• cyclonic and anticyclonic storms;
  • hail point-frequency;
  • sunward and sunshaded sides;
  • cloud splitting


We study herein the relationship between hailstorm vortex pair evolution and complex orography. In particular, the effects of differential heating are analysed to evaluate how they can affect the propagation of split hailstorms over complex terrain and impact on the spatial distribution of hail. For these reasons, this investigation is targeted at the characteristics of intense hailstorms over a particular area which is well-known as their source, as well as the hail point-frequency spatial distribution over such an area. The methods used include observations and a numerical cloud model. The investigations lead to the following findings.

The key mechanism for a favoured right-moving split hailstorm under strong low-level directional shear is found. This is differential heating of the sunward and sunshaded sides of the valley. For this reason, stronger convection over the sunward side of the valley compared to that over its sunshaded side leads to a weaker vertical wind shear. In such a scenario, additional vertical vorticity components are generated that weaken the former existing vortices. The resulting effect is stronger weakening of the anticyclonic part of the cloud than its cyclonic counterpart. Finally, we have established the conceptual model of the vortex pair modified by differential heating.

The other findings are as follows: the spatial distribution of the maximum values of hail point-frequency associated with air-mass hailstorms is well correlated with the tracks of split hailstorms. It shows the regular spatial pattern due to the corresponding cell-regeneration behaviour. As a consequence, hail point-frequency maxima more often occur in the valley than over the nearby areas. Hail point-frequency maximum locations associated with frontal clouds are more or less uniform in the along-line direction of a front. Copyright © 2011 Royal Meteorological Society