Raindrop Size Distribution Characteristics of Two Short-Term Heavy Precipitation Processes in Western Tianshan, China

Short-term heavy precipitation processes (8∙2 and 8∙18 processes) took place on August 2 and 18, 2020, at the Xinyuan station of West Tianshan, China. In this work, the raindrop size distribution (DSD) properties of these precipitation processes were investigated using high temporal and spatial resolution observation data from an automatic weather station, a GPS/MET water vapor detector, a two-dimensional video raindrop spectrometer, and the Fengyun-2G- satellite. The results show the following: (1) Both processes produce local short-term heavy precipitation under the favorable background of the Central Asia vortex. The 8∙2 process has strong stages and is caused by the isolated β mesoscale convective system (MCS) along the westerly flow in front of the vortex. The water vapor originates from the vortex itself, and its transport and convergence are relatively weak. The 8∙18 process lasts for 100 min without obvious periodicity and is caused by the β-MCS generated in stratus clouds. The water vapor comes not only from the low vortex but also from the abundant water vapor transport outside Xinjiang. (2) The two short-time heavy precipitation processes exhibit obvious differences in DSD. The contribution of the number concentration of raindrop particles with diameters (D_m) <2 mm in both processes is about 97%; that is, the concentration of small particles accounts for the vast majority in the precipitation process. In the 8·2 process, the contribution of particles with D_m < 2 mm to precipitation is 66.75%, and the contribution of large particles with D_m > 4 mm to instantaneous strong rainfall is large. The particle spectrum of convective precipitation is scattered: raindrops with large particles and small concentrations and those with small particles and high concentrations are observed. In the 8·18 process, the contribution of particles with D_m < 2 mm to precipitation is 86.34%, of which the contribution of particles with diameters 1–2 mm to precipitation is 71.99%, and there are no particles with D_m > 4 mm. The particle spectrum of convective precipitation is relatively concentrated, and the concentration of particles with D_m >2 mm is considerably lower than that observed for the 8∙2 process. The average and maximum values of horizontal polarization radar reflectivity (Z_H), differential reflectance factor (Z_DR), and specific differential phase shift (K_DP) are significantly smaller than those observed in the 8·2 process. (3) The two-dimensional video disdrometer detection data in the summer during the 2020–2021 period are used for the analysis of the DSD of convective precipitation. The raindrop spectrum characteristics of convective precipitation indicate the interannual difference, which is characterized by large raindrop particles and low concentration, exhibiting the nature of continental convective precipitation. It was found that the convective precipitation in the West Tianshan Mountains is characterized by small raindrop particles and high concentrations. Z_DR and K_DP are significantly smaller than the convective precipitation in the monsoon area. The fitting relationship of R (Z_H, Z_DR) and R (K_DP, Z_DR) with polarization is obviously better than that of R (Z_H) and R (K_DP).