Abstract
A study of very detailed spatial and temporal data on damaging hail, associated cloud-to-ground (CG) lightning, and storm echoes in the Midwest was pursued to define their relationships and gain insight on their formation processes. Lightning activity was always closely associated with hailstreaks: the surface embodiment (4.5 km long by 1.3 km wide) of a single volume of hail generated aloft. Lightning seldom occurred where the hail fell and generally formed and moved forward in a 10–15-km2 area on either the left or right flank of the hailstreaks. These lightning centers, areas with CG flashes closely associated with hail, typically developed 9 min before hail at a point 5 km upstorm from first hail, suggesting that CG flashes began as the hailstones were developing aloft. The centers then grew in areal coverage and flash frequency until hail began and diminished shortly after hail ended, with a duration of 26 min. The hailstorm's severity was found to be well correlated to the rate of flashing during the hailfall. More than 75% of all lightning centers in the storms on the studied hail days were not associated with damaging hailfalls, indicating little chance to use lightning activity as a predictor of hail. Multicellular echoes in lines produced hailstreaks with severities twice those generated in single cell storms, and severities were much higher when hail-generating cores exceeded 65 dBZ. Merging of cells produced one-third of the hailstreaks but did not affect hail severity. Hailstreaks and associated lightning centers were generally found in storm cores (>45 dBZ) that developed well before the lightning and persisted well after the hail and associated lightning ended. These were relatively strong storms typical of the summer season in the central United States. The findings may not be representative of storms in other climatic zones. Lightning with hailfalls occurred throughout individual cores and adjacent steep reflectivity gradients, whereas hail tended to fall either along the left or right flank of the cores. Hailstorms occurring with cold fronts had more lightning activity and less complexity than did stationary frontal storms.
