Hailfalls and Hailstorm Feeder Clouds—an Alberta Case Study

Lawrence Cheng Resource Technologies Department, Alberta Research Council, Edmonton, Alberta, Canada

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David C. Rogers Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado

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Abstract

Observational evidence from an Alberta hailstorm was examined in an attempt to demonstrate the link between feeder clouds and hailfalls. Radar data, time resolved surface collections of hail, and cloud photographs from a storm were analyzed. It was found that the streak events in the surface hailfall can he linked to small-scale radar reflectivity maxima in the new growth region of the storm. The results suggest that the hail growth process began with packets of hail embryos in distinct feeder clouds, and that the separation between feeder clouds was eventually manifested as distinct hail streak events at the surface. The feeder clouds formed approximately in a line parallel to the vertical ambient wind shear near the cloud base level. The spacings between feeder clouds were almost equal and estimated to he 3 km. Theoretical predictions indicate that convective spacing in a horizontally uniform atmosphere is determined by environmental wind shear, stability, and depth of the shear layer. The results of this and other observational studies lead to the speculation that the spacing between distinct hail streak events may be controlled by the same factors in the vicinity of the new growth zone of hailstorms.

Abstract

Observational evidence from an Alberta hailstorm was examined in an attempt to demonstrate the link between feeder clouds and hailfalls. Radar data, time resolved surface collections of hail, and cloud photographs from a storm were analyzed. It was found that the streak events in the surface hailfall can he linked to small-scale radar reflectivity maxima in the new growth region of the storm. The results suggest that the hail growth process began with packets of hail embryos in distinct feeder clouds, and that the separation between feeder clouds was eventually manifested as distinct hail streak events at the surface. The feeder clouds formed approximately in a line parallel to the vertical ambient wind shear near the cloud base level. The spacings between feeder clouds were almost equal and estimated to he 3 km. Theoretical predictions indicate that convective spacing in a horizontally uniform atmosphere is determined by environmental wind shear, stability, and depth of the shear layer. The results of this and other observational studies lead to the speculation that the spacing between distinct hail streak events may be controlled by the same factors in the vicinity of the new growth zone of hailstorms.

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