An Exploration of Hail Formation in Supercell Storms: Why do some Supercell Storms Produce Large Hail while others Generate Small Hail?

Accurate hailstorm forecasts are still elusive, despite their relatively frequent occurrence and large societal costs. Part of the hail forecasting challenge is a lack of a complete understanding of the processes and environmental factors controlling hail production. For example, there is no accepted conceptual model for why some supercells produce extremely large hail, whereas others produce large quantities of small hail. Our current understanding of hail processes is largely founded in work from several decades ago performed with limited computational and observational capabilities. Recently, improvements in both computational power (e.g., supercomputers) and observational tools (e.g., dual-polarization radar) allow us to overcome many of the challenges and shortcomings faced in previous studies.

A companion paper examines hailstone sizes using dual-pol radar observations (Kumjian and Lebo; this conference). In the present work, we examine the environmental conditions most suitable for the formation of large versus small hail in supercell storms using high-resolution numerical simulations with a state-of-the-art bin microphysics scheme, which is better suited to represent microphysical processes that are highly dependent on the ambient hydrometeor size distributions than traditional bulk modeling frameworks. The work builds off of a recently submitted paper in which the magnitude of the deep layer east-west shear was found to play a key role in elongating and enlarging the hail growth region, resulting in large hail mass mixing ratios. We further explore this problem by examining the shear-CAPE parameter space using size-resolved (i.e., bin) microphysics, permitting a more comprehensive analysis of environmental factors and an improved representation of hail sizes. We elucidate the portion of the parameter space that is deemed most critical for the formation of large destructive hail versus that of small accumulating hail, which can aid in forecast lead times in the event of severe weather.