基于RMAPS的一次局地强降水过程成因分析

Causal Analysis of a Short-Time Strong Rainfall Based on RMAPS and Observation Data

2017年8月8日午夜,天津北辰区出现局地短时强降水,小时雨强超过70mm·h^(-1)。利用RMAPS(Rapidrefresh Multi-scale Analysis and Prediction System)和观测资料,通过多种资料叠加、雷达回波参数量化和低层风场反演等方法分析了降水成因,结果表明:①降水是由M_βCS(Meso-β-scale Convective System)后向传播过程中尾部的多单体雷暴造成的,传播过程中单体合并对雷暴系统的结构及增强有重要影响,合并后系统的回波体积参数V_(40)(反射率≥40dBz的体积)增长了23.3%,在V_(40)增长之前,变率参数F_(V40-up6)(6km以上40dBz回波体积6min的变化)出现了"突降",这对雷暴系统即将发展有预警意义;②冷池出流与地面东南风形成的辐合线是对流系统维持的关键,中低层切变线也为对流的发生发展提供了有利的动力条件;伴随着强降水,对流有效位能CAPE由峰值急速下降,低层垂直风切变指数α骤增,这揭示了不稳定能量、垂直风切变与对流过程之间的关系;③降水落区与不稳定参数SWEAT的大值区对应,预警时间3h,来自渤海的地面暖湿带与假相当位温密集带的交汇处也是落区临近预报的关注点。

An exceptional short-time strong rainfall happened in the Beichen district of Tianjin at the midnight of 8 Aug 2017 and its intensity surpassed 70 mm·h-1.Using the methods of multi-data superposition analysis,quantitative analysis of radar echo parameters,and low-level wind field retrieval,etc.,RMAPS(Rapid-refresh Multi-scale Analysis and Prediction System)products and various observation data are used to analyze this rainfall.The results indicate:(1)The multi-cell storms in the southern extremity of MβCS(Meso-β-scale Convective System)during the back-propagation caused this rainfall.The merging process had an important impact on the structure and enhancement of the storm system.After merging,echo volume parameter V40(the volume of the reflectivity above 40 dBz)increased by 23.3%.Before the rise of V40,variable parameter FV40-up6(the change of strong echo volume of above 40 dBz above 6 km height in 6 min)suddenly dropped,indicating that the echo was about to develop explosively.(2)The ground convergence line formed by the cold pool outflow and surface southeast winds was the key to maintain the convective system;the low level wind shear also influenced the trigger and development of convection;accompanied by heavy rainfall,CAPE suddenly dropped from the peak and the low level vertical wind shear indexαincreased,indicating the relationship among unstable energy,vertical wind shear,and convection process.(3)The large value area of the instability parameter SWEAT had an early indication of the heavy rainfall area;the lead time was about 3 hours;and the location where the surface warm-wet area from the Bohai Sea and pseudo equivalent temperature dense zone met was the focus of falling area nowcast.