A New Method for Representing Mixed-phase Particle Fall Speeds in Bulk Microphysics Parameterizations

Here we present a simple method of improving bulk mixed-phase microphysical schemes to allow for a more realistic representation of partially rimed particles. The new procedure unifies the snow and graupel particles by assigning a single fallspeed to both that is weighted by the mixing ratios, and applying that fallspeed to both sedimentation and accretion processes. This avoids the problem of the species separating out by sedimentation as graupel forms, and the further problem of graupel then accreting snow too quickly because of its high relative fallspeed. Instead the unified graupel/snow moves together and evolves in its relative ratio due to riming, behaving as intermediate or partially rimed particles. Tests of the new method were carried out using the Weather Research and Forecasting (WRF) Single-Moment 6-class (WSM6) microphysics scheme in a high-resolution idealized simulation, and mesoscale heavy precipitation events in the summer and winter over Korea. The effect of the new accretion rates on cloud structure and precipitation was found to be greater than that of the changed sedimentation alone. Verification of these tests showed a much-reduced production of graupel and more snow, influencing the cloud structure and surface precipitation fields. The scheme shows promise in improving precipitation intensity and precipitation type forecasts.