Large Accumulations of Small Hail

Large accumulations of small (< 2.5 cm) hail can have disruptive impacts on society through agricultural damage, flooding, and road washouts and closures. Yet, such events are not well forecasted, and currently there is no conceptual model for why some supercell storms produce large amounts of small hail whereas others produce large hail. This study aims to better understand what environmental factors and storm structural features are conducive to large accumulations of small hail, and to identify any signatures in dual-polarization radar data that may be useful for operationally identify such storms.

To do so, we analyze polarimetric radar data and environmental and/or high-resolution model soundings from cases of large accumulations of small hail in different geographic regions, including Colorado, Alabama, Florida, Indiana, New Mexico, and Texas. Common low-level radar characteristics found in these cases include large reflectivity factor (> 70 dBz), large differential attenuation, and anomalously large specific differential phase (>10 deg km^-1). Electromagnetic scattering calculations reveal that such features can be explained by large concentrations of sub-severe hail. Further, these storms tend to be weakly tornadic or nontornadic and have compact forward flank precipitation echoes. Several of the cases exhibited midlevel minima in storm-relative flow in or near the prime hail growth region (-10 to -30 °C). In a companion study (Lebo and Kumjian, this conference), high-resolution simulations using a state-of-the-art bin microphysics model are conducted to examine the impact of midlevel storm-relative flow weaknesses in the context of different CAPE/shear environments on the resulting storm structure and hail production.