Analyzing the Characteristics of Raindrop Size Distributions of the'7·20'Torrential Rain in Zhengzhou on 20 July 2021

The microphysical characteristics is important for the formation mechanism of rainstorm which is the result of multi-scale conditions and the interactions of macro and micro physical processes.From 15:25 （Beijing Time， the same as after） to 17:27 on July 20， 2021， a strong mesoscale convective system affected Zhengzhou， and the accumulated precipitation was 312.1 mm， which was referred to as extreme rainstorm.Excluding the period of extremc rainstorm， from 08:00 to 20:00 on the July 20， the accumulated precipitation was 183.4 mm， which is referred to as rainstorm.Based on the observations data of disdrometer at Zhengzhou station， the characteristics of raindrop size distributions（DSDs） and integral parameters of extreme rainstorm and rainstorm are analyzed.The results showed that： （1）In rainstorm， the average DSDs increased with the increase of rain rate.The concentrations of diameter less than 1 mm increased less， and concentrations of large diameter which is greater than 2 mm increased faster.In extreme rainstorm， the average DSDs increased with the increase of rain rate， and the concentrations of each diameter increased significantly.Meanwhile， the differences between the concentrations of average DSDs at different rain rate were complex.Somewhile there were distinct differences in the concentrations of small raindrops， and there were also great differences in the concentrations of large raindrops.The differences of average DSDs of different rain rate categories lead to different parameters variation with the increase of rain rate.For rainstorm， the Dm increased with the increase of rain rate， while lgNw increased slightly， indicating that the main source for the increase of rainstorm intensity was the increase of particle diameter， and the secondary factor was increase of particle concentrations.For extreme rainstorm， when the rain rate was between 50 and 100 mm·h-1， Dm（lgNw） increased （decreased） with the increase of rain rate， while the rain rate was greater than 100 mm·h-1， Dm and lgNw increased slightly with the increase of rain rate， suggesting that the formation mechanisms of DSD in different rain rate categories of extreme rainstorm may be different.（2）The distribution of lgNw-Dm showed that the DSDs of rainstorm were mainly continental-like， and its formation mechanism was mainly warm-ice mixture process and a small amount of ice-based process.The scatter plot of lgNw-Dm ofextreme rainstorm with rain rate less than 100 mm·h-1 mainly clustered on the upper and right side of the continental-like area， and the formation mechanism of extreme rainstorm was mainly warm-ice mixture process and ice-based process； the scatter plot of lgNw-Dm of extreme rainstorm with rain rategreater than 100 mm·h-1were mainly distributed on the upperside of the continental -like area， and the formation mechanism was mainly warm-ice mixture process.（3）There were a small proportion（about 2.8%）of the equilibrium raindrop size distribution（EDSD） and a large proportion （about 60.5%）of transition DSDs in rainstorm.There was no EDSD in the extreme rainstorm， while the transition DSDs had a very high proportion （about 83.9%）， indicating that the breakup process played a much more role in the extreme rainstorm.