SIMULATION OF HYDROMETEOR SIZE SPECTRA EVOLUTION BY WATER-WATER, ICE-WATER AND ICE-ICE INTERACTIONS

Hydrometeors spectra evolution induced by coalescence processes is simulated using a microphysical model of the Berry and Reinhard (1974) type including hydrometeors of seven kinds: water drops, plate-like and columnar crystals, dendrites, snowflakes, graupel and hail. The sensitivity of spectra evolution to several parameters, such as air temperature, drop freezing rate, initial liquid water and ice particles content is studied. The Berry and Reinhard (1974) method was used for numerical integration of corresponding stochastic coalescence equations. A remarkable impact of ice processes on the evolution of hydrometeors of all types is demonstrated. The amount of large droplets in mixed phase experiments is significantly less than that in the experiment without the ice phase. The main factor causing the cessation of water spectrum broadening is connected with a crucial decrease of the coalescence rate between water droplets due to permanent elimination of the largest droplets from the drop spectrum as a result of ice-water interaction and drop freezing. The results of simulations show that when only coalescence processes are considered, relative amounts of snowflakes and graupel crucially depend on air temperature. Under comparably high temperatures it is snowflakes that are the dominating hydrometeor type, while under low temperatures graupel can be the dominating hydrometeor type due to an increased rate of drop freezing. A high sensitivity of snowflake/graupel content ratio to the initial size of crystals is demonstrated. Initial liquid water content in model experiments appears to be one of the most important factors determining the formation not only of graupel, but of snowflakes as well. Results indicate the existence of a ''critical'' minimum concentration of water drops and minimum broadness of the drop size spectrum, which means that when the corresponding values are smaller there is no large snowflake or graupel generation.