引发“6.25”天津极端雷暴大风的超级单体与中尺度对流系统分析

Analysis of supercell and meso-scale convective system causing an extreme thunderstorm gale in Tianjin on 25 June 2020

2020年6月25日天津地区发生一次极端雷暴大风过程(简称“6.25”大风过程),利用加密观测、多普勒天气雷达、风廓线雷达、微波辐射计、VDRAS(变分多普勒雷达分析系统)分析场等数据,对引发“6.25”大风过程的超级单体与中尺度对流系统进行了分析。结果表明:高空前倾槽与低层的暖湿气流叠加形成上干下湿的对流不稳定以及较强的垂直风切变、大风发生前超级单体回波形态由钩状转为弓形、对流系统前侧扰动温度梯度增大、冷池出流与环境风场形成的中尺度辐合中心与天津东南侧局地辐合区汇合,有利于中尺度对流系统的加强和维持;0-3km较强的垂直风切变促进西青站垂直涡度不断增长,对中尺度对流系统的发展起到重要作用;风暴后侧3km高度处入流气流与风暴内部的下沉气流合并有效增强了气流下沉速度,有利于雷暴大风的形成;中层径向辐合加强、中气旋直径减小,V40up6(即6 km以上大于40dBz的强回波总体积)及V40down6(即6km以下大于40dBz的强回波总体积)指标的变化比西青站极端大风出现时间提前近30min,具有一定预报指示意义。

An extreme thunderstorm gale event(hereinafter referred to as the"6.25"event)occurred in Tianjin on 25 June 2020.Based on the intensive stational data and the data from Doppler weather radar,wind profile radar,microwave radiometer,and VDRAS(Variational Dop-pler Radar Analysis System),we conducted an analysis of supercell and meso-scale convective system causing the“6.25”event.The main results are as follow:Before the occurrence of gale,the echo pattern of the supercell changed from hook to bow.The increase of the distur-bance temperature gradient in the front of mesoscale convective system(MCS)and the mesoscale convergence center formed by the cold pool outflow and the ambient wind field merge with the convergence area in the southeast of Tianjin,which is beneficial to strengthen and main-tain the strength of MCS.The strong vertical wind shear at 0—3 km enhanced the vertical vorticity at the Xiqing station,which also played an important role in the development of MCS.The merging of the incoming air flow at the height of 3 km behind the storm and the downdraft inside the storm enhanced effectively the downdraft velocity,which was conducive to the formation of thunderstorm winds.Mid-altitude Me-ridional Convergence strengthened,the diameter of mesocyclone decreased,and the variation of V40up6(total volume of strong echoes greater than 40 dBz over 6 km)and V40down6(total volume of strong echoes greater than 40 dBz below 6 km)was nearly 30 minutes earlier than the oc-currence of extreme gale at the Xiqing station,which has some value for prediction.