“海葵”台风残涡极端暴雨的对流组织特征及成因

Structure and organization of convection during extreme precipitation associated with the remnant vortex of typhoon Haikui

摘要:

基于多源观测数据揭示了台风“海葵”残涡极端降水过程中的对流组织和发展特征,并从环流热动力特征和涡度收支分析了大湾区极端降水成因。粤港澳大湾区极端暴雨发生在“海葵”减弱残涡环流背景下,在累计降水量、短时雨强等方面具有显著极端性特征。2023年9月7—8日台风残涡环流在珠江口北侧停滞回旋并重新增强,珠江口地区附近出现罕见的准静止“人”字形的组织化对流结构,对流单体在港岛南侧合并增强,并自南向北经过港深地区造成极端强降水。雨滴谱观测表明降水具有高粒子密度的海洋型对流降水特征,在降水增强时段雨滴直径显著增加,有效提升了暖云降水强度。雷达观测表明对流垂直结构具有低质心特征,云顶高度超过8 km,深厚暖云中的降水粒子的碰并增长特征明显,产生了高数密度、大直径的雨滴特征。在弱环境场引导气流下台风残涡移动缓慢,残涡低层环流的复苏增强与对流层中层潜热垂直梯度有直接关系;7日夜间开始粤东沿海边界层东风急流快速增强,在残涡旋转作用下与来自南海的西南暖湿季风气流汇合为涡旋东部的东南急流,边界层中尺度东南风急流脉动风速超过20 m·s^-1,急流出口区的中尺度辐合和锋生作用导致了气流交汇点附近出现持续性的对流触发和增强。

Abstract:

On September 7,extreme torrential rainstorms struckthe Greater Bay Area(GBA),including Hong Kong,Macau,and parts of Guangdong Province near the Peal River Estuary in South China.This event resulted in historical accumulated precipitation exceeding 800 mm within 24 hours in Hong Kong,causing severe social and economic losses.Operational numerical forecast models showed limited skill in predicting the intensity and location of this extreme rainfall.Using multi-source meteorological observations,this study analyzed the precipitation characteristics,convection initiation,organization,and underlying causes of the extreme precipitation through circulation diagnostics,thermodynamic analysis,and vorticity budgeting.The rainfall was primarily associated with remnant vortex of the decayed typhoon Haikui.Synoptic analysis revealed that the typhoon vortex decayed rapidly after landfall in Fujian Province,South China.Due to weak mid-level environmental steering,the westward-moving remnant vortex stagnated over the GBA,north of the Pearl River Estuary,from the morning of September 7 until early evening on September 8.On the night of September 7,an enhanced easterly boundary layer jet combined with southwesterly monsoon airflowfrom the South China Sea,forming an intensified southeast jets tronger than predicted in numerical models.This southeast jet injected large amounts of moist,unstable air into the remnant vortex circulation over the GBA,triggering excessive precipitation.Significant low-level convergence and frontogenesis provided favorable conditions for mesoscale lifting.Late on September 7,a mesoscale jet pulse with boundary-layer wind speeds exceeding 20 m/s was detected by a wind profiler in Shenzhen.The mesoscale convergence and frontogenesis near the jet exit intensified convection initiation and precipitation.Revival of the remnant vortex in the boundary layer was closely linked to latent heat gradients below the mid-level maximum heating center.Weather radar observations revealed that from 14:00 BST on September 7,convection and precipitation intensified,forming a quasi-linear convective belt composed of rapidly growing discrete convective cells along the west shore of the GBA.A secondary,weaker convective line formed along the east shore,organized along the boundary between shallow easterly airflow and southwesterly monsoon winds.These two linear mesoscale convective systems (MCS) merged near Hong Kong’s southern coast,persisting for over 12 hours.A series of northward-moving convective cells repeatedly passed over Hong Kong and Shenzhen along similar tracks,inducing localized extreme rainfall.A maximum rainfall intensity of 158 mm/h was recorded in Hong Kong.After 14:00 BST on September 8,the remnant vortex of Haikui gradually moved westward toward Guangxi Province,leading to weakened rainfall in the GBA.Observation of raindrop size distributions showed that the precipitation had characteristics typical of maritime convection,with significant increases in raindrop size during periods of enhanced rainfall.This increased precipitation efficiency in the deep moist layer.The convection exhibited a low echo centroid structure withstorm tops exceeding 8 km,facilitating rapid growth of raindrops through coalescence during descent.Embedded β- and γ-mesoscale vortices within the linear MCS contributed to the extreme rainfall intensity.These vortices,identified using Doppler radar observations in Shenzhen,warrant further investigations.