Influence of Irregular Coastlines on a Tornadic Mesovortex in the Pearl River Delta during the Monsoon Season. Part I: Pre-storm Environment and Storm Evolution

Abstract

The Pearl River Delta (PRD), a tornado hotspot, forms a distinct trumpet-shaped coastline that concaves toward the South China Sea. During the summer monsoon season, low-level southwesterlies over the PRD’s sea surface tend to be turned toward the west coast, constituting a convergent wind field along with the landward-side southwesterlies, which influences regional convective weather. This two-part study explores the roles of this unique land–sea contrast of the trumpet-shaped coastline in the formation of a tornadic mesovortex within monsoonal flows in this region. Part I primarily presents observational analyses of pre-storm environments and storm evolutions. The rotating storm developed in a low-shear environment (not ideal for a supercell) under the interactions of three air masses under the influence of the land–sea contrast, monsoon, and storm cold outflows. This intersection zone (or “triple point”) is typically characterized by local enhancements of ambient vertical vorticity and convergence. Based on a rapid-scan X-band phased-array radar, finger-like echoes were recognized shortly after the gust front intruded on the triple point. Developed over the triple point, they rapidly wrapped up with a well-defined low-level mesovortex. It is thus presumed that the triple point may have played roles in the mesovortex genesis, which will be demonstrated in Part II with multiple sensitivity numerical simulations. The findings also suggest that when storms pass over the boundary intersection zone in the PRD, the expected possibility of a rotating storm occurring is relatively high, even in a low-shear environment. Improved knowledge of such environments provides additional guidance to assess the regional tornado risk.

摘要

粤港澳大湾区独特的“喇叭口”海岸下垫面, 长久以来被认为在区域强对流天气的形成和发展过程中发挥着重要作用. 基于中国龙卷风历史个例统计, 该地区也是华南海岸带龙卷风暴相对集中发生的区域. 本研究以喇叭口海岸地形对夏季盛行西南季风气流的扰动为切入点, 探究大尺度季风气流与局地环流相互作用从而激发龙卷风暴的可能物理机制. 上篇首先通过南海夏季风活跃月份的长时段数值模拟, 揭示了喇叭口海岸在午后会形成有利于对流触发与升尺度发展的三种气团辐合交汇区, 即“三汇点”. 在午后海陆热力差异和下垫面粗糙度差异的影响下, 近地面季风上岸气流在珠江口海面会发生水平切变并向西偏转, 与陆地盛行西南气流在珠江口西岸形成准静止的低层辐合边界. 当北侧出现风暴冷池出流时, 在珠江口会形成比辐合边界具有更强动力辐合与环境垂直涡度的三汇点. 本文以2020年6月1日在珠江口生成的一个龙卷风暴为例, 诊断研究其生成环境及精细结构演变. 距龙卷风暴仅6公里的相控阵天气雷达显示, 钩状回波和中涡旋形成于珠江口西岸准静止辐合边界与风暴冷池出流形成的三汇点, 后者在此次龙卷风暴的形成过程中可能发挥了重要作用. 高分辨率数值敏感性试验进一步证明, 三汇点区域的低层环境具有局地强辐合与强垂直涡度, 为中涡旋的形成提供了低层涡度来源, 请详见下篇. 观测结果还表明, 即使在弱切变环境下, 当对流系统经过喇叭口海岸的三汇点时, 发展形成旋转性风暴的可能性更高.