Lili Xu1, Kun Zhao1, Guifu Zhang1,2, Long Wen1
1Key Laboratory for Mesoscale Severe Weather/MOE and School of Atmospheric Science, Nanjing University, Nanjing, China
2School of Meteorology and Advanced Radar Research Center, University of Oklahoma, Norman Oklahoma 73072, USA
In this study, the microphysical characteristics of a heavy rainfall event in Nanjing, an important city in East China, is examined by using the C-band polarimetric Doppler radar and a two dimensional video disdrometer (2DVD) observations during the 2015 OPACC field campaign. This event lasts for about 18 hour from 02:00 LST to 22:00 LST on 2 June 2015, causing the maximum accumulated rainfall over 100 mm and the significant flash flood in the urban area of Nanjing. Based on the wind profiler and surface observations in Nanjing, the whole precipitation process can be separated into two stages according to its position relative to the surface front: the warm sector (ahead of the cold front) and the frontal zone precipitation. Over 80% of the warm sector rainfall can be attributed to the embedded convection in stratiform precipitation, which is characterized with a large number of small- to medium-sized raindrops. The corresponding raindrop size distribution (DSD) for warm sector convection has a large number of small- to medium-sized raindrops (diameters smaller than 1.5 mm), with the mean diameters Dm and the generalized intercept parameter Nw of 1.45 mm and 4.53 m-3 mm-1, respectively. In contrast, the frontal convection is characterized with a slightly larger Dm (1.58 mm) and lower Nw (4.53 m-3 mm-1). The polarimetric radar observation shows that frontal convection has a larger differential reflectivity (Zdr) than warm sector convection for a given reflectivity (Z), consistent with its larger raindrop as revealed by 2DVD. The larger rain drop size in frontal zone convection can be attributed to the higher updraft indicated by the higher echo top. These differences in DSD and precipitation structure also suggest the different precipitation microphysics related to the warm sector and frontal zone convection. The warm sector convection is dominant by the warm rain process, while the frontal zone convection may have more chance to form from the melting of graupel.