Study of strong tornadoes in Central Europe: Various aspects of Forecasting and Nowcasting

Tornados pose a significant threat to life, property, and economy. Thus, an analysis of tornadoes is of high relevance. An understanding of historical events, e.g. regarding the characteristics of tornadic storms compared to multi-year storm statistics, may help to improve the situational awareness of future tornado events.

In this study, tornadic storms with a tornado intensity of F2 or stronger on the Fujita scale that occurred in recent years (2016 – 2020) in Germany were analyzed in detail. The four F3 tornadoes (Bützow, Affing, Bonndorf and Roetgen) and sixteen F2 tornadoes, which developed on 17 different days occurred in various parts of Germany. Most of the analysed tornadoes occurred from May to early September. The other three cases are typical winter cases that differ significantly from the summer cases in some aspects that are discussed where applicable. One case which happened in the third decade of September has characteristics form both, summer and winter, and is thus the only hybrid case. The great majority of all cases occurred during the second half of the day, most of them between 12 and 18 UTC. The most active hour was 16 to 17 UTC.

Regarding forecasting, similarities and differences of the prevailing synoptic and mesoscale conditions are assessed in addition to the convective environment of the events. Furthermore, the type of convection is analysed. The goal is to anticipate typical characteristics that enhance the threat of a potentially dangerous tornado situation. Using these findings may then help to strengthen the awareness of the forecaster. Two situations in mid- and upper-level flow are typical for the occurrence of strong tornadoes. On the majority of the analysed tornadic days, the event happened on the forward flank of a long wave trough that was slowly propagating eastward. The other typical situation is a vivid short wave trough passing rather fast over the area of interest from West to East.

Regarding nowcasting, a multi-source approach was applied to best analyse the events. For this purpose, radar reflectivity and rotation data were combined with lightning detection in order to analyse the tornadic storms with respect to storm mode and storm evolution as well as lightning and rotation characteristics. In many cases, radar radial wind data showed a persistent rotation track. The automatically detected mesocyclones had a vertical depth between 2.5 and 11 km at the time of the tornado, the diameter was above 8 km. The base of the rotation was low compared to multi-year statistics of all mesocyclonic storms. The lighting activity of the tornadic storms was high. In many cases, a lightning jump occurred between 5 and 120 minutes before the event.