Numerical Simulation Of The Aerosol And Terrain Effects On The Physical Processes Of Hail Formation In Xinjiang

Terrain and aerosols play important roles in the development of hailstorms.The Weather Research and Forecasting model with a spectral-bin microphysical scheme(WRF-SBM)was used to simulate a hailstorm occurred in summer of Xinjiang.The effects of terrain and aerosol(acting as CCN and IN respectively)on microphysical characteristics and precipitation of the hailstorm as well as formation mechanism of hail are studied by sensitivity tests,and the results are compared with the radar data.The main conclusions are as following:Terrain has an impact on the location,range and start-up time of hailstorm.When the terrain height of Tianshan reduced,the convection occurred in the south of Tianshan starts earlier,otherwise,the convection starts later.The existence of the Tianshan topography block the transportation of water vapor and airflow from the northwest and north,to a certain extent,it suppresses the development of hailstorm located in south of the Tianshan Mountains.Decreased terrain height leads the surface accumulated liquid precipitation to increase while the ice phase precipitation to reduce,and the results reverse when terrain height rises.Moreover,terrain also has an influence on the microphysical structure of the cloud,for example,the decrease of terrain height makes hail content increase significantly.CCN affects the formation of cloud droplets in the cloud.The convection of the hailstorm is stronger with larger CCN concentration.At the development stage of the hailstorm,the liquid water content increases with the increase of CCN concentration and the ice water content is the highest under moderate polluted condition.The hail mixing ratio increases first and then decreases with the increase of CCN concentration.Under moderate polluted condition,there is appropriate cloud droplet size and relatively sufficient supercooled water,which is favor for the transformation of water from liquid phase to ice phase and therefore contributing to hail growth.Hail is initially formed by the riming of supercooled liquid water by ice crystals,but this process is rapidly weakened after the development of hailstorm.Then the freezing of droplets becomes the main source of hail for a short while.However,once the hail is formed,it will grow rapidly by collecting the supercooled water,which becomes the dominant process of the hail growth.The severe polluted condition will postpone the onset of hail formation processes.With enhanced CCN loading,the surface accumulated liquid precipitation is increased while the ice phase precipitation is increased first and then reduced.Therefore,we propose “the optimal CCN concentration” that is most suitable for hail growth,which is also the concentration that should be paid more attention in the work of artificial hail suppression.IN affects the formation of ice crystals in the cloud.With enhanced IN loading,more liquid water is consumed to generate a large number of ice crystals.Moreover,the formation mechanism of ice crystals is different from that when CCN is increased: When CCN concentration increases,a large number of small cloud droplets are lifted to the higher altitude,and the ice crystals generated by the freezing process increase;while IN directly participates in the heterogeneous nucleation processes in the cloud,and more IN leads to more ice crystals which are generated in the warmer environment.The hail content decreases with IN concentration increases.Larger IN concentration will promote the process of ice crystals riming with supercooled liquid water to form hail,and inhibit the process of hail formation by immersion freezing.The surface accumulated liquid precipitation is increased with larger IN concentration,while the ice phase precipitation is decreased.