2017年5月7日,广州经历了一次罕见的局地特大暴雨事件,刷新了多个雨量历史纪录,造成了严重的国民财产损失.本文利用双偏振雷达、二维雨滴谱仪、微波辐射计和风廓线雷达等多种新型探测资料,分析这次短时暴雨的演变过程和降水特征,并通过大气环境诊断和双多普勒雷达风场反演方法研究其维持机制.结果表明,此次降水过程发生在弱天气系统强迫条件下,大气层结表现为弱对流抑制、低抬升凝结高度、中等对流有效位能、较厚的暖云层,低层受暖湿气流影响但无明显急流.强降水是由中尺度对流系统直接产生的.午夜至凌晨的初始对流主要由偏南暖湿气流与地形相互作用产生,对流单体不断在后部触发并逐渐形成准静止的对流雨带;黎明至早晨,新生对流单体沿着成熟的强降水风暴出流与低层偏南暖湿气流的交界不断激发,后向传播过程更为显著,形成回波列车效应;此后降水以组织化的对流雨带不断南移.此次暴雨过程中的对流云团为典型的低质心降水云团,降水雨滴谱在高湿环境中表现为暖性降水的特征(小雨滴浓度非常高),但同时存在部分大粒子,从而导致更高的降水效率和局地强降水.不断加强的低层偏南暖湿气流对于对流系统的发展和维持具有重要作用.%An extremely heavy rainfall event occurred in Guangzhou City on 7 May 2017.The record-break-ing heavy precipitation caused severe property damage.To investigate the reasons/mechanisms responsible for the severe rainfall,a detailed observation analysis was performed in this study,based on the dataset collected by ground-based Doppler radars,two-dimensional video disdrometer (2DVD),microwave radio-meter and wind-profiling radar.The research results show that the ambient conditions prior to this event are characterized by small value of convective inhibition (CIN),low lifting condensation level (LCL), moderate convective available potential energy (CAPE),deep warm layer,and moist southerly flow.From midnight to dawn of 7 May 2017,initial convection was generated mainly by the terrain-blocked southerly flow,leading to the formation of quasi-stationary rainbands.From sunrise to early morning,new convec-tive cells were repeatedly triggered along the precipitation-induced outflows boundary (i.e.,back building process) and were continuously propagated backward(i.e.,"echo training").The observaiton of 2DVD further suggests that the surface heavy precipitation was composed of a high concentration of small rain-drops and a few large raindrops.This microphysical information indicates that the high-precipitation-effi-ciency warm rain process was the main microphysical mechnism responsible for the heavy rainfall.This ar-gument is supported with the low-centroid cumulonimbus structures observed by Doppler radar.It sug-gests that the continuously intensifying southerly flow played an important role in sustaining the convective system and producing the local heavy rainfall.
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