Abstract: Graupel and frozen drops are the main source of deep convective precipitation.The terminal falling velocity of graupel and frozen drops are different because of the difference between their densities.As a result,cloud microphysical processes and the temporal and spatial distribution of precipitation will change.Based on the two-dimensional axisymmetric convective cloud model developed by Tel Aviv University in Israel with detailed treatments of both the warm and cold microphysical processes,the authors increased the bins of hydrometeors from 34 to 40 and modified the graupel and snow densities and developed a bin microphysical model that includes water drops,ice crystals,snow,graupel,and ice pellets.The authors used the improved model to simulate an ideal case of severe convective cloud and analyzed the characteristics of the dynamical fields and hydrometeor distributions.Results of the present study were compared with simulations by the original model.The results showed that:(1) Ice pellets can produce a large amount of ice crystals due to their high terminal velocities,which results in the falling of ice pellets into the ice multiplication zone that is determined by the temperature and concentration of cloud droplets;(2) there is a liquid water accumulation zone before the ice pellet formation,because the maximum area of liquid water is located above the maximum vertical velocity zone.At the stage of ice pellet formation,the liquid water accumulation zone is above the level of 0℃.The ice pellets are formed by water drop freezing and graupel riming with the water drop radius greater than 100 μm.At the stage of ice pellet growth,the ice pellets grow due to the accretion of supercooled water,leading to ice pellet water content increase and liquid water content decrease.The modeling results showed that the improved model could successfully simulate the ice pellet formation process.The improved spectral microphysical scheme will be coupled into the WRF (Weather Research and Forecasting model) to study the formation mechanism of hails under more complicated dynamical conditions.