European winter storms: Dynamical aspects and wind gust estimation based on results of regional climate model simulations

Extratropical cyclones in the North Atlantic – European sector are among the most perilous and damaging natural hazards affecting Europe. While most of the severe extratropical cyclones pass by Europe in northeastern direction, a small number of strong storms hit Europe each year. Their destructive power is mainly related to strong wind gusts, sustained high wind speeds or huge amounts of precipitation. Especially the relation between wind gusts and losses is a current topic of research. The focus of this thesis is to analyse severe extratropical cyclones affecting Europe during the winter half year (winter storms). The investigation of dynamical aspects and mesoscale processes associated with these hazardous extratropical cyclones is based on results from partly high-resolution mesoscale modelling approaches with the regional climate model COSMO-CLM. In the first part of this study, the ability of the COSMO-CLM to simulate severe winter storm events realistic is verified. With this aim, a total of 158 historical winter storms events between 1972 and 2008 are simulated. A new physically based wind gust estimation method, extended by a probabilistic approach, has been implemented to the COSMO-CLM to provide realistic areawide wind gust distributions during the storm passage. In the second part, two recent severe winter storms (Kyrill in January 2007, Xynthia in February 2010) that caused widespread damage and even fatalities are investigated in more detail. Particularly, the dynamical aspects and mesoscale processes affecting their development are considered. In general, the results approve the ability of realistic simulations of severe winter storm events by the COSMO-CLM. Further, the novel introduced wind gust estimation method provides comparable results to existing wind gust estimation methods. The probabilistic extension permits an estimation of the uncertainties of severe gusts at observational sites. This could be utilised as a valuable application when forecasting severe winter storm events to determine the possible range of maximum wind gusts and their related losses. This is of relevance for both society and for applications in insurance industry as well. The results for winter storm Kyrill reveal the genesis of a secondary cyclone along the occluded front of the parent cyclone. This is an uncommon location for secondary frontal development and has not been documented in recent review articles covering this field of research. The formation of the secondary cyclone was associated with negative deformation stretching and supported by diabatic processes in the lower and mid troposphere. The analysis of severe wind gusts associated with the strong cold front over Central Europe reveals the existence of a conditionally instable boundary layer in addition with a turbulent flow. This indicates that high momentum at the top of the boundary layer could have been mixed downward to the ground leading to the strong surface wind gusts. The analyses of winter storm Xynthia show that moist and warm air masses over the anomalously warm North Atlantic Ocean were incorporated into the cyclone. The realisation of sensitivity studies with modified (lowered) sea surface temperatures (SSTs) or reduced surface latent heat fluxes reveal their important influence on the intensity of the storm. A stronger reduction of SST or surfaces fluxes leads to a less intensive cyclone, which emphasizes the importance of warm and moist air near the ocean surface. This is also indicated by reduced diabatic heating rates at lower and mid levels and a weakening of the PV (potential vorticity) tower in case of altered surface conditions. These findings may be of relevance within the context of climate change and possible warming of SSTs. To conclude, the ability of the COSMO-CLM to provide realistic simulations (including realistic area-wide wind gust estimates) of winter storms over the North Atlantic – European sector is ascertained. The realistic representation of near surface wind gusts by the model permits the possibility of estimation of losses and thus is of potential importance e.g. in the insurance business. Furthermore, the outcomes of this thesis extends the current knowledge and provides a substantial basis for the understanding of dynamical aspects and mesoscale mechanisms being relevant during the genesis, development and the passage of individual winter storms like Kyrill (January 2007) and Xynthia (February 2010) over Europe. Finally, the understanding of physical mechanisms and the effects of atmospheric conditions associated with individual winter storms are essential to improve the accuracy of the prediction of future storm events