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

Abstract

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.

Investigation of the temporal variability of thunderstorms in central and western Europe and the relation to large‐scale flow and teleconnection patterns

“…This situation often implies southerly advection of moist and warm air at low levels with sensible‐heat fluxes towards the surface water accounting for the positive SST anomaly. Associated southerly flow aloft also has an increased likelihood for the advection of an elevated mixed layer (EML), known as the Spanish Plume over western France, which produces a conditionally unstable environment favourable to the development of severe convective storms (Carlson and Ludlam, ; Carlson et al ., ; Lanicci and Warner, ; Lewis and Gray, ; Kunz et al ., ). Overall, the influence of the SST anomalies is spatially extended and affects most parts of France and Spain in the case of SST − , and entire western and central Europe in the case of SST + .…”

Relationship between atmospheric blocking and warm‐season thunderstorms over western and central Europe

“…This supports the statement of Piper et al (2019) that convective predisposition is decisively influenced by the state of mid-tropospheric flow steering the large-scale thermodynamic and dynamic conditions relevant for convection. Flow patterns, for example, associated with advection of warm air masses at low levels from southwesterly regions from Spain to France and central Europe in combination with an elevated mixed layer (Carlson et al, 1983;Lanicci and Warner, 1991) and a Spanish plume event (Morris, 1986;van Delden, 2001;Piper, 2017) often produce a conducive environment for the development of (severe) thunderstorms (Kunz et al, 2018;Piper et al, 2019).…”

Improving the precipitation forecast over the Eastern Mediterranean using a smoothed time‐lagged ensemble

“…The convection permitting model used is COSMO‐IL with a 2.5 km resolution, based on the primitive thermo‐hydrodynamic equations that describe non‐hydrostatic compressible flow in a moist atmosphere. The model uses a rotated geographical Arakawa C‐grid and generalized terrain following height coordinates with a Lorenz vertical staggering method (Steppeler et al ., ; Baldauf et al ., ; Doms et al ., ; Kunz et al ., ). Its vertical extension reaches 23.5 km (about 30 hPa) with 60 model levels in the atmosphere.…”

“…During recent years, numerous studies have been devoted to exploring the effect of the resolution of NWP models on the quality of precipitation forecasts for different synoptic conditions and geographical areas. The most remarkable feature of numerical models with a grid spacing <2–4 km is the possibility to treat deep convection explicitly instead of using a traditional convection parameterization (Noppel et al ., ; Kunz et al ., ). Many studies have shown that such models provide quantitatively better results in terms of the simulated precipitation amount, its structure and timing (e.g.…”