Supercell Sensitivities to Graupel and Hail Fall Speeds

Supercell storm simulations are known to be very sensitive to physics parameterizations and the microphysics scheme in particular. Previous studies have noted differences for wholesale changes in large ice hydrometeor characteristics, but here the focus is narrowed to fractional fall speed variations. Hydrometeor fall speeds play a central role in microphysical production rates (e.g., accretion, precipitation rate) and mass distribution throughout the storm. The Collaborative Model for Mesoscale Atmospheric Simulation (COMMAS) is employed with two microphysics schemes: the NSSL 2/3-moment bulk scheme (with either fixed or predicted density of graupel and hail) and the Takahashi bin scheme (fixed density graupel and hail). Fall speed factors (in the range of 0.7 to 1.4) correspond to substantial changes in total precipitation, cold pool area and intensity, hail production, and storm evolution. For example, for the environment of the 29 May 2004 "Geary, OK" supercell storm shows a monotonic relationship between integrated cold pool perturbation temperature and graupel/hail fall speeds (Figure, for NSSL bulk scheme with fixed particle density). Some effects of fall speed changes can be partially offset by compensating adjustments to collection efficiencies between graupel/hail and droplets. Hail production is sensitive to graupel fall speed because of the wet growth requirement to convert graupel to hail. Microphysical differences are also examined with simulated dual-polarization radar variables such as differential reflectivity, correlation coefficient, and specific differential phase. Results are also applicable to other models that include the 2-moment NSSL scheme (WRF and CM1).