SENSITIVITY OF A SIMULATED SQUALL LINE TO THE MICROPHYSICAL REPRESENTATION OF GRAUPEL
Steven Naegele1, Sarah Tessendorf2, Greg Thompson2, Trude Eidhammer2.
1University of Illinois at Urbana-Champaign, Urbana, IL, 2Research Applications Laboratory, National Center for Atmospheric Research, Boulder, CO.
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 vapor, rain, snow, 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 does not vary much, like rain or ice, but it is not very realistic for the graupel/hail category since the density of graupel and hail has been observed 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 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 increased from graupel-like to hail-like, simulations showed a slower squall line with weaker maximum reflectivities aloft and stronger updrafts. There was also a decreased graupel melting rate, which created more graupel and less rain. Less rain meant less evaporation and therefore a less intense cold pool, causing a decreased propagation speed. The sensitivity of this simulated storm to the prescription of graupel density is motivation for microphysics parameterizations to have the density of graupel as a predicted variable to more accurately model storm formation characteristics such as precipitation, cold pool formation, and subsequent evolution.