Sensitivity of a Simulated Squall Line to the Microphysical Representation of Graupel

In order to accurately simulate storms and their precipitation within atmospheric models, we need to ensure that atmospheric processes are well parameterized. In the case of the Thompson microphysics parameterization, the rain, snow, cloud drop, cloud ice and graupel/hail hydrometeor categories have their particle densities set to a constant value. This is a good assumption for particles where the density doesn't vary much, like rain or ice, but it is not very realistic for the graupel/hail category since the density of graupel and hail is known to vary greatly between and within storms. This study assessed the sensitivity of an idealized simulated squall line to the prescription of graupel density using the Weather Research and Forecasting (WRF) model. The range of graupel density was varied from 200 kg m-3 to 800 kg m-3, representing particles more characteristic of graupel to those of hail, respectively. As the density of graupel particles was decreased from being hail-like to graupel-like, simulations showed a faster squall line with less graupel and more ice. There was a notable increase in the graupel melting rate, which resulted in more latent cooling and therefore a more intense cold pool in the lower density cases. This led to the faster storm propagation speed, indicating a sensitivity of this simulated storm to the prescription of graupel density. These results provide motivation to use graupel density as a predicted variable when modeling storm formation characteristics such as precipitation, cold pool formation, and subsequent storm evolution.