Forecasting Hail Using a One-Dimensional Hail Growth Model within WRF

Abstract

he HAILCAST hail growth model has been integrated into the Advanced Research version of the Weather Research and Forecasting (WRF-ARW) Model to predict hail size at the ground. Significant updates to the physics of the hail growth model are added, including variable hail density for both wet and dry growth regimes, an updraft multiplier that parameterizes advection of the hail embryo across an updraft, temperature-dependent ice collection efficiency, mass growth by vapor deposition or condensation, and an improved liquid water shedding threshold. Sample hail trajectories from three different updrafts are presented showing the effects of these physical updates. The updraft multiplier in particular improves the representation of the hail growth by not requiring a hail embryo to be locked in the center of an updraft until it grows large enough to fall. Five weeks of hail diameter forecasts are verified using a maximum expected size of hail (MESH) product. At points where WRF successfully forecasts convection, the forecasted hail size is within 0.5 in. 66% of the time.