Central European tornado environments as viewed from a potential vorticity and Lagrangian perspective

ECMWF analysis data in conjunction with infrared satellite imagery and surface weather analyses from the German Weather Service are used to investigate 15 significant central European tornadoes (F2 intensity on the Fujita scale) that occurred in 2005 and 2006. The primary goals of the work are to: (i) determine the typical synoptic and mesoscale environments that are conducive to European tornadogenesis; (ii) compare and contrast the said environments with those found in the central United States (US), with a specific focus on severe storm predictors; and (iii) elucidate a methodology for the real-time forecasting of these destructive storms that, in addition to the use of severe storm predictors, leans heavily on the potential vorticity (PV) and Lagrangian frameworks of analysis.With the caveats that there is significant case-to-case variability and the sample size is relatively small, the results illustrate that most European tornadoes form close to (within 200 km of) a distinct upper-level PV anomaly and a majority under the cyclonic side of an upper-level jet streak. Lower-level forcing, in the form of surface fronts, is also found to be present in a number of cases. With regards to severe storm predictors (convective available potential energy, storm-relative helicity and the energy helicity index), this study confirms the earlier findings that, while representative values for European tornadic environments are substantially lower than their US counterparts, they do provide useful predictive information in that their values tend to be markedly higher than the local, monthly climatology. A subsequent Lagrangian analysis that isolates the coherent air streams present in US and European tornadoes provides significant insight into the discrepancies in both the synoptic environments and the absolute magnitude of the severe storm predictors. Backward trajectories launched from the tornado genesis time and position, illustrate that low-level flow blocking by the Alps and the relatively-colder sea surface temperatures found over the Atlantic Ocean (in contrast to the Gulf of Mexico) play a primary role in reducing the dynamic and thermodynamic instabilities in European tornado environments.

Research Highlights
► Most Central European tornadoes occur close to an upper-level PV anomaly.
► Severe storm predictors are on average much lower in Europe than in the US.
► Predictors for Europe should be used with respect to the local climatology.
► Most low-level backward trajectories of European tornadoes start over the Atlantic.