Numerical Simulation of Ice-Phase Processes Using a Double-Moment Microphysical Scheme and a Sensitivity Test of Ice Nuclei Concentration

In the context of the Weather Research and Forecasting (WRF) single column model (SCM), a double-moment bulk microphysics scheme, Morrison 2-mon (MOR), is selected to simulate a case of tropical convection from the Tropical Warm Pool International Cloud Experiment (TWP-ICE) field experiment. We then compare the results with observations and cloud resolution model results to evaluate the ability of the MOR scheme to describe the microphysics. The evidence shows that the MOR scheme is capable of properly reproducing the time and space distribution features of liquid and ice particles in the tropical cloud from both macro and micro points, as well as the characteristics of the temporal evolution of surface downward longwave radiation and outgoing longwave radiation (OLR) at the top of atmosphere. From our analysis of microphysical characteristics, we found that the dominant ice crystals cloud microphysical processes during the monsoon active period are the depositional growth of ice crystals, sedimentation of ice crystals, autoconversion of ice crystals to snow, and accretion of ice crystals by snow. Ice crystals occur predominantly in environmental temperatures less than ―20℃, and indirectly contribute to rain formation. The cloud microphysical processes of snow are predominantly its simultaneous sedimentation and depositional growth. Snow's strong deposition growth, which results in the consumption of much water vapor, may suppress ice crystal growth. In addition, the active melting of snow is an important factor for precipitation. During the monsoon depression, ice-phase microphysics become relatively simple and weaken, with the dominant cloud microphysical processes being sublimation, sedimentation, and depositional growth. Results from MOR scheme sensitivity tests, in which we changed the total number of ice nuclei in deposition mode and condensation freezing mode (N_dep), suggested that the responses of the macro and micro characteristics in ice-phase cloud to N_dep present marked linear features during the monsoon depression period. During this period, the N_dep increase favors the growth of ice crystals, but leads to smaller ice crystals in ice clouds and a higher cloud top. This results in reduced OLR decreases during the monsoon depression period for those microphysical processes primarily related to ice crystals, and vice versa. In the active monsoon period, the ice-phase cloud characteristics vary irregularly.