A very extreme rainfall event occurred on 7 May 2017 in Xintang Town,Zengcheng District of Guangzhou with maximum hourly precipitation exceeding 180 mm and 3 h rainfall exceeding 330 mm (shortly"the 5·7 extreme rainfall event"),causing severe economic damages.The rainfall process can be divided into two stages:Huadu rainfall stage and Zengcheng rainfall stage.The severe rainfall was mainly concentrated in two or three hours.The maximum minutely rainfall was high up to 5.0 mm.About 120 mm of the rain poured between 05:30 and 06:00 BT for the extreme hourly precipitation of 184.4 mm in Xintang Town of Zengcheng.Some negative lightning was observed during the Huadu rainfall stage and only several lightning occurred during the Zengcheng convection stage.Both radar reflectivity and satellite images show that the severe convective rainfall system was characterized by small-scale and rapid develo-ping.The radar vertical profiles show the convection featured low-echo-centroid warm-cloud precipitation. There was remarkable spatial inconsistency between radar maximum reflectivity and minimum TBB of sat-ellite image during the mature stages of the convection.The strong updraft was the cause of the spatial in-consistency between radar maximum reflectivity and minimum TBB.The topographic radiation cooling formed the surface cold center near Huadu.The terrain combined with large-scale weak cold air blocked the north-moving warm,moist flow,and the convection was finally triggered near Huadu.The continu-ously transport of warm,moist air and blocking of Huadu terrain maintained the mesoscale convective sys-tem (MCS) during 01:00-03:00 BT in Huadu.The south-moving large-scale weak cold air enhanced the cold pool,and pushed the MCS to move southward rapidly in 03:00-04:00 BT.The combination of south-moving MCS and local convection enhanced the convection over Zengcheng Region.The cold pool driven theory can explain the long-time maintenance and development of the MCS over Zengcheng.Both weak ambient flow and southward surface flow made the MCS slowly move during the two heavy rainfall stages.Thus,the extremely severe rainfall over Huadu and Zengcheng of Guangzhou took place.%2017年5月7日,广州市增城区新塘镇等地出现了小时雨量超过180 mm、3 h雨量超过330 mm的极端强降水事件(简称"5·7"极端强降水事件),导致了严重的经济损失.这次过程的高强度降水分为两个主要阶段:花都区降水和增城区降水,每个阶段的强降水均集中在2~3 h内,最大分钟级降水达到了5 mm的强度,增城区新塘镇184.4 mm的极端小时雨量中约120 mm的雨量是在05:30—06:00的半小时内产生的.地闪监测显示,对流发展的第一阶段伴有较少的负地闪,第二阶段仅伴有几个闪电.雷达和卫星资料显示,强降水对流系统具有空间尺度小,发展迅速的特征;但发展成熟阶段的反射率因子大值区和卫星低TBB区在空间上出现明显偏离.强倾斜上升气流可能是造成反射率因子大值区和卫星低TBB区空间偏离的原因.雷达资料垂直剖面显示,对流具有回波顶高较低、云底高度低、强回波质心低等低质心暖云降水的特征.地势分布和辐射降温是花都北部低温中心的主要成因,大尺度弱冷空气和冷中心伴随的地形的共同作用,使得偏南暖湿气流向北移动受阻后,在花都地形的强迫抬升下触发了对流.偏南暖湿气流的持续输送、花都地形的阻挡和冷池的作用是01—03时对流维持的主要原因,弱冷空气的南下对03—04时对流系统的快速南移起到了重要作用,而冷池驱动的对流发展模型可以解释增城地区05—06时对流的较长时间维持.弱的环境引导气流和偏南暖湿气流使得高效的低质心、高效率强降水对流系统较长时间影响同一局地区域,从而导致了花都和增城两地局地极端强降水的出现.
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