Double-moment approach for snow and graupel in the WDM6 scheme and its effects on simulated precipitation

The Weather Research and Forecasting (WRF) Double-Moment 6-class (WDM6) microphysics scheme only predicts the number concentrations for CCN and liquid-phase hydrometeors such as cloud water and rain. Although the double-moment approach for the cloud ice is recently introduced in the WDM6 scheme by Park and Lim (2023), the single-moment approach, in which only mixing ratio is prognosed, is still employed for solid-phase precipitating hydrometeors such as snow and graupel. In this study, the double-moment approach is introduced to WDM6 for all hydrometeors by adding prognostic number concentration of snow and graupel. To evaluate the effects of prognostic snow and graupel number concentrations, simulated results between the new and original versions of WDM6 scheme are compared. The four summer-precipitating (cold-type and warm-type; Kim et al. 2019) and seven winter-precipitating convection cases (cold-low type and warm-low type; Ko et al. 2022) are selected to evaluate the new scheme. In comparison to the original WDM6 scheme, the new scheme exhibits increased snow mixing ratio, except for cold-type summer cases. Additionally, the new scheme reduces the graupel mixing ratio and rain number concentration for all cases. In the new scheme, the raindrop size becomes larger due to the reduced rain number concentration, which is more consistent results with the observation data from 2DVD. Furthermore, larger raindrop size in the new scheme makes the evaporation inefficient. Therefore, the new scheme produces more surface precipitation than the original one. Meanwhile, among total 11 cases, the new scheme improves the equitable treat score (ETS) for eight cases and probability of detection (POD) for seven cases.   *This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government. (MSIT) (RS-2023-00208394)