Simulation of a Hail Event Set for Central Europe

Since 1980, severe thunderstorms have caused nearly € 3 billion in damages annually in Europe. Hailstorms account for a significant portion of both the costliest individual events and total thunderstorm-related losses, yet the climatological aspects of large hail are not well understood. While the European Severe Weather Database (ESWD) contains the most comprehensive archive of hail observations in Europe, spatial and temporal inhomogeneities and discontinuities preclude the database from being used to generate a representative long-term hail climatology. We therefore propose a method to generate a hail event set for central Europe by: 1) objectively identifying recent hail swaths from high-resolution radar data, and 2) combining the identified hail swath properties with a probabilistic convective hazard model and ESWD hail observations to simulate individual hailstorms. Such a hail event set will facilitate the evaluation of the climatological aspects of large hail and time-dependent changes in the frequency of hailstorms. In addition, the hail event set will be combined with exposure and vulnerability data to produce detailed loss estimates and better understand the risk of large hail in Europe.

Hail swaths occurring in Germany over a 3-year period (2015–2017) were identified using the vertically integrated ice (VII) product from the Deutscher Wetterdienst (DWD). Specifically, we isolated continuous regions where 6-h accumulated VII satisfied minimum intensity (25 kg m⁻³) and area (25 km²) criteria, and then used Python image processing to generate hail swath ellipses and analyze their spatial characteristics. Next, for each 6-h interval during the 1979–2017 period, we applied the convective hazard model to ERA-Interim reanalysis data to simulate the occurrence/non-occurrence of hail in each reanalysis grid cell. If hail was simulated in a given grid cell, we also predicted the number of hail swaths in the grid cell during the 6-h period. For each simulated hail swath, we estimated the area, length, and orientation based on the spatial properties of the VII hail swath ellipses. In addition, we estimated the maximum hail diameter based on the statistical relationship between observed hail diameter and the combination of instability and deep-layer shear.

Results from the VII hail swath detection algorithm suggest that large hail in Germany occurs primarily after 12 UTC, with a maximum frequency during the afternoon and early evening (12–18 UTC). While most identified hail swaths affected areas less than 100 km², tracks exceeding 500 km² were not especially uncommon. Hail swaths generally followed trajectories between 225° (northeastward) and 300° (east-southeastward) and were typically associated with right-moving thunderstorm cells (with respect to the mid-tropospheric flow). The simulated hailstorms exhibit a meridional gradient in the predicted frequency of large hail, with the lowest frequencies near the North and Baltic Seas, and the highest frequencies over the eastern Alps.