华北局地大暴雨过程中多个β中尺度对流系统发生发展对比分析

Comparative Analysis of Occurrence and Development of Multipleβ-Meso Scale Convective Systems During a Localized Severe Torrential Rain Event in North China

利用常规地面高空观测、多普勒雷达、风廓线、VDRAS(Variational Doppler Radar Analysis System)和NCEP再分析资料,对2018年8月5—6日副热带高压(以下简称副高)控制下华北一次局地大暴雨过程中多个β中尺度对流系统触发和发展机制进行了分析。结果表明:这次大暴雨发生在副高控制下,处于高温、高湿气团中,大气层结极不稳定。暴雨由多个相继发展的中尺度对流系统造成,分别是太行山迎风坡上西南—东北向、华北平原地区保定一带南北向、保定至霸州附近西南—东北向和以雄安新区为中心东西向原地生消的准静止MCS-Ⅰ、MCS-Ⅱ、MCS-Ⅲ和MCS-Ⅳ,均属于β中尺度。在相似的环境中,不同中尺度对流系统触发机制有较大差异,太行山迎风坡上的MCS-Ⅰ是由近地层偏东暖湿气流在迎风坡与山风形成的辐合抬升触发;由辐射差异和前期强降水形成的局地冷池受MCS-Ⅰ影响再次加强后,其出流与环境风形成的两条地面辐合线分别触发了MCS-Ⅱ和MCS-Ⅲ,并组织对流沿辐合线呈带状发展;而超低空偏东风增强叠加冷池出流在地形抬升作用下促使沿山暖湿气团进一步抬升,使得原本消亡的MCS-Ⅰ再次重建。MCS-Ⅳ发展最旺盛、持续时间最长,是大暴雨中心的直接制造者,一方面MCS-Ⅱ与MCS-Ⅲ、MCS-Ⅰ与MCS-Ⅳ的两次合并过程,是MCS-Ⅳ增强、持久的重要原因;另一方面边界层偏东风急流为MCS-Ⅳ的发展提供了水汽和不稳定能量等有利条件,同时推动其左前方中尺度涡旋的发展,导致MCS-Ⅳ所在地的气旋性涡度大大增加,加强了以急流轴为中心的垂直次级环流发展,造成MCS-Ⅳ的发展维持,形成华北平原地区以雄安新区为中心的东西向大暴雨带。

Based on the conventional surface and upper-air observations,Doppler radar products,wind-profiling radar data,VDRAS and NCEP reanalysis data,the triggering and development mechanisms of multipleβ-meso scale convective systems under the influence of subtropical high in North China from 5 to 6 August 2018 are analyzed.The results show that this severe torrential rain occurred under the control of subtropical high,in a high temperature and high humidity air mass,with extremely unstable atmospheric stratification.The torrential rain was caused by multiple successive developing mesoscale convective systems,namely MCS-Ⅰ,MCS-Ⅱ,MCS-Ⅲand MCS-Ⅳ,which were quasi-stationaryβ-meso scale systems on the windward slope of Taihang Mountains in southwestnortheast direction,in Baoding Area of North China Plain in north-south direction,near Baoding to Bazhou in the southwest-northeast direction and centered on Xiong’an New Area in east-west direction,respectively.In a similar environment,different mesoscale convective systems had significant differences in triggering mechanisms.MCS-Ⅰon the windward slope of Taihang Mountains was triggered by near-surface easterly warm and humid airflow converging and lifting on the windward slope with mountain wind.After being influenced by MCS-Ⅰ,the local cold pool formed by radiation difference and earlier-stage severe precipitation was strengthened again.Its outflow formed two surface convergence lines with environmental wind,which triggered MCS-Ⅱand MCS-Ⅲand organized convection along convergence line into band-like development.The enhanced easterly wind at ultra-low altitude superimposed the outflow of the cold pool,promoting further uplift of the warm and humid air mass along the mountain under the effect of terrain uplift,triggering MCS-Ⅰreconstruction.MCS-Ⅳdeveloped most vigorously and lasted longest,and was the direct maker of the severe torrential rain center.On one hand,the two merger processes of MCS-Ⅱand MCS-Ⅲ,MCS-Ⅰand MCS-Ⅳwere important reasons for the strengthening and persistence of MCS-Ⅳ.On the other hand,the boundary layer easterly jet provided favorable conditions such as water vapor and unstable energy for MCS-Ⅳ’s development,and also promoted the development of mesoscale vortex in its left front side,resulting in a significant increase of cyclonic vorticity in MCS-Ⅳ’s location,enhancing the vertical secondary circulation development centered on jet axis,causing MCS-Ⅳ’s continuous maintenance,and forming an east-west severe torrential rain belt centered at Xiong’an New Area in North China Plain.