An Observational And Simulation Study Of The Vertical Evolution Of Raindrop Size Distribution On Mt. Huang

Raindrop size distribution(RSD)described the trend of number concentration of raindrops with scale in a certain size range,it is very important for improving the precision of radar quantitative estimation of precipitation and optimizing of precipitation parameterization scheme in climate models.However the most study of raindrops focuses on the analysis of individual cases or certain cloud systems,and the domestic model research on the vertical evolution of raindrops is relatively small.This paper based on the RSD data observed with PARSIVEL distributions deployed at different altitudes on Mt.Huang from June to July 2011,several microphysical parameters and the Gamma function fitting to raindrop spectra as well as the falling velocity of raindrops at different altitudes for different rain types(stratiform and convective)were compared and analyzed.In order to examine the role of microphysical processes included evaporation,collision coalescence and breakup,and the sedimentation of drops in vertical evolution of RSD,a one-dimensional bin model for raindrops were developed,finally we explained some observation phenomena through sensitivity experiments.The main conclusions are as following:The RSDs from all sampling positions were in good agreement with the gamma distribution,so we used this function to describe the initial distribution in the model.Then result satisfactorily reproduced the vertical evolution features of drop spectrum in Mt.Huang.According to the results of simulations,sedimentation provided drops larger than 1mm for lower levels in the early stage of precipitation,then we found two peaks produced by collision coalescence and breakup occurred at diameters of 0.25 and 0.60mm,these two peaks would be smoothed out by evaporation and at the same time generated drops below 0.1mm.Convective precipitation had higher rainwater content,more intense rainfall rate,larger raindrops and higher raindrop concentrations compared with the stratiform precipitation at the same height.In terms of evolutionary features,during the stratiform precipitation which has lower rain water content(or rain rate),the evaporation and collision induce small significant changes to the RSDs,result in a more stable evolution in vertical direction then the convective precipitation.For both convective and stratiform precipitation,the RSDs in difference altitudes mainly deferred in drop size.The mountaintop is often in the cloud with the smallest average scale,in addition the raindrops at the mountain bottom is smaller than the mountainside because of the breakup under the cloud base.According to the analysis of velocity,we found there are large spreads at virtually all the diameters,besides drop sizes,raindrop fall velocity is also affected by many other factors,such as air density,updrafts and downdrafts,raindrop coalescence/breakup,turbulence and so on.