Simulations of an extreme rainstorm event (1056.7 mm/day) along the South China coast: Cloud microphysical processes and maintenance mechanism of rainstorm

Abstract

The coastal area of Guangdong Province underwent an extreme rainstorm in Aug. 2018. In this paper, using the convection-permitting WRF simulation results, a comparative analysis was given to the cloud microphysical conversion processes simulated under four cloud microphysics schemes, i.e. WSM6, WDM6, WSM7 and WDM7, as well as the accompanying latent heat budget. The maintenance mechanism of the rainstorm is discussed. In all four schemes, water vapor condensing into cloud water and accretion of cloud water by rain water were the top two conversion processes. In WSM6/WDM6 scheme, the third largest conversion process was accretion of rain water by graupel; in WSM7/WDM7, however, the third largest was cloud water evaporating to water vapor. The most important source of rain water was accretion of cloud water by rain, but the differences in hydrometeors conversion between rain water and graupel, snow and hail were the leading cause of differences in simulated rainfall under different schemes.The top three releasing latent heat processes were water vapor condensing into cloud water, water vapor deposited to cloud ice, and snow sublimation. The top two absorbing latent heat processes were cloud water and rain water evaporating to water vapor. In WSM6/WDM6/WSM7 schemes, the third major process of absorption of latent heat was snow sublimating to water vapor, in WDM7, however, it was hail melting into rain water. Compared with ice-phase processes, liquid-phase processes provide more latent heat. Differences in the release and absorption of latent heat were found in the mid-high troposphere. Hydrometeors in the troposphere maintain efficient conversion and release large amounts of latent heat that formed positive feedback with convection, which was the critical reason for the development and maintenance of the extreme rainstorm.