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 ro 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).