Numerical modelling and forecasting of oblate hailstones

HAILCAST consists of a steady-state cloud model combined with a time-dependent hailstone growth model to simulate the development and growth of a hailstone. The intial conditions for HAILCAST are extracted from the Canadian operational GEM model to forecast the largest hail size. This study investigates the effects of non-spherical rough-surfaced hailstones. The mass growth and heat budget equations of a hailstone model were extended to allow for oblate forms with a rough surface. This hail growth model was coupled with a one-dimensional steady-state cloud model to predict the maximum hail size for Alberta hailstorms. Model runs were initialized with proximity soundings recorded during the Alberta Hail Project (AHP) area for the summers of 1983, 1984 and 1985. The results showed that oblateness of stones can be included with virtually no additional computational resources. Comparing model sizes of typically oblate hailstones with spherical ones indicates that the oblateness added about 10% to final hailstone equivalent diameter. In general, we found a monotonic relationship between hail size and oblateness. The effects of hailstone surface roughness were comparable to those of hailstone oblateness. Comparison of model predicted hail sizes with AHP surface observations suggests that the model for rough oblate stones is suited to assist in forecasting maximum hail sizes.