Geoscience Reference
In-Depth Information
Figure 6.57. Idealized depiction of a vortex sheet, which is located near the dashed line, where
the azimuthal wind speed suddenly jumps to a higher value at larger radius. Curved lines with
arrows represent the horizontal component of the flow around the two-cell vortex; the length of
the arrows is proportional to the azimuthal wind speed.
Vortex sheet instability may affect not only the strongly horizontally sheared
region in two-cell vortices, but also vortices produced along shear lines associated
with surface boundaries such as fronts, outflow boundaries, the dryline, and the
sea breeze/land breeze front.
If the swirl ratio is then decreased, there is hysteresis, such that the transition
in the number of secondary vortices decreases, but at higher critical swirl ratios.
The transition from a one-cell to a two-cell vortex occurs at swirl ratios of
0.5-0.7 in a laboratory simulator, depending upon the Reynolds number.
Typical swirl ratios in laboratory vortices range from 0.1-1.5; in a multiple-
vortex tornado, swirl ratios of 2-6 have been estimated from analyses of
ground-based, mobile, Doppler radar data; in the parent mesocyclone of a
tornado, swirl ratios of 0.7-8 have been estimated from airborne Doppler radar
analyses and have increased rapidly with time when a tornado was forming.
When a vertically oriented vortex line is introduced into a larger scale
barotropic vortex, the vortex line moves along with the flow and becomes con-
torted. For a larger scale cyclonic vortex, there is frictionally induced vorticity in
the radially inward direction as a result of the increase in azimuthal wind with
 
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