Simulation of deep tropical clouds using a spectral microphysics cloud model HUCM

A deep tropical convective cloud is simulated using a spectral microphysics Hebrew University Cloud Model (HUCM). Water drops, plate and columnar and branch ice crystals, snowflakes, graupel and hail/frozen drops, as well as atmospheric aerosols are described using the number density size distribution functions, calculated in course of model integration. All size distribution functions contain 33 mass bins. The model takes into account the following microphysical processes: nucleation of cloud condensation nuclei; formation (nucleation) of ice crystals, condensation growth/evaporation of droplets; growth /sublimation of ice particles; accretion; freezing of drops; melting of ice, coalescence of drops, drops and ice and ice particles; ice multiplication, riming of ice, breakup of drop and snowflakes. The model is specially designed to take into account the effects of atmospheric aerosol on the cloud development and precipitation formation. The stochastic equation of collisions is used for the description of the collision growth of cloud hydrometeors. A precise description of droplet nucleation is based on analytical calculations of supersaturation with respect to water and ice. Using the values of supersaturation, the sizes of aerosol particles to be activated and the corresponding size of nucleated droplets are determined. Using the model we study the mechanisms of ice formation and its contribution to precipitation formation.

Special set of experiments is dedicated to simulation of cloud-aerosol interaction. It is shown that cloud evolution, precipitation rate and accumulated rain are very sensitive to the concentration and the size distribution of aerosol particles.