Observations and theoretical aspects of tornadogenesis.

Abstract

The direction and degree of tilt offers insights, hitherto unexplored, into the character of the velocity field in which the tornado is embedded. This paper departs from previous research in that it deals with the vector form of the vorticity equation, written in terms of cartesian tensors, instead of the vertical component form. The traditional vertical vorticity equation hides the 'physics' of the tilting term because of cancelled terms, and this has led to misconceptions. It is shown that the convergence and tilting terms are associated with the diagonal and nondiagonal elements, respectively, of the rate of strain tensor, and that both stretch and reorient vortex tubes. Shearing deformation plays the same role in the tilting term, as convergence in the convergence term. In general, the intensifying vortex is tilted toward a principal axis (the one of maximum stretching) of the rate of strain tensor. (from paper)