Effects of cloud microphysics on monsoon convective system and its formation environments over the South China Sea: A two-dimensional cloud-resolving modeling study

The Weather Research Forecast (WRF) model imposed by the large-scale advective forcing derived from observed data representing an average of gridded data fields over the South China Sea Monsoon Experiment (SCSMEX) was used to study the effects of cloud microphysics on monsoon convective systems and its formation environment over the tropical ocean. The temporal variation of the WRF model-simulated rain rate is in good agreement with the observed data. The sensitivity experiments with ice runs and ice-free runs show that the ice run can produce less rain prior to and during the onset of the monsoon period: the mean decrease can be -5 mm d^-1 and the maximum decrease can be more than 60 mm d^-1 during the post onset of the monsoon period, and about 6% decrease of the domain-averaged accumulative rainfall. The mechanism that induces this phenomenon was investigated through the budget of cloud radiation forcing, the variations of latent heating and cooling, and that of terminal fall velocities of cloud hydrometeors due to phase change in this paper, and show that at the top of atmosphere the net cloud radiation forcing on both ice cloud and water cloud can be neglected. However, on the ocean surface the ice run may reduce about 14% of downward shortwave radiation and increase about 1.6% of downward longwave radiation on average, which can produce a net cooling process and directly reduce the sensible and latent heat fluxes on the ocean surface and result in the decrease of precipitation over the whole region. Therefore, the effect of ice microphysics is dominated through the radiation processes instead of microphysics itself. In addition, ice cloud can lead to the atmosphere being warmer and drier at middle and upper levels and stabilize the atmosphere and suppress the further development of convections.