Impact of Parameters Related to Ice Crystal on the Simulation of Winter Precipitation over the Korean Peninsula

Cloud microphysics parameterization has several ice-crystal-related parameters that define the characteristic of ice crystal. Weather Research and Forecasting (WRF) Double-Moment 6-class (WDM6) parameterization scheme adopts the fall velocity-diameter and mass-diameter relationships from Heymsfield and Iaquinta (2000, HI00 hereafter) with the assumed single-bullet shape of ice crystals, and the mean mass-weighted terminal velocity-mixing ratio relationship from Heymsfield and Donner (1990, HD90 hereafter). There are a total five parameters that define ice-crystal characteristics, and these parameters vary according to different shapes of ice crystals, contributing to uncertainties of simulated precipitation. To assess these uncertainties, we generate 50 sampling sets using Latin hypercube sampling within the recommended range from previous studies. Numerical experiments are conducted for two major types of winter precipitation, namely Air-mass Transformation (AT) and Ease-coast Terrain effect (ET) types, over the Korean peninsula. The simulation results indicate that parameters defining the mass-diameter relationship are most sensitive for simulating precipitation in the AT type, while parameters defining the fall velocity-diameter relationship are most sensitive for the ET type. Sensitivity experiments are designed by adjusting the sensitive parameters for each type by ±20% to mitigate biases in surface precipitation observed in the control experiments. In the AT type, the sensitivity experiment simulates more solid-phase precipitable hydrometeors, such as snow and graupel, resulting in increased precipitation over the region with a negative bias. Conversely, in the ET type, the sensitivity experiment reduces the amount of snow and graupel, leading to a decrease in precipitation over the area with a positive bias. Our analysis underscores the high priority of tuning parameters related to ice-crystal characteristics to reduce uncertainty in precipitation simulations, depending on the type of winter precipitation.

 

Key words: Ice crystal, Uncertainty parameter, WDM6, Winter precipitation

 

Acknowledgement: This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (RS-2023-00272424) and Korea Meteorological Administration Research and Development Program under Grant (RS-2023-00240346)