Tornadoes - Severe Convective Storms and Tornadoes: Observations and Dynamics - page 341

Figure 6.29. Vortex lines in a tornado simulator. (Top) For steady-state flow: (left) when there
is no azimuthal velocity (no swirl), but only radial and vertical velocity; (right) when there is
swirl. (Bottom) From left to right, time-dependent flow characteristics: air converges toward
the center and exhausted outward at the top, without any surface friction; a vertically oriented
vortex line is introduced at the outer edge; it is advected inward and becomes tilted; sinking
motion develops at the center and the vortex stops getting any closer to the center, along the
axis where there is no vertical motion (from Rotunno, 1980).

( Figure 6.29, top left); these vortex lines are associated with radial inflow that
increases with height (as a result of a forced updraft and surface drag). Radial
convergence associated with the updraft brings the rings radially inward and
upward. With the addition of swirling motion in the vortex chamber, vortex lines
do not form rings, but instead enter the domain of the tornado near the surface
and then spiral counterclockwise with height ( Figure 6.29, top right).
To spin up a tornado, vertically oriented vortex lines that point upward (for a
cyclonic tornado) ( Figure 6.29, bottom) converge radially inward from the ''en-
vironment'', and then are tilted radially outward, as the vortex line encounters
decreasing radial inflow with height as it travels radially inward. The azimuthal
vorticity vector near the surface points in the clockwise direction and radial vorti-
city points radially inward. Eventually the vortex line cannot be advected any
farther radially inward. The vortex lines then curl in a clockwise direction with
height around the central axis. Tornado-like vortices in a vortex chamber are pro-
duced with no pre-existing downdrafts.

6.5.2 Stretching of pre-existing vertical vorticity

We first consider how pre-existing vorticity can be increased to tornado intensity.
Vorticity in a tornado is 1s 1 , since the radial shear of the azimuthal wind
50-100m s 1 /100m. Another way of estimating vorticity in a tornado is to use
circulation:

the maximum azimuthal wind speed is 50m s 1

if at a ''core''