helicity(VEH)was positive and it was obviously greater than 2×10^(-4)J·m·kg^(-1)·s^(-2),which were the favorable energy conditions for the formation of the heavy precipitation with quasi-linear MCSs.In the process of heavy precipitation of PS type MCSs,the upper level horizontal divergence strengthened the pumping effect,and it allowed the large ascent rate to be maintained.The superior dynamic condition was one of the important reasons for the longer du⁃ration heavy precipitation.