Tornadoes in Germany 1950-2003 and their relation to particular weather conditions

Kurzfassung

The objective of this statistical–climatological study is to identify characteristic weather situations in which tornadoes in Germany preferably occur. Tornado reports have been taken from the TorDACH database for the period 1950–2003. Weather situations are defined by the objective weather types classification of the German Meteorological Service (DWD), described by the direction of air mass advection, the cyclonality and the humidity (precipitable water) of the troposphere. In addition the number of thunderstorm days and aerological parameters, derived from temperature and dewpoint in 850 and 500 hPa, are taken into account. Higher frequencies of tornadoes in Germany are found in recent years 1998–2003, particularly in the years 2003 (40 tornadoes) and 2000 (warmest year in Germany since the beginning of the 20th century). There is no shift of the seasonal cycle detectable for the period 1980–2003 compared to 1950–2003 and also no shift of the intensity distribution. The majority of the tornadoes can be attributed to 3 specific weather types, all with a south-westerly advection and high humidity. This can be found for weak as well as for strong tornadoes. Anti/cyclonality in 950 hPa reduces/enhances the tornado frequency roughly by a factor of 2, while in 500 hPa cyclonality shows no significant influence. The fraction of the number of registered tornado days to the number of thunderstorm days has a maximum of one tornado in 50 thunderstorm days in the north-western part of Germany. Specific ranges of temperature and dewpoint in 850/500 hPa, which are not the highest values, are favoured for the occurrence of tornadoes.