Microphysical Process Study of Mei-Yu Precipitation Events over Central China

The multi-scale monsoon frontal system is responsible for majority of heavy rainfall and flooding events in central China during the meiyu season (June-July). Raindrop Size Distribution (DSD) is a fundamental microphysics of precipitation, however, there continues to be lack of comprehensive observations of meiyu season over central China. This study analyzes the altitudinal and temporal evolution of rainfall microphysical characteristics for meiyu rain in 2018 over central China using the measurements and retrievals from the rain gauge (RG), micro rain radar (MRR), wind profile radar, and S-band weather radar over central China. The S-band radar retrieved reflectivity over MRR site at first elevation (1km) suffered from beam blockage issue by mountains, resulting severe underestimation of rain rate. Compared with the S-band radar, the MRR retrievals show more detailed vertical structures of rain below 3km under the assumption of zero vertical wind and provide reliable rainfall estimates in near surface. According to rain rates and radar reflectivity, the rain samples can be divided into convective rain (CR), straiform rain (SR) and light rain (LR). The MRR retrievals present slight variance and little attenuation for the LR and SR categories, but show significant vertical variance and suffer strong attenuation above 1 km for CR category because significant wind shear and vertical wind. As raindrops fall down, the number concentrations of both smaller and larger raindrops present decreasing trends for the LR category, which suggests evaporation process and break-up process. The slight increases in the number concentrations of larger raindrops for the SR category suggest weak coalescence process. For the CR category, the collective process with smaller raindrops below 1 km leads to rapid increases in raindrop sizes and sharp decreases in number concentrations. Among three categories, the LR category has the smallest raindrop size but the highest number concentration, the SR and CR categories have comparable raindrop number concentrations and the CR category has the largest raindrop size, which shows some differences with East China and roughly matches the maritime-like clusters. The rain rates of LR and SR categories are mainly determined by number concentrations, while the rain rates of CR category are determined by both raindrop sizes and number concentrations. The height-variance raindrop characteristics found in this study can help improving operational radar rainfall estimates and rain microphysical parameterizations of meiyu rainfall over central China.