Abstract
Abstract. The synoptic evolution and some meteorological impacts of the European winter storm Kyrill that swept across Western, Central, and Eastern Europe between 17 and 19 January 2007 are investigated. The intensity and large storm damage associated with Kyrill is explained based on synoptic and mesoscale environmental storm features, as well as on comparisons to previous storms. Kyrill appeared on weather maps over the US state of Arkansas about four days before it hit Europe. It underwent an explosive intensification over the Western North Atlantic Ocean while crossing a very intense zonal polar jet stream. A superposition of several favourable meteorological conditions west of the British Isles caused a further deepening of the storm when it started to affect Western Europe. Evidence is provided that a favourable alignment of three polar jet streaks and a dry air intrusion over the occlusion and cold fronts were causal factors in maintaining Kyrill's low pressure very far into Eastern Europe. Kyrill, like many other strong European winter storms, was embedded in a pre-existing, anomalously wide, north-south mean sea-level pressure (MSLP) gradient field. In addition to the range of gusts that might be expected from the synoptic-scale pressure field, mesoscale features associated with convective overturning at the cold front are suggested as the likely causes for the extremely damaging peak gusts observed at many lowland stations during the passage of Kyrill's cold front. Compared to other storms, Kyrill was by far not the most intense system in terms of core pressure and circulation anomaly. However, the system moved into a pre-existing strong MSLP gradient located over Central Europe which extended into Eastern Europe. This fact is considered determinant for the anomalously large area affected by Kyrill. Additionally, considerations of windiness in climate change simulations using two state-of-the-art regional climate models driven by ECHAM5 indicate that not only Central, but also Eastern Central Europe may be affected by higher surface wind speeds at the end of the 21st century. These changes are partially associated with the increased pressure gradient over Europe which is identified in the ECHAM5 simulations. Thus, with respect to the area affected, as well as to the synoptic and mesoscale storm features, it is proposed that Kyrill may serve as an interesting study case to assess future storm impacts.
Highlights
Mid-latitude winter storms rank, after tropical cyclones, as the second highest cause of insurance loss related to a natural disaster, and are the most frequent and costly natural hazards for Central Europe (e.g., Swiss Re, 2000)
Individual meteorological factors that favour the development of strong extratropical cyclones over the Eastern North Atlantic Ocean and that steer these powerful storms to Central Europe are wellknown: an enhanced north-south tropospheric temperature contrast associated with a very strong upper-tropospheric jet stream directed towards Europe (e.g., Ulbrich et al, 2001; Wernli et al, 2002), upper-level divergence at the left exit region of the jet stream, sometimes augmented by the colocated right entrance region of a downstream second jet streak (e.g., Uccellini and Johnson, 1979; Baehr et al, 1999), upper-level dry air intrusions overrunning existing frontal
Pinto et al (2009) demonstrated that a strong intensification of Atlantic cyclones is frequently linked to the occurrence of extreme values of the above-mentioned growth factors in the immediate vicinity of the cyclone centre. It appears that these favourable conditions happen more frequently and persistently during positive phases of the North Atlantic Oscillation (NAO; e.g., Defant et al, 1924; Walker, 1924)
Summary
Mid-latitude winter storms rank, after tropical cyclones, as the second highest cause of insurance loss related to a natural disaster, and are the most frequent and costly natural hazards for Central Europe (e.g., Swiss Re, 2000). An enhancement of loss potentials for Europe may be expected (Leckebusch et al, 2007; Pinto et al, 2007a) Some of these above-described results are, not consensual: for example, even though the majority of the General Circulation Models (GCMs) included in the IPCC fourth assessment report (4AR) show an increase of synoptic activity over the North Atlantic and Europe as does ECHAM5, this is not true for some GCMs (cf Ulbrich et al, 2008). 1.875◦ latitude-longitude), and data is available every 6 h These simulations have been investigated by the authors for changes of synoptic activity and surface winds with increasing greenhouse gas forcing in previous studies (e.g., Pinto et al, 2007a,b, 2009). 10-m maximum wind speed between 1970–1999 and 2070– 2099
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