Raindrop Size Distribution and Z-R Relation during the Black Rainstorm Warning in Hong Kong

The passage of typhoon Chanthu from 21st July to 23rd July is followed by heavy rain that caused flash floods in large areas in Hong Kong. The Hong Kong Observatory issued an amber, a red and a black rainstorm warning within a few hours. The flash flood resulted in four deaths in Hong Kong. Analyses of rain drop size distribution (DSD) sampled at 1 second interval measured by a newly installed optical Meteorological Particle Sensor (MPS) installed at the Fok Ying Tung Remote Sensing Center at the Chinese University of Hong Kong showed two breakpoints with droplet diameter of around 0.5 mm and 2.5 mm, respectively, where the slopes of the DSDs changed. These break points are consistent with theoretical simulations from cloud models. The MPS also measures drop fall speed and hence rainfall rate. Using a threshold of 5 mm/hr as a separation between convective and stratiform rain the DSD showed a clear distinction between convective and stratiform rain, with Z-R relations Z = a R^b where (a, b) is around (800, 1.05) for convective and around (1600, 1.1) for stratiform rain. These values deviates substantially from the climatological estimates derived from a Marshall Palmer Distribution of (a, b) = (200, 1.6) which are used operationally.