桂林一次强下击暴流成因分析

2019年3月21日广西桂林市临桂区发生一次极端大风天气过程(以下简称"3·21"临桂大风),当日21:13临桂观测站记录最大阵风风速为60.3 m·s^-1(17级)。通过风灾现场调查判断这是强度为EF2级的微下击暴流过程。应用常规观测资料以及加密自动气象站、探空、多普勒雷达等资料,分析了"3·21"临桂大风的环流背景与影响系统及其形成原因。结果表明:低层暖湿气流活跃,中层显著干层,强的低层垂直风切变是大风发生的有利条件,地面中尺度辐合线、冷锋南压为其提供了触发机制。"3·21"临桂大风由2个超级单体风暴合并加强造成,在下击暴流发生前,风暴单体最强反射率因子核心高度(HGT)超过6 km,有中等强度中气旋伴随,中层径向辐合明显,辐合值达36 m·s^-1;当反射率因子减弱、风暴顶高下降、HGT下降时,下击暴流发生;当HGT剧降,一个体扫间隔下降3.5 km,17级极端大风发生,低层0.5°仰角在强中气旋的出流区观测到强的径向辐散,其值达27 m·s^-1;中气旋表现出最强切变加强,底高迅速下降到1 km以下等特征。本次下击暴流发生与极端强降水和冰雹的拖曳作用有密切关系,冰雹与雨水粒子的拖曳和融化蒸发作用使下击暴流加强。当分钟降水量大于3 mm时,风速超过12级;当分钟降水量大于6 mm时,则出现17级极大风速。

Cause Analysis of a Severe Downburst in Guilin

An extreme gale event,hereinafter referred to as“3.21 Lingui gale event”,occurred in Lingui,Guangxi on 21 March 2019.Maximum gust of 60.3 m·s^-1(Grade 17)was recorded at the Lingui Station at 21:13 BT.Based on the results of the wind damage survey with video and meteorological data,this paper shows that the serious wind damage was caused by a severe microburst.Using the data from conventional meteorological observations,dense automatic weather station,radiosonde,Doppler weather radar,this paper analyzes the environmental conditions and influencing systems of the“3.21 Lingui gale event”.Results are that the“3.21 Lingui gale event”occurred under some favorable conditions such as active low-level warm and humid air flow,mid-level significant dry layer and strong low-level vertical wind shear.The event was triggered by the surface mesoscale convergence line and the southward cold front.The“3.21 Lingui gale event”was caused by the merging of two supercells into one supercell.Before the downburst occurred,the supercell developed to the most powerful stage.The maximum reflectivity core height(HGT)exceeded 6 km.There was a medium intensity of the mesocyclone accompanied by the(mid-altitude radial convergence,MARC)value of 36 m·s^-1.When the downbursts occurred,reflectivity factor began to weaken,the height of storm top dropped and HGT dropped.When HGT dropped sharply,the scanned volume interval dropped by 3.5 km,and the extreme gale of Grade 17 occurred.At this time,the low-level 0.5°elevation was observed in the high-altitude area of the cyclone,with a high radial divergence value of 27 m·s^-1.The mesocyclone had the strongest wind shear strengthening,the bottom height rapidly dropped to less than 1 km,and so on.In addition,the occurrence of downburst was closely related to the dragging effect of extreme severe precipitation and hail.Usually,when the minutely rainfall is higher than 3 mm,wind force is higher than Grade 12;when the minutely rainfall is higher than 6 mm,the extreme gale of Grade 17 occurs.