A Numerical Study For The Microphysical Processes Of Ice Pellets With A Spectral(Bin) Cloud Model

Hail is solid precipitation resulting in severe weather disasters in china. In order to mitigate the damage of hailstones, we should further study the growth mechanism of hails. The Graupel and the ice pellet as the hail embryos are the main source of the deep convective precipitation. The density difference between the graupel and the ice pellets lead to the different terminal falling velocity. As a result, the cloud microphysical processes and the spatial and temporal distribution of precipitation will change. However, most of the bin cloud models do not consider the microphysical processes of the ice pellet.Based on the two-dimensional axisymmetric convective cloud model developed by Israel Tel Aviv University with detailed treatments of both the warm and the cold microphysical processes, we developed a bin microphysical model including water drops, ice crystals, snow, graupel and ice pellets. The density of ice pellets was assumed to be 0.9g/cm’3. The ice pellet spectrum was divided into 34 bins, with mass doubling in each bin. The following ice pellet microphysical processes were computed:formation of ice pellets, ice multiplication, collision coalescence with themselves and with other species of particles, deposition/sublimation of ice pellets, melting and sedimentation of ice pellets. We used the improved model to simulate the ideal case of the severe convective cloud and analyzed the characteristics of the dynamical fields and hydrometeor distributions compared with simulations by the original model. Meanwhile, the impact of aerosol on cloud dynamical and microphysical processes was studied by the improved model.We used the improved model and the original model to simulate and analyze the hydrometeor distributions, respectively. The results showed that the improved model could successfully simulate the ice pellet formation process. The formation of ice pellets could produce a large amount of ice crystals due to their higher terminal velocities, which results in themselves falling into the ice multiplication zone that is determined by temperatures and concentrations of cloud droplets. Consequently, graupel concentrations decreased due to the reduction of formation pathways compared with the original model.For the ice pellet formation process, the characteristics of the dynamical fields and the microphysical processes were analyzed. The results showed that there was a liquid water accumulation zone before ice pellets formation, because maximum area of liquid water located above the maximum vertical velocity zone. At the stage of ice pellet formation, the liquid water accumulation zone was above the level of 0℃. The ice pellets were formed by water drop freezing and graupel riming with water drop radius greater than 100μm. At the stage of ice pellet growth, the ice pellets grew by accretion of supercooled water, leading to ice pellet water content increasing and liquid water content decreasing. The maximum of the number concentration and the water content of ice pellets were 4L-1 and 5g/kg, respectively, At the stage of ice pellets falling, the number concentration and the water content maximum of ice pellets reached 0.1L-1 and lg/kg, respectively.In order to study the effects of aerosols on the characteristics of the dynamical fields and the ice pellets formation process, sensitivity tests were conducted. The results showed that the warm rain was inhibited due to the enhanced concentration of small cloud droplets in the polluted background conditions. The large amount of liquid cloud water promoted the growth of ice particles. In the clean condition, higher concentration of large aerosol particles was favor to produce large cloud droplets and promoted precipitation. As a result, the concentrations and sizes of raindrops, graupel and ice pellts increase. Larger ice pellets and graupel particles with higher terminal falling velocities can even fall into the ground.The modeling results showed that the improved model could successfully simulate the ice pellet formation process. The improved spectral microphysical scheme is expected to be coupled into the WRF (Weather Research and Forecasting model) to study the formation mechanism of hails in more complicated dynamical fields.