Entropic Balance Theory and Radar Observation for Prospective Tornado Data Assimilation - Data Assimilation for Atmospheric, Oceanic and Hydrologic Applications - page 436

Fig. 18.10 Wrap-around
mechanism (two
dimensional) analogous to
the nonlinear Baker's
transformation (one
dimensional)

The wrap-around mechanism explains the observation that all tornadoes hit the
ground in the perpendicular direction, and also it is found favorable for the drastic
transition from the supercell to tornadic stages. The wrap-around mechanism based
on the entropic balance theory creates axially-symmetric structure of a tornado
and suggests the transition from supercell to tornado as that from baroclinic to
barotropic states. The barotropic state is horizontal due to the axial symmetry along
the vertical core axis of tornado. We will further discuss the background reasons for
this suggestion.

18.7.1 Tornado Hits the Ground in the Perpendicular Direction

Some theories speculated that a tornado is formed from a horizontally laying vortex
tube by tilting upward by a storm updraft ( Davies-Jones et al. 2001 ). However,
it is known from many visual observations that a tornado vortex always hits the
ground in the perpendicular direction. It is easily understood from fluid mechanics
that a vortex tube of finite diameter does so at a wall surface, because the normal
component of the flow velocity should vanish at the wall surface.
Therefore, there only exist two cases: (a) the vortex tube lays on the ground in
the parallel direction, or (b) the tube hits the ground in the perpendicular direction,
but not in a slanted direction.