Development of the Single-Moment Cloud Microphysics Scheme with Prognostic Hail for the Weather Research Forecasting Model

This study examines the effect of hail processes on microphysical processes and precipitation. A 4-ice scheme with additional hail category, which is called the Weather Research and Forecasting (WRF) Single-Moment 7-class Microphysics (WSM7), has been developed by adding prognostic hail. The sensitivity simulations are performed for 2D idealized squall line and 3D cases. 3D real-case experiments are also conducted to investigate the effect of density of hail and interaction between hydrometeors in WSM7. The variable hail densities used in sensitivity simulations are 500, 700, 912, and 1100 kg m-3. Hail with larger density increases the accretion and precipitation rate because of the faster sedimentation velocity. Owing to hail addition, the mixing ratio of graupel decreases below 10 km. Graupel and snow with decreased sedimentation velocity reduce the accretion and remain at higher altitude. The lessened sum of graupel and hail leads to decrease in a mixing ratio of rain at the melting level. The precipitation based on hail is concentrated in the convective leading line, and the precipitation regime is narrow. Moderately falling graupel is easily transported backward. The area of the trailing stratiform region decreases. The frequency of heavy rainfall increases when the WSM7 scheme is used. The evolution of the surface precipitation in the squall line in the WSM7 experiment is fairly comparable to that of the WSM6 experiment for both 2D and 3D squall line cases.