Influences on Hail Size as Inferred from Hailstone Growth Trajectory Model Calculations

Damaging hailstorms have become increasingly frequent and costly across the world, yet the skill in forecasting hail size remains poor. In part, this is because climatological studies have found no strong connections between hail size and commonly used bulk environmental indices. However, recent modeling studies have suggested environmental factors like deep-layer wind shear and midlevel relative humidity can play a significant role in hail production, though were unable to directly assess hailstone size given limitations of the modeling framework used therein.

We have developed a detailed hailstone microphysical growth model that explicitly calculates hail sizes produced in a given storm. The calculations are driven by seeding thousands of hailstone embryos in cloud-resolving model simulations of convective storms. The resulting hail fallout patterns and distribution of sizes are consistent with expectations in supercells and squall lines and are found to be robust to small perturbations in storm kinematic and microphysical fields. Additionally, hail size increases with increasing deep-layer wind shear in idealized quarter-circle hodograph cases, supporting our previous findings. Millions of additional hailstone growth trajectories are computed, covering a range of storms in environments with differing wind shear profiles and CAPE. We explore relevant sensitivities of model parameters, including hailstone embryo size and density, drag coefficients, and collection efficiencies. Further, sensitivities to storm motion and storm evolution are found, highlighting how a storm in a given environment can produce a range of maximum hail sizes throughout its lifetime.