Analysis of Hailstone Physical Properties from the IBHS Field Project 2012-2017

Hailstorms cause billions of dollars of damage, on average, in the United States every year. They cause significant damage to buildings and cars, yet anticipating the damage risk is challenging largely owing to gaps in the understanding of the damage potential of hailstorms. In order to better understand the damage potential that these storms have, the physical properties of the hailstones themselves must be understood. This will also aid in better understanding the scattering properties of hailstones for radar-based detection and sizing algorithms, and can help improve modeling of this costly hazard.

To this extent, starting in 2012, the Insurance Institute for Business and Home Safety (IBHS) has conducted a field experiment every spring, with their efforts mainly focused across the Great Plains. In this study, we analyze measurements from over 3500 hailstones collected from 2012 through 2017. These measurements include tri-axial dimensions and mass, and in recent years, involve 3-D infrared laser scanning of some hailstones. We look first at the overall distribution of hailstone maximum, intermediate, and minimum axes, with emphasis on the calculated axis ratios. Hailstones tend to be ellipsoidal, with a modal minor-to-major axis ratio of 0.63 and a modal intermediate-to-major axis ratio of 0.85. Further, we look at how these hailstone properties vary across the hail swath, with a particular focus on these axis ratios. The axis ratios tend to decrease with the time the measurements were collected by IBHS members, which implies that melting is ongoing in the field as the hailstones are being collected. Calculations for some of the hailstones produce unrealistic densities over that of solid ice (~0.9 g cm^-3), suggesting biases in the mass measurements and/or poor representation of hailstone shape by ellipsoids. Although the 3-D scanning technology does alleviate some of the latter concern by providing precise volume measurements, further improvement in taking the mass measurements in the field is necessary. Our results may aid in radar detection and hailstorm modeling by providing more realistic constraint on hailstone shapes and sizes across the hail swath.