Comparison of Scattering Properties of Real Hailstones and Spheroids

Severe hail causes over $5 billion in damage annually. However, detection of giant hail is still a challenge in radar observations. Typically, spheroids are used to represent hailstones in radar forward operators and to inform radar hail-sizing algorithms. However, natural hailstones can be irregularly shaped and have lobe structures, and these shape details significantly influence the hailstone’s scattering properties.

The high-resolution 3D structure of real hailstones was obtained using a 3D laser scanner for hail collected during the Insurance Institute for Business and Home Safety (IBHS) Hail Field Project in 2015 and 2016. In addition, plaster casts of several record hailstones (e.g., Vivian, South Dakota 2010) were scanned. S-band scattering properties of the detailed hailstones are calculated with Discrete Dipole Approximation (DDA). The scattering properties of spheroidal approximations of each hailstone are calculated with the T-Matrix for comparison. Spheroidal approximations to these hailstones are made such that they have the same maximum and minimum dimensions, assuming the density of solid ice.

On average, the polarimetric radar variables (Z_H, Z_DR, K_DP, δ, ρ_hv) can be biased using spheroids to represent the real hailstones, even for relatively small hail. The variability of each polarimetric variable is generally smaller for the spheroids than for the irregular hailstones (an exception is in cases when the spheroid size is in the region where significant resonance scattering occurs such that the backscatter cross-section decreases with increasing spheroid size). The increased variability is one reason why ρ_hv tends to be lower in observations than forward-simulated using spheroids. Additionally, δ has a large variance, particularly for large hailstones.