上海地区不同类型短时强降水的大尺度环流背景特征分析

Characteristics Analysis of Large-scale Circulation Background for Different Kinds of Short-time Heavy Rainfall in Shanghai

作为一种强对流天气,短时强降水的监测诊断和预报预警已成为气象预报部门的业务重点,也是当今社会各界的关注重点。为此,以上海短时强降水为例,利用上海地区2004—2012年11个基本气象站的逐时观测资料、NCEP/NCAR第二套逐日再分析资料及实况天气图等资料,基于对所有短时强降水的影响系统进行分类,重点分析了不同影响系统下短时强降水的大尺度环流背景。研究结果表明:1)上海汛期(6—9月)平均每年发生8次短时强降水,其中,8月份发生次数最多,占总次数的近一半。降水强度的空间分布呈现城市雨岛特征,大值区位于中心城区及其下风向的近郊。2)产生短时强降水的主要影响系统有准静止锋、热带气旋、低压倒槽、冷锋及中尺度对流系统。其中,准静止锋导致的短时强降水发生次数最多,发生比例达67%,且多出现在6—8月;热带气旋影响下的短时强降水发生比例占17%,多出现在8—9月;其他影响系统导致的短时强降水发生次数较少。3)各类影响系统为短时强降水提供了不同的大尺度背景及作用:准静止锋类型有利于形成上冷下暖的不稳定层结;热带气旋与高空急流辐散场相配合导致强烈的水汽辐合和垂直上升运动;暖性低压倒槽使地面增湿减压,高空冷平流有利于高空辐散;冷锋有利于形成北侧下沉、南侧上升的经向垂直环流圈;中尺度对流系统则主要与非均匀加热导致的局地垂直环流及其伴随的冷空气卷入相联系。

As a severe convective weather,the monitoring and diagnosis,forecast and warning of short-time heavy rainfall have become the key businesses of meteorological department and the focuses of all social sectors. For that reason,taking short-time heavy rainfall of Shanghai for example,using hourly observed data from 11 basic meteorological stations of Shanghai during 2004—2012,NCEP / NCAR daily reanalysis data and real-time weather map,on the basis of classifying influencing systems of all short-time heavy rainfall events,the large-scale circulation backgrounds of different kind of rainfall events are investigated in this paper. Results suggest that： 1） 8 short-time severe rainfall events usually take place during flood season（ June—September） in Shanghai. And it occurs more often in August,which accounts for about one half of the total number. From the point of spacial distribution,rainfall intensity exhibits characteristics of urban rainfall island with large intensity areas locating in central city and its leeward suburb.2） The main synoptic systems that lead to short-time heavy rainfall include quasi-stationary front,tropical cyclone,low pressure inverted trough,cold front and the mesoscale convective system. The short-time heavy rainfall caused by quasi-stationary front happens most frequently,while occurring probability is67%,mainly during June—August. The next is those influenced by tropical cyclone,occurring probability is 17%,mainly take place in August—September. And other affecting systems induce few short-time heavy rainfall events. 3） Each affecting system provides different large-scale background for short-time heavy rainfall. Quasi-stationary front is in favor of the formation of unstable stratification with up cold down warm. Tropical cyclone combining with upper and lower jet brings strong water vapor convergence and vertical ascending motion. The warm low pressure inverted trough makes the humidity increase and pressure decrease near the ground,and the upper-level cold advection enhances the upper divergence.The cold front is beneficial to form meridional vertical circulation loop that descends north and ascends south. The mesoscale convective system is in favorable to the local hot air rising through non-uniform heating and accompany cold air intrusion.