The severe hailstorm in southwest Germany on 28 July 2013: characteristics, impacts and meteorological conditions

At the end of July 2013, a series of severe thunderstorms associated with heavy rainfall, severe wind gusts and large hail affected parts of Germany. On 28 July 2013, two supercells formed almost simultaneously in southern Germany, from which only the more southerly cell produced hailstones up to 10 cm in diameter on a hailswath approximately 120 km long and 15-20 km wide. For the insurance industry, this event, with losses of more than EUR 1 billion, was one of the most expensive natural disasters that has ever occurred in Germany. This article investigates the creation, temporal evolution and effects of the most severe supercell that day by considering and merging radar and satellite data, eyewitness reports, insurance loss data and numerical model studies. Observations of hail at ground level fit very well with a cold-ring-shaped structure in the cloud-top brightness temperature observed by a geostationary satellite imager. Various simulations conducted with the convection-permitting COnsortium for Small-scale MOdeling (COSMO) revealed that the track of the hailstorm could be reproduced only when convection was triggered artificially by two warm bubbles that produced single cells that were precursors of the supercell. The model results suggested that the supercell developed near a pre-existing single cell through low-level flow convergence in an environment with moderate CAPE but substantial wind shear and storm-relative helicity, both of which persisted for several hours in the area in which the supercell moved.