Prognostic Treatment of Snow and Graupel Number Concentrations in the WDM6 Scheme and Its Impact on Simulated Precipitation

The Weather Research and Forecasting (WRF) Double-Moment 6-class (WDM6) microphysics scheme only predicts the number concentrations for liquid-phase hydrometeors. Even though Park and Lim (2023) recently revised the WDM6 scheme by implementing the prognostic number concentration of cloud ice, the precipitating solid-phase hydrometeors such as snow and graupel are still treated as the single-moment approach. This study introduces the new version of WDM6 scheme by treating the number concentrations of snow and graupel as the prognostic variables. Therefore, the new WDM6 scheme predicts the number concentrations of all hydrometeors. By comparing simulation results from the new and original versions of the WDM6 scheme, we can identify the impact of prognostic snow and graupel number concentrations. The model integration has been conducted for four summer-precipitating (warm type and cold type) and seven winter-precipitating (cold-low type and warm-low type) convection cases. Our analysis shows that the new scheme produces a larger size of raindrops with a reduced number concentration than the original scheme, which is more consistent feature with the observation data from 2DVD. Therefore, the new scheme hinders raindrop evaporation and produces more surface. The new scheme improves the statistical skill scores such as the equitable threat score and probability of detection for all warm-type summer cases, which occur most frequently during the summer season over the Korean Peninsula.