212 Forecasting Hail Aloft

Monday, 23 January 2017
Donald W. McCann, McCann Aviation Weather Research, Inc., Overland Park, KS
Manuscript (2.1 MB)

Hail aloft can significantly damage flying aircraft. Hail is mostly thought to occur in supercell storms. However, twenty-one gathered aircraft hail strike cases suggest the problem is not limited to supercell environments. Therefore, a comprehensive hail aloft forecast must address all thunderstorms types. Hail forecasting is implemented with a basic hailstone growth model. It needs just an atmospheric sounding as input. From the sounding’s most unstable parcel and potentially unstable layers, vertical motion and cloud liquid water at all atmospheric levels can be determined. Simple time-dependent cloud physics equations describe hailstone growth from an initial embryo in any environment. Any thunderstorm with a sufficiently high updraft will produce some hail because it is very easy to keep small hailstones aloft so they can continue to grow. If ascending hailstones can grow large enough, they may descend through the updraft and become very large. The developed hailstone growth model is optimized by empirically fixing important variables to fit known characteristics of three separate hail strike events. The resulting hailstone growth model is implemented on global numerical forecast model data, and forecasts of maximum hail size potential are distributed to users worldwide.
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