Statistical Analysis Of Convective Precipitation Microphysical Characteristics Over The Yangtze-Huaihe River Basin During Meiyu Season Based On Polarimetric Radar Observations

Meiyu precipitation over the Yangtze-Huaihe river (YHR) basin is the main precipitation system over eastern China in the summer, which often causes natural disasters, leading to huge losses. With the advance of the observation systems and forecasting technology, the forecast accuracy of Meiyu front has been significantly improved, but progress on the forecast of the fine structures of Meiyu precipitation, especially convective precipitation, is still limited. One important reason is the lack of understanding of the Meiyu convective precipitation microphysics. Polarimetric radar is an important tool to observe the three-dimensional structures of precipitation. Using the S-band polarimetric radar and 2 Dimensional Video Disdrometer (2DVD) data collected over the YHR basin during the OPACC field experiment in 2013 and 2014, combined with the Droplet Size Distribution (DSD) retrieval techniques, this study statistically analyzes the microphysical characteristics of stratiform and convective precipitation. Then, the microphysical characteristics of convection for precipitation of different scales is analyzed. At last, this study analyzes the microphysical characteristics of the extreme convective precipitation events (top 1% precipitation rate), and the extreme deep convection events (top 1% echo height).First, this study counts the frequency of stratiform and convective precipitation during the Meiyu season and analyzes their microphysical characteristics. The frequency occurrence of stratiform precipitation is dominant, but the contribution of convective precipitation to the total precipitation is high. The ZDR of stratiform precipitation is larger than that of convective precipitation above the 0° level, because stratiform precipitation consists mainly of irregular ice-phase particles, while graupels dominate in the convective scenarrio. When ice-phase particles melt at the 0° level, the Z and ZDR of stratiform clouds increases, forming the bright band. The raindrops of stratiform break up below 0° level, corresponding to the decrease of Z and ZDR, then the ZDR increases with the process of collision coalescence and evaporation. The ZDR of convective precipitation increases all the way with decreasing height below the 0° level, corresponding to the process of collision coalescence, and possibly evaporation near the ground. Overall, the diameter (Dm) of stratiform and convective precipitation are approximately 1.21 and 1.39 mm near the ground, but the number concentration (Nw) of convection is three times as large as that of stratiform precipitation, leading to larger precipitation intensity.Secondly, based on the horizontal scale of precipitation features (PFs), this study analyzes the convective microphysical characteristics of large (>1000 km2), medium (200-1000 km2) and small (<200 km2) PFs. Results show that convection in large PFs develops deeper with stronger Z, and the ZDR has more concentrated distribution with a maximum value of 1.7 dB. The distribution of ZDR is discrete in small PFs with a maximum value greater than 3 dB. The vertical structure shows that the increase of ZDR in three types of PFs occurs at different heights. The ZDR in large PFs increases mainly at 8-4 km and particles grow by cold cloud processes; while The ZDR in small PFs increases mainly at 4.5-3 km level and particles grow by warm cloud processes. The increase of ZDR in medium PFs is between large and small PFs. The result of the retrieved DSD shows that convection in small PFs has widely distributed DSD with smaller concentration, corresponding to weaker precipitation, but large and medium PFs has concentrated smaller raindrops with high concentration, corresponding to stronger precipitation.Finally, this study investigates the linkage between top 1% extreme precipitation convective PFs (R,>46.2 mm/h) and top 1% extreme deep convective PFs (H>14.5 km), and analyzes the microphysical characteristics of them. Results show a weak link between them and the overlap is only about 30%. Near the ground, ZDR in R-only convective PFs is always 0.2 dB smaller than that in H-only convective PFs, resulting in stronger precipitation. In the vertical structure, ZDR of both R- and H-only PFs increases mainly at 8-4 km, and the H-only convective PFs have stronger vertical velocity, so more water vapor and super-cooled liquid water are transported above the 0℃ level, leading to cold cloud processes. The larger melted ice-phase particles have higher efficiency to collect small raindrops and cloud droplets, finally producing larger raindrops. The result of retrieved DSD shows the presence of smaller raindrops with higher concentration in R-only convective PFs, resulting in stronger precipitation, consistent with the rain gauge observations. The case study verifies the statistical result well. In the meaning time, the hydrometeor classification results confirm that graupel develop deeper in H-only convective PF.