High-resolution assessment of the hail hazard over complex terrain from radar and insurance data

Abstract

A new method for the assessment of the hail hazard in high spatial resolution and its application for Southwest Germany are presented. By applying a cell tracking algorithm to 3D radar data, tracks of severe hailstorms are detected and merged with loss data from a building insurance company. Besides the detection of hailstorm tracks between 1997 and 2007, maximum reflectivity is projected on a 10 km × 10 km grid and analyzed by extreme value statistics. In doing so, radar reflectivities are related to statistical return periods. The results confirm a high spatial variability of both track density and hail hazard. Accordingly, severe hailstorms occur most frequently, and consequently also with highest intensities, in the region south of Stuttgart, whereas hail activity is lowest over the Rhine valley and the low mountain ranges of the Black Forest and Swabian Jura. Based on a conceptual approach, it is discussed how spatial differences in hail incidence can be explained plausibly by low Froude number flow (Fr < 1) around the mountains of Black Forest and the northwestern edge of the Swabian Jura, which causes downstream horizontal convergence. © Gebrüder Borntraeger, Stuttgart 2010.