Numerical Study on Microphysical Processes of Two Different Snowfall Cases in Northern China

In this paper, two snowfall cases under different weather conditions in northern China are simulated by using the meso scale model MM5. Two-way nesting structure of domains is designed for each case. Among the explicit schemes of MM5 , the Reisner graupel scheme is selected to describe the microphysical process.The simulated snow-bands of two cases are basically consistent with observations. The simulated results of microphysical processes are mainly discussed. The hydrometeors and their sources and sinks under different weather backgrounds are described. The feedback effects of microphysical processes on the thermal and dynamic processes are also discussed. Method that outputs the accumulative sources and sinks per hour is used to analyze the distribution characteristics of hydrometeors during the strongest snowfall period. Two sensitivity tests (called heat test and drag test) are conducted to examine the effects of microphysical processes on cloud produced by the latent heat and drag force. Results have shown that the distribution of particles has a close relation with temperature. The tem- perature of Beijing snowfall is under 0℃ and there exist vapor and solid phase particles, while Liaoning snowfall has vapor, liquid, and solid phase particles due to the warm temperature. The distribution of these particles is not the same at different development stages. From the analyses of the characteristics of sources and sinks, it is found that snow is mainly produced by the deposition and accretion with ice. Cloud water is crucial to graupel. The melting of ice-phase particles enhances the rain production. The results of heat tests and drag tests reveal that the microphysical processes have interacted with the dynamic and thermal processes. Latent heat release of hydrometeors feeds back positively on snowfall while the drag force not.At last, comparisons of simulated results have been done between the two different kinds of snowfall cases.The microphysical processes of Liaoning snowfall case is more complicated than those of Beijing snowfall case. The values of the cloud variables are larger and the interactions between the microphysical processes and the thermal and dynamic processes of Liaoning snowfall case are stronger than those of Beijing snowfall case.