Observation and Numerical Simulation of Cloud Physical Processes Associated with Torrential Rain of the Meiyu Front

Cloud micro-physical structures in a precipitation system associated with the Meiyu front are observedusing the balloon-borne Precipitation Particle Image Sensor at Baoshan observatory station, Shanghaiduring June and July 1999. The vertical distributions of various cloud particle size, number density, andmass density are retrieved from the observations. Analyses of observations show that ice-phase particles(ice crystals, graupel, snowflakes, and frozen drops) often exist in the cloud of torrential rain associatedwith the Meiyu front. Among the various particles, ice crystals and graupel are the most numerous, butgraupel and snow have the highest mass density. Ice-phase particles coexist with liquid water dropletsnear the 0℃ level. The graupel is similarly distributed with height as the ice crystals. Raindrops belowthe 0℃ level are mainly from melted grauple, snowflakes and frozen drops. They may further grow largerby coalescence with smaller ones as they fall from the cloud base. Numerical simulations using the non-hydrostatic meso-scale model MM5 with the Reisner graupel explicit moisture scheme confirm the mainobservational results. Rain water at the lower level is mainly generated from the melting of snow andgraupel falling from the upper level where snow and graupel are generated and grown from collection withcloud and rain water. Thus the mixed-phase cloud process, in which ice phase coexists and interacts withliquid phase (cloud and rain drops), plays the most important role in the formation and development ofheavy convective rainfall in the Meiyu frontal system.