一次低空急流加强下的暴雨过程成因分析

Analysis on Causes of a Rainstorm Process Under Enhanced Low-altitude Jet Condition

基于地面、高空常规气象观测资料及风廓线雷达资料和WRF模式资料,分析江西一次低空急流加强下暴雨过程的环境场及成因。结果表明:(1)暴雨发生在副热带高压(简称“副高”)北抬、西南急流加强的天气背景下,此次暴雨是副高、西风带短波槽、中尺度辐合系统、低层切变、季风等多系统作用的结果。(2)低层增湿、增暖和对流层中层冷平流的侵入以及异常的各项热力不稳定指数,使江西北部成为高能、高湿、高不稳定区,并在江西东北部形成中尺度假相当位温θse能量锋区和露点锋区,而锋生正是此次暴雨过程原因之一。(3)低空急流加强一方面带来了充沛水汽,另一方面,中尺度辐合系统沿着急流发展形成中尺度能量、温度锋区,而对流性降水释放的凝结潜热反馈又促使中尺度对流系统进一步加强,利于强降水发生。1.5~3 km 16 m·s-1以上急流可提前1~3 h指示西南风下风方向地区有强降水。(4)500 hPa正涡度平流的发展使得对流层中层气旋性涡度增加,导致正涡度柱发展,并与涡度柱北侧下沉运动构成次级环流。同时高位涡中心向下发展,有利于低层气旋性涡度加强、锋生加剧,促进上升运动,有利于暴雨维持。

Using conventional surface and upper meteorological observations,wind profile radar data and WRF model data,this paper analyzed physical field and trigger conditions of a rainstorm process with low-altitude jet enhancing in Jiangxi.The results are as follows:(1)The rainstorm happened under the weather situation of subtropical high jumping northward and southwest jet strengthening,and it was caused by joint action of multi-systems such as subtropical high,short-wave troughs in the westerlies,monsoon,mesoscale convergence system and low-altitude shear.(2)Increase of both temperature and humidity in the low-level layer,invasion of cold convection in middle troposphere and abnormal value of all kinds of thermal instability indexes took high energy,high humidity and high instability over north of Jiangxi Province,and formed mesoscaleθse energy front and dew point front over northeast Jiangxi.Frontogenesis was one of causes of this rainstorm.(3)Low-altitude jet strengthening brought enough water vapor and mesoscale convergence system developing along the jet produced mesoscale energy and temperature front.And condensation latent heating released by convective precipitation promoted meso-scale convective system.That helped strong rainfall.The jet pulsation with the speed more than 16 m·s-1 from 1.5 to 3 km would forecast strong rainfall in downstream zone of southwest wind 1-3 hours in advance.(4)The 500 hPa positive vorticity advection developing increased the cyclone vorticity in middle troposphere,which caused positive vorticity column to grow,and made a secondary circulation with descending motion in the north of vorticity column.Meanwhile,high potential vorticity center developed downwards,which was favorable to cyclonic vorticity and frontogenesis development in low layer,promoting rising motion,and rainstorm keeping.