Determination of tornado intensity from forest damage

The dissertation of Johannes Letzmann (1923) (cf. Dotzek et al., 2008) provides an analytical near-surface tornado wind field model. Holland et al. (2006) describe a simple model for forest damage caused by tornadoes using a tree model based on Peltola and Kellomäki (1993).

The main object of our study is the numerical implementation of Letzmann's analytical model and its application to determine tornado intensity from forest damage. Especially in Europe, tornado damage to forests occurs quite often. Focussing on the patterns of the fallen trees, this model can be widely used and allows for comparison to tornado vortex theory.

The model generates a two-dimensional velocity field depending on the parameters G_max (ratio of circular and translation velocity component), α (angle between wind direction and pressure gradient) and the maximum velocity. Radar observations are used to estimate the translation speed of the tornado. In addition, various analytical tornado core models used by Letzmann can be visualized. By means of the Peltola tree model, the Letzmann damage patterns are simulated and adapted to real cases.

First the presentation will compare the analytical Letzmann tornado model with well-known models and illustrate the convergence and divergence lines first identified by Letzmann. Then the design of the model will be outlined, and the determination of the tornado intensity from forest damage will be demonstrated by examples from Europe.