Geoscience Reference
In-Depth Information
horizontal advection of precipitation from the
rear of the mesocyclone. Rotation develops where
a thunderstorm updraft interacts with the
horizontal airflow. Provided that the wind speed
increases with height, the vertical wind shear
generates vorticity (Chapter 6C) around an axis
normal to the airflow, which is then tilted
vertically by the updraft. Directional shear also
generates vorticity that the updraft translates
vertically. These two elements lead to rotation in
the updraft in the lower-middle troposphere
forming a meso-low 10-20km across. Pressure
in the meso-low is 2-5mb lower than in the
surrounding environment. At low levels, hori-
zontal convergence increases the vorticity and
rising air is replenished by moist air from
progressively lower levels as the vortex descends
and intensifies. The meso-low shrinks in diameter
and the conservation of momentum increases the
wind speed. At some point, a tornado, sometimes
with secondary vortices (
Figure 9.34
), forms
within the meso-low. The tornado funnel has been
observed to originate in the cloud base and extend
towards the surface (
Figure 9.34
). One idea is that
convergence beneath the base of cumulonimbus
clouds, aided by the interaction between cold
precipitation downdrafts and neighboring up-
drafts, may initiate the funnel. Other observations
suggest that the funnel forms simultaneously
throughout a considerable depth of cloud, usually
a towering cumulus. The upper portion of the
tornado spire in this cloud may become linked
to the main updraft of a neighboring cumu-
lonimbus, causing rapid removal of air from the
spire and allowing a sharp pressure decrease at the
surface. The pressure drop is estimated to exceed
200-250mb in some cases, and it is this that
makes the funnel visible by causing air entering
the vortex to reach saturation. Over water,
tornadoes are termed waterspouts; the majority
rarely attain extreme intensities. The tornado
vortex is usually only a few hundred meters in
diameter and in an even more restricted band
around the core the winds can attain speeds of
50-100m s
-1
. Intense tornadoes may have multiple
vortices rotating anticlockwise with respect to
the main tornado axis, each following a cycloidal
path. The whole tornado system gives a complex
pattern of destruction, with maximum wind
speeds on the right-side boundary (in the
T
Translational speed of tornado center
Translational speed of suction vortex
(Min. V-T : Max. V+T)
V
Rotational speed around tornado center
Wind directions
Suction
vortex
T
Tornado
center
V
Figure 9.34
Schematic diagram of a complex tornado with multiple suction vortices.
Source: After Fujita (1981, p.1251, fig. 15). Courtesy of the American Meteorological Society.
