Tornadogenesis

In this paper we use a simple numerical model to study vortex growth in a flow configuration which broadly simulates the principal characteristics of a severe tornadic storm system, i.e. strong vertical forcing by intense cumulus or cumulonimbus convection in the presence of an organized field of rotation on the cloud scale. The model is similar to one described by L. M. Leslie in which the updraught in the main cloud cell is modelled by an imposed body force, but differs in that the rotation field is determined by specifying the vertical profile of swirling velocity at the lateral boundary of the flow domain and air is allowed to enter or leave the computational region through its radial and upper boundaries. In particular, we compare situations in which the imposed swirl is concentrated aloft, primarily above cloud base, and when it extends to lower levels. In the former case, solutions exhibit genesis to a steady suspended vortex provided that the forcing strength lies within a certain range, depending on the level of rotation. However, if the imposed swirl extends sufficiently far below cloud base, the vortex continues its downward growth and establishes contact with the ground. We believe our results provide a plausible and consistent picture of the growth of a tornado beneath the main updraught of a severe thunderstorm and indicate why only a relatively small proportion of such clouds spawn pendant funnel clouds and why only a fraction of these develop into tornadoes. Moreover, they appear to be consistent with the observed development of circulation patterns deduced from single‐pulse Doppler radar measurements of a tornado‐producing storm system by Burgess, Lemon and Brown.