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Thus far, a few important
new findings have been made. We have found that boundaries, which
are the leading edges of pools of cooler air left behind by thunderstorms,
are prime locations for later tornado formation. Evidence suggests
that the temperature contrast along these small-scale "fronts"
supplies the air with horizontal rotation like a rolling pin.
Then, when a mature storm moves across a boundary, the rotation
is tilted upward into the storm's updraft so that the spin has
the orientation of a top, while at the same time being stretched
and intensified. This process imparts strong rotation to the lower
levels of the storm updraft, which seems to be a necessary, but
not sufficient, condition for tornado formation.
Tornado formation
itself seems to be strongly linked to the character and behavior
of a downdraft at the back side of the supercell storm, recognized
for many years as the "rear-flank downdraft." In tornadic supercells
observed in VORTEX, this downdraft straddles two regions of opposite
rotation: the developing mesocyclone, with its cyclonic, or counter-clockwise
spin, and a region of anticyclonic, or clockwise spin, that spirals
around the outside of the downdraft. As this downdraft develops,
it carves its way into the main storm updraft in the shape of
a hook. In fact, this downdraft often contains enough rain to
produce the hook-shaped echo seen on radar reflectivity displays.
In effect, the downdraft draws rotation downward from aloft, while
at the same time focusing it toward a common center. Once the
rotation is focused enough, it becomes strong enough to develop
a funnel cloud and raise dirt and debris at the ground, becoming
a tornado.
Most of the above
process occurs in a very small area, perhaps a couple of miles
across. This was not known during VORTEX, when field teams were
deployed across a large region of each storm. In subsequent, small
focused field efforts, field teams supported by NSSL and the University
of Oklahoma through the NSF have been attempting to operate in
this small region, mainly in the hook echo and inside it. This
region has been called the "bear's cage" by storm chasers for
many years-- even experienced storm chasers would rather not be
there! But with knowledge gained through VORTEX, and with an extra
degree of caution, field teams have gathered data in this region
in many more supercells. The goal of this ongoing work is to determine
what sorts of rear-flank downdrafts are supportive of tornado
formation as opposed to those that actually work to hinder or
prevent tornado formation.
VORTEX has produced
a number of troubling new findings. For example, it appears that
perhaps many fewer supercells and mesocyclones produce tornadoes
than scientists originally believed. At one time, researchers
felt that tornadoes somehow were caused directly by mesocyclones,
and that perhaps one-half of all mesocyclones produced tornadoes.
We now know that this is not the case, and that tornado formation
is a complicated process that depends perhaps only indirectly
on the presence of a mesocyclone. Further, we have learned that
the difference between tornadic and non-tornadic mesocyclones
can be very, very subtle. We are examining a case in which a storm
shows all indications of being tornadic on WSR-88D, and in fact
in mobile Doppler radar data it has a vortex with a hook and an
"eye" in the hook... indicative of very strong rotation and the
centrifuging of raindrops, within about 100 m of the ground. This
pattern of reflectivity and velocity is in most respects the same
as observed in the tornadic supercells. Yet, no tornado formed.
In the near future,
VORTEX follow-on experiments will continue to focus on the subtle
differences between tornadic and non-tornadic supercells. Increasing
attention will be paid to the degree of buoyancy of the rear-flank
downdraft; is it warm, so that it can readily rise when ingested
into the tornado, or is it cold so that it spreads away from the
storm; and the tornado, if it can form, expends much energy lifting
the dense air? To explore this, novel new data-gathering techniques
must be developed. Don't be too surprised to see NSSL scientists
launching small rockets through these downdrafts to measure their
temperature structure in the future!
For
more information contact Erik Rasmussen at: rasmussen@nssl.noaa.gov
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