Raindrop Collision Outcomes in Persistent Precipitation over central China during Mei-yu Front

      The statistical characteristics of the raindrop size distribution (RSD) and raindrop collision outcomes over central China are investigated by using a two-dimensional video disdrometer (2DVD), micro-rain radar (MRR), and wind tunnel experiments abroad. By analyzing the evolution of each RSD in different precipitation, it is found that the RSDs have a higher mass-weighted mean diameter (Dm) and a lower normalized intercept parameter (log10Nw) for precipitation over 5 mm h-1, and the number concentration of small raindrops in convective and stratiform precipitation in this region is one order of magnitude smaller than the RSD of southern China. Moreover, the evolution of warm cloud microphysical processes based on the theoretical model, including collisional merging, spontaneous breakup, collisional breakup, bouncing, and the differences of warm cloud microphysical processes in different precipitation are also studied. The results show that the warm cloud processes in Mei-yu front precipitation are mainly dominated by raindrop merging, where the merging process includes raindrop merging and neck breakup, followed by the breakup process. The merging and neck-breakup of raindrops mainly occur in clouds with raindrop diameters between 1-2 mm, and no spontaneous breakup occurs. However, spontaneous breakup can easily occur for raindrops with diameters larger than 4 mm, and this phenomenon is conducive to the generation and development of heavy precipitation. The interesting findings of our study show the importance of large raindrops’ evolution in the development of persistent heavy rainfall, which indicates that the description of large raindrops is essential to a variety of applications such as maintaining the mechanism of extreme precipitation and forecasting.