The sensitivity of simulated supercell precipitation to microphysical parameters

Supercell thunderstorms are often associated with heavy rainfall, large hail and strong winds. Predicting such storms, the amount and type of precipitation they will produce and their flash flooding potential are important forecasting issues. Producing accurate simulations of precipitation distribution and totals has proved challenging to the modeling community. It appears that simulated precipitation may be sensitive to several of the microphysical parameters in numerical models. If we are to produce more accurate forecasts of the precipitation produced by severe storms then the impact of these microphysical parameters needs to better understood. The aim of the research presented here is to investigate the sensitivity of the precipitation produced by severe thunderstorms to changes in mean hail diameter and hydrometeor distributions, to the inclusion of the ice species, and to the use of the single-moment or two-moment bulk microphysics.

Idealized simulations have been conducted using the Regional Atmospheric Modeling System (RAMS) developed at Colorado State University. A single model grid with 1 km grid spacing in the horizontal and variable spacing in the vertical is utilized. Convection is initiated using a warm bubble that is 10 km in diameter and the simulations are run out for 2 hours. The bulk microphysical species include vapor, cloud droplets, rain, pristine ice, snow, aggregates, graupel and hail. Increasing the mean hail diameter from 3mm to 2cm results in almost doubling the surface precipitation. Turning off all ice processes produced the least condensate at the ground. Changes in the mean hail diameter impact the distribution of the hail and rain with respect to the updraft, an important criterion in the classification of the supercell type. Varying the mean hail diameter also affects the low-level vorticity of the supercells simulated, which in turn affects the precipitation produced. The influence of changes in the hydrometeor shape parameter and the use of the two-moment scheme on severe storm precipitation will also be discussed.