Simulation of a Hail Event Set for Central Europe.

Since 1980, severe thunderstorms have caused nearly USD 3 billion in damages annually inEurope. Hailstorms account for a significant portion of thunderstorm-related losses, yet theclimatological aspects of large hail are not well understood. While the European SevereWeather Database (ESWD) contains the most comprehensive archive of hail observations inEurope, spatial and temporal inhomogeneities and discontinuities preclude the database frombeing used to create a representative long-term hail climatology. We therefore propose amethod to generate a hail event set for Central Europe by: 1) objectively identifying recenthail swaths from high-resolution radar data, and 2) combining the identified hail swathproperties with a probabilistic convective hazard model and ESWD hail observations tosimulate individual hailstorms. Such a hail event set will facilitate the evaluation of theclimatological aspects of large hail and time-dependent changes in the frequency ofhailstorms. In addition, the hail event set will be combined with exposure and vulnerabilitydata to produce detailed loss estimates and better understand the risk of large hail inEurope. Hail swaths occurring in Germany over a 4-year period (2015–2018) were identifiedusing the vertically integrated ice (VII) product from the Deutscher Wetterdienst (DWD).Specifically, we isolated continuous regions where 6-hour accumulated VII satisfied aminimum intensity (25 kg m−3) criterion, and then used Python image processing to generatehail swath ellipses and analyze their spatial characteristics. Next, for each 6-hour intervalduring the 1979–2018 period, we applied the convective hazard model to ERA-Interimreanalysis 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 thegrid cell during the 6-hour period. For each hail swath, we simulated the length,area, and orientation based on the spatial properties of the previously identified hailswaths. In addition, we simulated the maximum hail diameter based on the statisticaldistributions of observed hail diameter under various instability and deep-layer shearscenarios. Results from the VII hail detection algorithm suggest that large hail in Germany occursprimarily after 12 UTC, with a maximum frequency during the afternoon and early evening(12–18 UTC). While the majority of identified hail swaths affected areas less than 100km2, hail swaths exceeding 500 km2 were not especially uncommon. Hail swathsgenerally followed trajectories between 225˚ (southwest-to-northeast) and 315˚ (northwest-to-southeast) and were typically associated with right-moving thunderstorm cellswhen the synoptic-scale environment was moderately-to-strongly sheared. The simulated hailswaths exhibit a meridional gradient in the predicted frequency of large hail, with thelowest frequencies near the North and Baltic Seas, and the highest frequencies overthe eastern Alps. The number of simulated hail cases peaks in July and August,whereas large hail as reported in the ESWD is most prevalent between May and July. [ABSTRACT FROM AUTHOR]