Investigating the Impact of Graupel and Hail Parameterizations on Idealized Supercell Thunderstorms in the WRF Model

Accurate forecasting of supercell thunderstorms remains one of the preeminent areas of severe weather research. Numerical simulations of these storms utilizing convection permitting models including the Weather Research and Forecasting (WRF) model are a powerful tool for predicting timing, placement, and intensity of severe convection. Within these models, single moment microphysics schemes are a popular choice to represent hail and graupel, where size distributions and number concentrations of hydrometeors are parameterized based on limited observations. While these schemes are computationally inexpensive, they are oversimplifications of true hail growth processes. Documenting the response of simulated storms to these parameters is paramount for understanding their dynamics and improving skill in weather prediction models.

We examine the effects of changing size distributions (i.e., intercept parameter) and densities of graupel and hail in the WRF model for simulated supercells using an idealized sounding based on previous research. The impacts of changing microphysical parameters are documented for environments with varying buoyancy, utilizing several available single-moment microphysics schemes. Specifically, we investigate the development and evolution of storm updrafts early in the numerical simulations as a function of individual buoyancy terms within the WRF model.

Preliminary results suggest a sensitivity of vertical motion on choice of the size distribution and density of graupel and hail. We hypothesize that these differences are attributed to enhancements in latent heating for particle size distributions that describe numerous, smaller graupel particles with overall more surface area that maximizes freezing. Understanding upscale impacts of graupel and hail parameterizations on supercell structure and intensity may elucidate under what environmental conditions microphysics matter most in numerical simulations of these storms.