Development of a Single-Moment Cloud Microphysics Scheme with Prognostic Hail for the Weather Research and Forecasting (WRF) Model

Abstract

This study examines the effect of hail on microphysical processes and precipitation. The Weather Research and Forecasting (WRF) Single-Moment 7-class Microphysics (WSM7) is developed by introducing the hail hydrometeor as an additional prognostic water substance within the WSM6 scheme, which are four-ice and three-ice schemes, respectively. In an idealized 2D squall case, the WSM7 scheme with hail tends to enhance the accretion rate of ice particles due to the faster sedimentation of hail than that of graupel in the WSM6 scheme. The amount of hail is largely compensated with the reduction of graupel, but its maximum at lower altitudes. Weakened accretion of graupel by snow at higher altitudes maintains the snow aloft, and increases of it at the mid-level. The reduced sum of graupel and hail at the melting level leads to a decrease in the mixing ratio of rain in the WSM7 experiment, which is compensated by falling hail. In 3D squall line experiments, the WSM7 scheme tends to enhance convective activities in the leading edge of the squall line, whereas the precipitation intensity in the trailing stratiform region decreases. This is due to the fact that the addition of hail plays a role in suppressing light precipitation and increasing heavy precipitation activities.