Near-surface tornado wind field reconstruction from forest damage

Kurzfassung

The presented Diploma thesis describes the development of a method for the reconstruction of tornado near-surface wind fields from forest damage. For this, forest damage patterns are simulated with a tornado model and compared to observed damage patterns in order to determine the intensity and the velocity components of the near-surface wind field. The numerical model for the simulation of tornado-induced forest damage patterns consists of a model for the simulation of the near-surface wind field based on the analytical tornado model developed by Letzmann (1923) and a mechanical tree model adapted from Peltola and Kellomäki (1993) for the simulation of forest damage. For the analysis of observed forest damage patterns, aerial photos of forest damage patterns produced by tornadoes in Europe are used. The developed model is validated by simulated forest damage patterns from Holland (2006) and furthermore adapted to European tree species. Different tornado near-surface wind field models are implemented and compared to the variations of Letzmann (1923) tornado model. By analyzing the tornado near-surface wind fields and the forest damage patterns, the existence of the converging and diverging lines predicted by Letzmann (1923) is examined. Finally, the model is applied to real forest damage cases from tornadoes in Europe and both the intensity and the velocity components of the near-surface wind field of the tornadoes are determined. Conclusively, by comparing the simulated with the observed damage patterns and determining the translation velocity of the tornado from radar observations it came out, that detailed information about the parameters describing a tornado wind field can be obtained and the tornado intensity can be determined with an accuracy of 10 m/s.