Torrential rainfall responses to ice microphysical processes during pre-summer heavy rainfall over southern China

In this study, the effects of key ice microphysical processes on the pre-summer heavy rainfall over southern China during 3–8 June 2008 were investigated. A series of two-dimensional sensitivity cloud-resolving model simulations were forced with zonally uniform vertical velocity, zonal wind, horizontal temperature, and water vapor advection data from the National Centers for Environmental Prediction (NCEP)/Global Data Assimilation System (GDAS). The effects of key ice microphysical processes on the responses of rainfall to large-scale forcing were analyzed by comparing two sensitivity experiments with a control experiment. In one sensitivity experiment, ice crystal radius, associated with depositional growth of snow from cloud ice, was reduced from 100 μm in the control experiment to 50 μm, and in the other sensitivity experiment the efficiency of the growth of graupel from the accretion of snow was reduced to 50% from 100% in the control experiment. The results show that the domain-mean rainfall responses to these ice microphysical processes are stronger during the decay phase than during the onset and mature phases. During the decay phase, the increased mean rain rate resulting from the decrease in ice crystal radius is associated with the enhanced mean local atmospheric drying, the increased mean local hydrometeor loss, and the suppressed mean water vapor divergence. The increased mean rain rate caused by the reduction in accretion efficiency is related to the reduced mean water vapor divergence and the enhanced mean local hydrometeor loss.