Tornadoes - Severe Convective Storms and Tornadoes: Observations and Dynamics - page 401

increased, so that it may now be closer to its critical value. Consider (6.45) again.
If the influx of angular momentum is cut off at the large radius, then v

2

r in the
corner region decreases in magnitude. Therefore, the radially inward acceleration
u

=

2

r is decreased.
Thus, a reduction in the inward flux of angular momentum allows for greater pene-
tration of rings of air inward and thus the potential for an increase in azimuthal
velocity as a potential vortex is brought radially inward farther. The increase in
intensity of a vortex as the influx of angular momentum into the corner region is
impeded is known as ''corner flow collapse''. It has been suggested that corner
flow collapse may be triggered when the rear-flank downdraft in a supercell wraps
around a mesocyclone at the surface and acts as a barrier to radially inward-
flowing air. This hypothesis differs from the one that explains vortex intensifica-
tion due to enhanced production of surface vorticity when convergence is
produced as the RFD wraps around the mesocyclone or as higher angular
momentum from aloft is advected downward.
We conclude that the intensity of a tornado is determined both by the
intensity of the parent mesocyclone that is produced by storm-scale processes and
by boundary-layer processes that allow for greatest radially inward penetration
and greatest enhancement resulting from surface friction. The downward-directed
perturbation pressure gradient force associated with the vertical decrease in
vorticity is a dynamic feedback that limits the amount of frictional enhancement
of vortex intensity.
If a vortex is non-steady there may be local ''transients'' that are characterized
by even higher wind speeds than possible through corner flow collapse. If the
vortex is asymmetric because, for example, it is translating, then the drag of the
air on the surface is greater on the side in which the translational motion vector is
added to the azimuthal wind field and less on the opposite side. Therefore, S c may
be decreased in a ''medium-swirl'' vortex on one side toward S c . Thus, a vortex
may experience corner flow collapse if it translates along, but the same vortex may
remain relatively weak if it does not translate along. In this case, the reference
frame of the parent supercell is important. Furthermore, a vortex that translates
along and encounters changes in surface roughness may or may not undergo
corner flow collapse depending on how S c is changed.
If the thermodynamic speed limit is based on faulty physics and almost always
exceeded as much as a factor of 2, or maybe more, why refer to it all? The reason
is that it might be useful as a forecast tool in that the maximum possible wind
speed may be predicted based on a forecast of CAPE: take the square root of
CAPE and multiply it by 2.

@ u =@ r must be greater in magnitude when the magnitude of v =

6.7 ECONOMIC AND SOCIETAL IMPACTS

We now leave the realm of science and enter the non-scientific realm of how
society reacts to tornadoes as a weather problem. It is argued that in the U. S.
taxpayers contributing to government-funded research projects should have access