甘肃长江流域一次极端短时强降水特征及成因分析

Analysis on Characteristics and Causes of an Extremely Short-term Heavy Rainfall in the Yangtze River Basin in Gansu Province

为研究短时强降水形成的原因与机理,提高对短时强降水强度和落区短期预报的准确率,进一步减少因短时强降水诱发滑坡泥石流等地质灾害造成的损失,利用NECPFNL逐6h1°×1°再分析资料、常规气象资料、区域自动气象站逐小时降水资料,对2017年8月6日—7日甘肃长江流域近10a来最强的一次极端短时强降水天气进行中尺度天气学诊断分析。结果表明:(1)此次极端短时强降水时空分布不均,主要有2个带状中心,分布特征与陇南地形山脉的形态走向有着一定的关系,时间分布呈双峰型,最大小时雨强达68.8mm/h;(2)此次极端短时强降水是在副热带高压东退的过程中,高原槽和低涡在较强的偏西气流带冷空气下进一步发展东移南压影响甘肃东南部所形成的,其中500hPa高原槽、700hPa低涡、切变线及西南气流线提供了良好的动力抬升条件,同时副热带高压外围西南气流以及高空下滑冷空气为此次短时强降水的形成提供了重要的水汽、能量及不稳定条件;(3)水汽通量散度辐合中心、湿Q矢量散度辐合和垂直速度大值区与短时强降水中心有较好的对应关系,湿Q矢量散度对短时强降水的发生有一定的指示作用,低层暖平流,中高层冷平流的不稳定层结及风向的垂直切变在低涡切变线的触发下,更有利于短时强降水的发生,同时地形的触发和地形辐合抬升作用对此次短时强降水的形成有着重要的作用。

In order to study the causes and mechanisms of the formation of short-term heavy rainfall,improve the intensity of short-term heavy rainfall and accuracy of short-term forecast in the falling area,and further reduce the losses caused by geological disasters such as landslide and debris flow induced by short-term heavy rainfall,the mesoscale meteorological diagnosis analysis on the most intense extremely short-term heavy rainfall in the Yangtze River Basin in Gansu Province on 6 th of August 6-7,2017 is performed in this paper by using the NECP FNL 6 hour 1°×1° reanalysis data,conventional meteorological data,and hourly precipitation data of regional automatic weather station,the results show that:(1) The temporal and spatial distribution of the extremely short-term heavy rainfall is uneven,mainly with two strip-shaped centers.The distribution characteristics have a certain relationship with the morphological trend of the landform mountains in Longnan.The time distribution is bimodal,with the maximum hourly rainfall intensity up to 68.8 mm/h.(2) The extremely short-term heavy rainfall is in the process of the eastward retreat of the subtropical high.The plateau trough and the low vortex are formed due to further development by eastward and southward shifting under the strong westward airflow with cold air,which affects the southeastern part of Gansu.The 500 hPa plateau,the 700 hPa low vortex,the shear line and the southwest airflow line provide good conditions for dynamic lifting.At the same time,the southwest airflow around the subtropical high and the descending cold air in the upper air provide important water vapor,energy and unstable conditions for the formation of this short-term heavy rainfall.(3) The convergence center of water vapor flux divergence,the convergence of wet Q vector divergence and the large value area of vertical velocity have a good correspondence with the short-time strong rainfall center,and the wet Q vector divergence has a certain indicative effect on the occurrence of the short-term heavy rainfall.The unstable stratification of low-level warm advection and the middle and upper-level cold advection and the vertical shear of the wind direction are more conducive to the occurrence of short-term heavy rainfall under the trigger of the low-vortex shear line.At the same time,the trigger of the terrain and the topographic convergence uplift play an important role in the formation of this short-term heavy rainfall.