Abstract
Tornadoes are a notorious, common threat to human life and property in the United States. Although less common, violent tornadoes are also reported in Europe. The authors aim for an estimation of the average annual loss ratio of European buildings due to tornadoes. An aggregated loss model is used that takes as input the tornado intensity distribution over the Fujita scale (F scale), the distribution of tornado footprint sizes per F class, the vulnerability of European buildings, and the average occurrence rate of tornadoes. Information about these variables is taken from the European Severe Weather Database and, where needed, from the U.S. Storm Prediction Center tornado database, which contains about 16 times more records. However, both databases are biased. Weak tornadoes are underrepresented. Therefore a bias-corrected tornado intensity distribution is used, with its uncertainty taken from the literature. Individual tornadoes are modeled as moving Rankine vortices creating elliptic footprints with correlated length and width. This allows for the estimation of the area fraction of a tornado footprint with lower wind speeds than the maximum wind speed, which is generally used to attribute an intensity. This approach is applied to define effective vulnerability functions of European buildings. A major result is that an expected 90% of the tornadoes contribute only about 1% to the average loss, while the rare F4 tornadoes contribute more than 40% of losses. Given that most national tornado databases in Europe contain tornado records for recent years only and few (if any) violent tornadoes, observed losses can lead to a remarkable underestimation of tornado risk in Europe.
