Tornado damage analysis of a forest area using site survey observations, radar data and a simple analytical vortex model

This paper deals with the application of a methodology to characterize tornado damage in forests based on a simple two dimensional stationary tornado vortex to describe the surface wind field. The basic vortex model is built over the traditional approach of a combined Rankine velocity profile with radial and azimuthal components plus a constant translational field. Several additions are considered such as producing theoretical swath patterns including absolute velocity values (to compare more easily with Fujita damage rating) or using radar data to estimate the translational speed of the vortex. The methodology is demonstrated with the Castellcir tornado that took place in Catalonia (NE Spain) on the 18th October 2006. The site survey indicated a 4 km path and 260 m maximum width as well as F2 damage. Further analysis suggests the existence of three stages in the tornado life cycle: 1) an organising stage with predominantly inflow pattern; 2) a mature stage with predominant tangential circulation of the vortex and maximum damage and width path –possibly influenced by the complex topography of the terrain–, and 3) a dissipating stage showing weakening and narrowing of the damage path but no outflow patterns. The methodology also helped to confirm the tornadic character of the damage discarding possible microbursts in some parts of the area surveyed.