Content area
Full Text
ABSTRACT
Windstorm Kyrill affected large parts of Europe in January 2007 and caused widespread havoc and loss of life. In this study the formation of a secondary cyclone, Kyrill II, along the occluded front of the mature cyclone Kyrill and the occurrence of severe wind gusts as Kyrill II passed over Germany are investigated with the help of high-resolution regional climate model simulations. Kyrill underwent an explosive cyclogenesis south of Greenland as the storm crossed poleward of an intense upper-level jet stream. Later in its life cycle secondary cyclogenesis occurred just west of the British Isles. The formation of Kyrill II along the occluded front was associated (i) with frontolytic strain and (ii) with strong diabatic heating in combination with a developing upper-level shortwave trough. Sensitivity studies with reduced latent heat release feature a similar development but a weaker secondary cyclone, revealing the importance of diabatic processes during the formation of Kyrill II. Kyrill II moved farther toward Europe and its development was favored by a split jet structure aloft, which maintained the cyclone's exceptionally deep core pressure (below 965 hPa) for at least 36 h. The occurrence of hurricane-force winds related to the strong cold front over north and central Germany is analyzed using convection-permitting simulations. The lower troposphere exhibits conditional instability, a turbulent flow, and evaporative cooling. Simulation at high spatiotemporal resolution suggests that the downward mixing of high momentum (the wind speed at 875 hPa widely exceeded 45 m s21) accounts for widespread severe surface wind gusts, which is in agreement with observed widespread losses.
(ProQuest: ... denotes formulae omitted.)
1. Introduction
In January 2007, windstorm Kyrill1 (cf. Fink et al. 2009) swept across large parts of western, central, and eastern Europe resulting in 54 fatalities and overall insured losses of 4.6 billion Euro (EUR) in Germany, the United Kingdom, Belgium, and the Netherlands (economic losses even reached 7.6 billion EUR; Swiss Re 2008). As described in Fink et al. (2009) Kyrill underwent explosive cyclogenesis [a pressure drop of more than 24hPa in 24 h at 608N, cf. Sanders and Gyakum (1980)] over the northeastern Atlantic between 1200UTC16January(998hPa)and1200UTC 17 January (968 hPa). Most of the extratropical systems affecting Europe originate and intensify in this region, known as the North Atlantic storm...