Identification of Hail and Size Estimation Using Weather Radar Spectral Reflectivity: Insights From the RELAMPAGO Field Campaign

Dual-polarization Doppler weather radars operating at S- and C-bands are proven tools for hail detection. However, this research focuses on using C-band spectral polarimetric variables observations to refine the identification of hail signatures as well as the determination of maximum probable hail sizes in convective storms. In this context, a retrieval algorithm has been introduced that minimizes the difference between measured and modeled spectral reflectivity. The spectral reflectivity model is formulated using hailstone terminal fall velocity-diameter relation, scattering cross-sections computed using T-matrix, and the information on effective spectral broadening. The retrieval algorithm uses C-band radar measurements from CSU-CHIVO, a dual-polarization Doppler weather radar deployed during the RELAMPAGO field campaign in Argentina. The algorithm also uses a combination of radiosonde and ERA5 reanalysis data to determine the vertical Doppler velocity range for the reflectivity spectrum as well as estimate the vertical distribution of hail sizes in a convective storm. The hail size distribution at various altitudes has been found to be primarily in the range of 2 to 14 cm during convective storms, with varying mean diameters. The uncertainty associated with this distribution is 0.4 to 1.6 cm if the uncertainty for Doppler velocity is 2-4 m/s. The maximum probable hail size has been found to be 5 cm, with 95% confidence intervals ranging from 3 to 10 cm, confirming the dominance of hail sizes$>2.5$cm. The sensitivity analysis of the retrieval algorithm shows that it is sensitive variations in measurements, backscatter cross-section, vertical wind velocity and turbulence. The results highlight spectral reflectivity coupled with a physics-based retrieval algorithm as a robust framework for quantifying hail size and microphysical properties.