On the basis of the precipitation and microphysics verification of the MM5 model numerical simulation of two wintertime precipitation events occurring in the Sierra Nevada, the roles of cold microphysical processes in the surface precipitation variability are investigated and the sensitivity tests of surface precipitation to various microphysical factors associated with cold microphysical processes are made.
Besides the verification of the simulated precipitation and wind field for these two winter precipitation cases, the verification of the microphysical properties simulated by the GSFC ice microphysics scheme in the MM5 model is unprecedented and pioneering. This work describes the method to relate model simulated cloud microphysical variables such as the mixing ratios of rain, snow, graupel, cloud ice, and cloud water to the observed or estimated variables from cloud microphysical probes such as 1D-C, 2D-C, and 2D-P.
It is found that without the Hallett-Mossop ice multiplication process the GSFC ice microphysical scheme fails to catch the observed dense cloud ice particle region around -5„aC in two cases. The inclusion of this second cloud ice production processes in the model microphysics scheme helps to reproduce this detected region with the order of magnitude of number concentration of cloud ice close to the detected ones by the 1D-C and 2D-C probes, indicating this ice-producing process is vital for accurate simulation of microphysical properties although its existence only increases mean area precipitation in the Folsom Lake basin not significantly, only 5% for two cases.
However, the simulation of number concentration of precipitating particles depends on the accurate selection of the intercept N0 of snow particle size distribution. This parameter can modify mean area precipitation in the Folsom Lake basin more significantly than the Hallett-Mossop processes, about 7-10%. The surface precipitation amount and its spatial variability are more sensitive to the selection of formula of the terminal velocity of snow particles than the value of the intercept N0 of snow particle size distribution, with the precipitation difference in the windward slope and in the lee side up to 20% and 50%, respectively.
The comparison between the simulations of with/without cold microphysical processes reveals that the inclusion of cold microphysical processes in the model microphysics scheme is vital. It not only modifies the magnitude of surface precipitation but also the spatial distribution of surface precipitation especially in the mountainous area. The larger differences existed in the higher elevation area of windward slopes and the lee sides of mountain areas with the warm rain schemes underestimating or failing to produce any surface precipitation in these areas.