Towards understanding wind-driven hail events, with a focus on Canada

An important aspect of severe thunderstorms is that they are frequently accompanied by damaging winds. Traditionally, however, strong winds and the other phenomena associated with severe storms (such as hail) are considered in isolation. When combined with hail, strong winds not only increase the impact energy (i.e., damage potential) of the hail, but the resultant change in impact angle of wind-driven hail also impacts surfaces that would typically be protected from the hail. This results in extreme damage to the siding and windows of buildings, potentially resulting in building envelope failures, severe damage to vehicles, as well as decimation of agricultural crops of all kinds. The majority of the literature on hail impacts (rightly) focuses on hail damage to roofs, vehicles and crops, from mainly vertical hail trajectories, focusing on terminal fall velocity and other indicators of hail damage. No such widely available research and guidance exists for impacts to siding, windows and building envelopes from wind-driven hail events (WHEs). This lack of research represents a gap in our understanding of WHE risk. Additionally, for risk modelling, we need to ascertain whether WHEs are more common in some hail-prone regions than in others. WHEs have been documented in North America, Europe, and Australia. Here we posit that, globally, wind-driven hail events are more common and are responsible for more damage than traditional hail report data would suggest. Estimates of wind speed and hail size collected during the Alberta Hail Project (AHP), which ran from 1956 to 1985, indicated that there was a tendency for winds at the time of hailfall to increase with increasing hail size. However, the conclusion of the AHP represented the end of dedicated hail research in Canada, and this finding was never explored in detail. Drawing on detailed forensic surveys of WHEs, we intend to develop a hail intensity scale (similar to the EF-scale for winds) that can be applied in Canada and elsewhere. This will be coupled with an analysis of the antecedent meteorological conditions associated with WHEs, will provide an understanding of the underlying mechanisms resulting in WHEs, which in turn will also help to improve weather alerts for WHEs.