Hailstone S-band Radar Signatures and Comparisons to Spheroids

Severe (>2.5 cm) hail causes >$10 billion in damage annually in the U.S. However, radar sizing of hail remains challenging, in part owing to the irregular shapes of natural hailstones. Despite this complexity, spheroids are often used to represent natural hailstones in radar forward operators and to inform radar hail-sizing algorithms. High-resolution 3D structures of ~100 real hailstones (see examples in the figure below) were obtained using laser scans of hail collected during the Insurance Institute for Business and Home Safety (IBHS) Hail Field Project during 2015-2018, along with scans of plaster casts from several record hailstones (e.g., Vivian, SD, 2010). We perform scattering calculations using these detailed 3D structures to evaluate the assumption of spheroidal shape. S-band polarimetric radar scattering properties of these hailstones were calculated with the Discrete Dipole Approximation (DDA) technique and compared to the scattering properties of spheroids with identical maximum and minimum dimensions and mass as the natural hailstones, which were calculated using the T-matrix.

The calculations show errors in the polarimetric radar variables when using spheroids, even for the smallest hailstones. Spheroids also had generally less variability in the polarimetric radar variables compared to the real hailstones, with the variability determined both from a range of orientations for a given particle and across particles of similar size. This increased variability is one reason why the correlation coefficient ρ_hv tends to be lower in observations than in simulations using spheroids. Backscatter differential phase δ was also found to have large variance, particularly for large hailstones. Irregular hailstones with a thin liquid layer produced enhanced and more variable reflectivity factor at horizontal polarization Z_H, differential reflectivity Z_DR, specific differential phase K_DP, linear depolarization ratio LDR, and δ compared with dry hailstones, along with significantly reduced ρ_hv. Implications for hail size discrimination algorithms using dual-polarization radar will be discussed.