Precipitation Properties of Supercell Hook Echoes

Recent studies have suggested that thermodynamic properties of supercell rear-flank downdrafts (RFDs) can affect whether or not tornadogenesis occurs. The thermodynamic characteristics of RFDs are determined in part by microphysical processes such as evaporation of raindrops and melting of hailstones. Whereas in situ measurements of hook-echo particle size distributions (PSDs) are exceedingly rare, polarimetric radars can be used to determine the bulk characteristics of these PSDs remotely. A preliminary analysis of polarimetric radar data from a small sample of supercell hook echoes reveals unusual drop size distributions compared to typical rainfall in Oklahoma, as well as spatially inhomogeneous structures. The inner edge of the hook echo is often characterized by low or moderate reflectivity factor at horizontal polarization (ZH), with very high differential reflectivity factor (ZDR), indicating a sparse population of very large drops. The southern and/or western (back) portion of the hook is characterized by moderate to high ZH and rather low ZDR, indicating a surplus of small drops and/or a lack of larger drops. Hypotheses explaining the unusual drop size distributions are presented. Additionally, the time evolution of these characteristics is explored using data collected with a special rapid scanning strategy.