“5.7”广州局地突发特大暴雨中尺度特征及成因分析

Mesoscale characteristics and mechanism analysis of the unexpected local torrential rain in Guangzhou on 7 May 2017

2017年5月7日广州局地突发特大暴雨,降水集中爆发于广州北部复杂地形区,单点小时雨量大、强降雨持续时间长。然而降水发生于副热带高压边缘、无明显的低空急流等天气系统配合,为弱强迫背景下的华南前汛期暴雨,加之珠三角地形复杂,其触发和组织维持机制等问题引起了气象科研和预报工作者的广泛关注。针对其降水特点,采用5 min自动气象站观测、分钟雨量、风廓线雷达、葵花8号气象卫星红外等高时空分辨率观测数据探讨中尺度对流系统的触发和组织维持过程,发现:中纬度入海高压南侧偏东风和低层切变系统为珠三角边界层南风风速辐合提供了有利的天气背景,喇叭口地形增强了风速辐合。小尺度地形辐射降温配合城市热岛在山前形成高温度梯度区,山风与南风对峙使地面辐合线在山前移速变慢有助于热带云团的生成。地形阻挡抬升和高温度梯度加强上升运动,南风风速脉动使云团迅速向山前移动,最终对流爆发。以暖云降水为主的对流系统产生弱冷池驱动对流系统连续传播,使强降水回波面积增大并在小尺度地形影响下稳定位于增城附近,产生极端小时雨强;中尺度对流系统的单体移动方向和传播方向近乎相反导致系统移动非常缓慢,后向传播明显,最终导致长时间强降水。

Guangzhou was hit by an unexpected local torrential rain on 7 May 2017, which occurred without any obvious synoptic weather system on the edge of the subtropical high. The torrential rain that mainly occurred in complex terrain area showed obvious mesoscale characteristics including large hourly local rainfall intensity and long duration. The triggering and maintenance mechanisms have attracted great attention of researchers and forecasters. Mesoscale characteristics and mechanism analysis for the triggering and maintenance of the mesoscale convective systems （MCS） were studied and discussed based on high spatial and temporal resolution data such as observations at 5 rain intervals collected at automatic meteorological stations, 1 min rainfall observations, and observations of wind profile radar and Satellite Himawari 8, etc. Results show that easterly winds on the southern side of a high-pressure system in the mid-latitude that was moving towards the sea were accompanied with the development and intensification of a shear line to the west of Guangzhou, leading to a favorable condition for the convergence of southerly winds that were enhanced by the trumpet-shaped topography in the Pearl River Delta. Dense isotherms in northern part of Guangzhou were caused by urban heat island effect and the underlying surface cooling. Mountain winds and southerly winds slowed the movement of the convergence line, which is helpful for the formation of tropical cloud clusters. Topographic blocking and lifting along with large temperature gradient further strengthened the ascending motion. The southerly wind velocity fluctuation enabled the cloud clusters to move towards the mountain quickly and finally resulted in convection outburst. Weak cold pool outflow brought by untypically deep convective storm under the condition of subtropical high made MCS to propagate continuously, which enabled the strong rain echo to enlarge quickly. The center of convection remained near Zengeheng due to effects of the small-scale terrain, finally leading to extreme hourly rainfall. The moving direction of the MCS was almost opposite to its propagating direction, resulting in slow movement of the MCS. Meanwhile, backward prorogation gave rise to the long-term heavy rainfall.