Impact of Drop Breakup and Hail Fall Speed on Supercell Cold Pools in a Multiparameter WRF/DART EnKF Simulation

An ongoing study is considering the ability of numerical weather prediction microphysical parameterizations to properly simulate supercell cold pools. Inaccuracies in these parameterizations have led typically to an overestimation of high-level clouds, precipitation amounts, and magnitude of evaporative cooling, which impact the evolution and strength of the supercell updraft, vorticity generation, and cold pool. Two-moment microphysical schemes address several issues with particle size distributions, but challenges remain to further improve the effectiveness of these parameterizations. For instance, the method of raindrop breakup and hail fall speeds used in these schemes lead to a large difference in cold pool temperature and evolution.

This study investigates a case from the Verification of the Origins of Rotation in Tornadoes Experiment 2 (VORTEX2) to determine the effect of changes in the aggressiveness of a drop breakup scheme and hail fall speed present in the Morrison microphysics parameterization on a supercell cold pool. The full physics simulations are run with the Data Assimilation Research Testbed (DART) software, using an EnKF technique coupled with the Weather Research and Forecasting (WRF) model, where WSR-88D and mobile radar radial velocity data are assimilated onto a 1 km domain every two minutes. The EnKF technique is used in order to minimize the initial condition error and otherwise best produce the observed atmospheric state, allowing for a focus on differences that can be attributed to the changes in drop breakup and/or changes in the fall speed of hail. Each ensemble member has varying drop breakup aggressiveness and a different hail fall speed leading to a larger spread of realistic values. Results will show that varying the drop breakup parameter and changes to the hail fall speed lead towards a more accurate cold pool in the ensemble mean or a subset of the ensemble mean when compared to observations taken at a high spatial and temporal resolution by 12 StickNet probes that gathered thermodynamic and kinematic data of the supercell as it passed over them. The changes in drop breakup and hail fall speed lead to differences between the observations and the simulations in terms of areal coverage, shape, and magnitude of the cold pool.