Tornadoes and Tornadic Storms: A Review of Conceptual Models
Tornadoes and Tornadic Storms: A Review of Conceptual Models
Tornadoes and Tornadic Storms: A Review of Conceptual Models
), Geophysical
Monograph 79, Amer. Geophys. Union, 161-172. NOTE: The references have been updated from the original to
include page nos. for "in press" articles and to make some minor corrections. There may be some slight variances
between this text and the paper as it appeared.
3.1. Landspouts
In an analogy with the common waterspout (Bluestein 1985), most of which
develop from non-supercell storms,5
many non-supercell tornadic events (e.g.,
Fig. 5) arise via intensification of preexisting, shallow vertical vortices near the
surface, through simple vortex stretching
cerns the names for cloud features, its flavor is characteristic of terminology debates in general.
5
Of course, some waterspouts do arise
from supercells. They have been called
tornadic waterspouts by Golden (1971)
and appear to be virtually identical to tornadoes associated with supercells over
land. The distinction between a tornado
and a waterspout is basically of little or no
scientific value.
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3.3. Gustnadoes
Very small scale, shallow vortices
(Fig. 6) may develop near the surface
along outflow boundaries and/or cold
At the risk of being repetitious, it is the
presence of a deep, persistent mesocyclone which defines a supercell, not the
depth of convection. When the mesocyclonic circulation exists through a substantial fraction of the depth of the storm,
it doesn't matter if the storm is relatively
shallow; it is a supercell. Storms poleward of, say, 45 latitude often have low
tops because the environment is relatively
cold, with a correspondingly low tropopause.
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(see Idso 1974). Dust devils arise in association with dry, rather than moist convection, of course.7
Meteorologists operating on storm
intercept teams have observed relatively
long-lived funnel clouds in association
with quite ordinary cumulus clouds (Fig.
7). A rather different phenomenon has
been observed on fair weather days, the
so-called "horseshoe vortices" (Fig. 8).
These may arise in much the same way as
"mountainadoes" (Bergen 1976): tilting
and the associated stretching of an enhanced region of horizontal vorticity over
some upward-protruding object, or perhaps by an isolated updraft (a small cumulus-scale version of the process depicted in Fig. 3a of Klemp 1987).
With most of these fair-weather vortices, it seems unlikely they ever would
reach damaging proportions at the surface, and so it is improbable that they
would (or should) be classified as tornadoes. Knowledge that they exist may be
important in responding appropriately to
citizen reports of such events, however.
4. CLASSIFICATION OF VORTICES
Interestingly, some citizens observing
the deadly Cheyenne, Wyoming tornado
of 16 July 1979 thought they were seeing
a dust devil; this confusion may have
arisen because of the relative rarity of tornadoes in Wyoming, along with the absence of a visible condensation funnel for
the early part of the tornado's life.
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ture that renews itself from instant to instant via one or more dynamic processes.
It is not a "thing" in the sense that a table
or a book (neglecting atomic or molecular
fluctuations) is the same from one moment to the next. Much confusion about
tornadoes comes from thinking of tornadoes as objects rather than as the kinematic manifestation of dynamic processes. The actual physical processes are
not heedful of our somewhat arbitrary
classification schemes and, as scientists,
we need constantly to remind ourselves
that our understanding of tornadoes and
tornadic storms can be clouded by an inappropriate classification scheme (see the
discussion by Doswell 1991a).
The only scientific justification for a
classification scheme is if that scheme
proves to be useful in developing our understanding and/or in application of that
understanding. While we probably have
muddied the waters by mentioning additional difficulties with event classification,
we believe that an appreciation for classification problems is needed in any proper
use of the data derived from classification.
The more we learn about tornadoes
and tornadic storms, the more they seem
to be terribly complicated processes. It is
possible that some insight we have yet to
find will simplify our understanding of
tornadoes and tornadic storms. On the
other hand, new observations may not result in some simple reconciliation, but will
raise new and even more confusing issues
with which to deal. There is nothing that
guarantees simplicity in nature.
Despite the confusion it has caused,
however, our new understanding developed since the last Symposium as a result
of radar, storm chasing, and numerical
and laboratory modeling has been applicable in both a research and an operational sense. The recognition of a range
of processes at the scale of the convective
storm and at the tornado scale has been
valuable to our science and to society as a
whole. It is likely that numerical cloud
models soon will be able to resolve tornadic flows, offering the chance for new
insights into tornadoes. As new operational and research observing systems are
implemented, it is virtually certain that we
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