Cloud physical process in heavy rainfall of Mei-yu frontal system in South China

Using the non-hydrostatic mesoscale model MM5 with explicit scheme to treat precipitation physics simulated an IOP heavy rainfall case during 23-24 May 1998 in HUAMEX project at South China. The vis5D system was used to view the structure and evolution of cloud and rain physical processes. It shows that in the convective region, the rain water started and reached down to ground when ice phase (graupels) appeared and coexisted with cloud water; then the column of rain water moved with the mixed phase; finally the rain water disappeared with the ice phase disappearing even there was still cloud water. There was only weak rainfall where cloud water existed but no ice phase coexisted at the non-convective region. These show that cold cloud process in which ice phase (ice, snow, graupel) is coexisted with super-cooled liquid phase (cloud water and rain) was playing the most important role in the formation and development of convective heavy rainfall in South China. The warm cloud process (condensation and aggregation among liquid droplets) may play a role only for that non-convective precipitation with larger scale but not very strong intensity.

For verifying numerical simulations, a video¨Csounding system was used to observe the cloud micro-physical structures in Mei-yu front in June and July 1999 at Shanghai. The observations include cloud particle phase, size, concentration and electric charging. WSR-88D Doppler radar in Shanghai was operated to support getting the location and intensity of the heavy rain and their evolution. During the periods of the observation, ordinary radio sounding with four times per day was launched. The most heavy rainfall of Mei-yu front in recent 100 years happened during this period, 14 video-sounding system were launched into six heavy rainfall processes which provided a valuable data set for studying the cloud physical process of Mei-yu front. Results from preliminarily analyzing show that numerous ice-phase particles: ice crystals, graupels, snow flakes existed above 0°æ level and up to 10km height. Mixed phase particles existed near 0°æ level (above and below) where the cold cloud physical process may form larger rain drops below and rainfall maximum on ground. The observations proved the results of numerical simulation.