Numerical Simulation of Aerosol Effects on the Physical Processes of Hail Formation in Xinjiang

The weather research and forecasting model with a spectral-bin microphysical scheme (WRF-SBM) was used to simulate a hailstorm that occurred in Xinjiang during a summer. The effects of aerosol concentration on the microphysical characteristics and precipitation of the hailstorm and hail formation mechanism were studied via sensitivity tests. The results show that the convection of the hailstorm increases in strength with increasing aerosol concentration. At the development stage of the hailstorm, the liquid water content increases with increasing aerosol concentration, and the ice water content is the highest under moderate pollution conditions. With the increase in aerosol concentration, the hail mixing ratio first increases and then decreases. Under moderate pollution conditions, the cloud droplet size is appropriate, and the amount of supercooled water is relatively sufficient, which are favorable conditions for the transformation of water from the liquid to ice phase, thus 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 hailstorm development. Further, the freezing of droplets becomes the main source of hail formation for a short while. However, once the hails are formed, they grow rapidly by collecting the supercooled water, which becomes the dominant process of the hail growth. The severe pollution condition will delay the start of hail formation processes. With enhanced aerosol loading, the surface accumulated liquid precipitation increases, whereas, the ice-phase precipitation first increases and then reduces. The moderate aerosol concentration leads to a large amount of hail mixing ratio and a high percentage of hail in the ice-phase precipitation. However, with further increase in aerosol concentration, both values are reduced. Therefore, we propose “optimal aerosol concentration,” which is the most suitable condition for hail growth.