Computer modelling of close-to-ground tornado wind-fields for different tornado widths.

Tangential velocity (V t) of tornadoes is the major parameter that causes building damage. In-field tornado measurements are less reliable at less than 20 m above ground level (AGL). Laboratory tornado simulators suggest that swirl ratio (S) and radius (r o) are the major tornado parameters that influence the V t. However, due to scaling problems, the laboratory simulators also report the V t at greater than 20 m AGL. Well-refined computational fluid dynamics (CFD) models can evaluate the V t at less than 10 m AGL. However, the CFD models are limited to r o = 1.0 km, and the effect of r o on V t is not investigated. The aim of this study is to investigate the maximum V t for different r o close to ground. Simulation results show that increasing r o decreases the maximum V t with respect to V ro. Moreover, by increasing ro, the corresponding elevation of occurrence of maximum Vt (zmax) will increase. However, for all tornado radii, the zmax is between 20 m and 64 m AGL. In addition, results show that for all r o , the radial V t profile has two peaks at z < 10 m AGL due to strong shear force close to the ground and at higher elevation the profile transits to Rankine Combined Vortex Model (RCVM). • When the tornado or tornado chamber radius (r o) increases the touchdown swirl ratio increases. • o The increase in radius (r) reduces the ratio of the maximum tangential velocity to radial velocity. • Close to the ground, the radial profile of the tangential velocity has two peaks. • Close to the ground the radial velocity profile is not same as the Rankine Combined Vortex Model. [ABSTRACT FROM AUTHOR]