Evaluation of Sounding-Derived Thermodynamic and Wind-Related Parameters Associated with Large Hail Events
Severe-convective hailstorms are one of the most frequent weather hazards across the United States. However, studies evaluating the ability of various environmental indices to differentiate lower-end severe hail (≤1.25 in, 32 mm) from significant hail (≥2.0 in, 51 mm) prior to storm formation are limited and typically overlap very little with microphysically based research. To bridge this gap, this study builds a database of 520 hail reports that sort into one of four hail-diameter ranges. For each report, various thermodynamic and wind-related fields are then extracted from Rapid Update Cycle (RUC) model analysis to create a parameter-based hail climatology.
Analysis of these environmental indices indicates most wind-based parameters display weaker magnitude winds and resultant shear for the smallest hail-size bin compared to the three largest. Further, the three largest hail diameter bins reveal nearly identical parameter values in the lowest 6 km AGL. In contrast, non-traditional shear layers that include winds in the upper portions of a storm (>6 km AGL) display some skill to differentiate larger hail sizes, especially for ≥3.5-in (89-mm) hail. Thermodynamic variables produced mixed results, with variables such as CAPE displaying a slight tendency to increase as binned hail size becomes larger but still with significant overlap. On the other hand, non-traditional parameters such as the hail-growth-zone thickness reveal a relationship toward decreased depth as the binned hail size increases, but with little to no increase in hail-growth-zone CAPE. Finally, evaluation of the significant severe parameter (SSP) and a new index called the large hail parameter (LHP) display mixed results. Skill at delineating ≤1.25-in (32-mm) report from 2.0-3.25-in (51-83-mm) cases for LHP (SSP) is slightly better (worse) than 0-6-km AGL bulk vector shear. However, the LHP displays improved skill over any other parameter to differentiate ≥3.5-in (89 mm) reports from those with less than 2.0-in (51-mm) diameter hail. The LHP formula creates improved skill by including non-traditional environmental parameters typically associated with storm longevity, precipitation efficiency, and hail-growth rates.
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