A record-breaking nocturnal rainfall event (543 mm in 16-h) under weak synoptic forcing occurred in the metropolitan city of Guangzhou, China, during 6-7 May 2017. The evolution and microphysical structures of this torrential rainfall event are investigated using S-band polarimetric radar datasets. The torrential rainfall concentrated in two cores: one over Huadu District (HD) in which the storms were initiated between urban areas and mountains at mid-night, and the other over Huangpu and Zengcheng District (ZC) which was characterized by locally triggered storms merging with the storms from HD. The two heavy precipitation regions show some similarities, including strong reflectivity factor for horizontal polarizations (Z(H)) magnitude, low centroid cumulonimbus structures, and column shape of differential reflectivity (Z(DR)). But obvious differences can also be viewed between them. Compared to HD, ZC has higher precipitation intensity, longer precipitation duration, and larger accumulated rainfall. Besides, ZC also has a relatively lower Z(DR) value of similar to 0.2 dB and a higher specific differential phase (K-DP) of approximately similar to 0.35 degrees km(-1), which indicates the larger population of medium-sized rain droplet and higher water content in ZC. The radar-retrieved drop size distributions (DSDs) (i.e., mass-weighted diameter, logarithmic normalized intercept, and liquid water content) show that small size particles and high particle number concentration are more obvious in the storm over ZC. Combined with the retrieved DSDs, the merger process brings more medium-sized raindrops to ZC, and increases the possibility of raindrop growth via the accretion of cloud water by rain, which leads to enhancement of precipitation. In addition, strong K-DP may be a good indicator of intensity for extreme precipitation.
