Tornadoes - Severe Convective Storms and Tornadoes: Observations and Dynamics - page 353

Figure 6.36. Example of tornadogenesis at the leading edge of a bulge in the rear-flank gust
front. Radar reflectivity (dBZ e ) from the U. Mass. W-band, mobile Doppler radar in north
central Nebraska on June 5, 1999. (Top left) White line points to bulge in rear-flank gust front
(U-shaped echo); (top right) about 30 s after the image on the top left, showing a spiral echo
just ahead of the bulge; (bottom) about 30 s after the previous image and showing a tightly
curled spiral echo just ahead of the bulge. The ''emerging'' tornado was headed in the general
direction of the radar. Range rings shown every 1 km.
capable of initiating tornadoes ( Figure 4.16 ). Recent observations do not show
that DRCs are always associated with tornadoes and that tornadoes are not
always preceded by DRCs. There is also recent observational evidence from a
tornadic supercell during VORTEX2 that a DRC may alter the way circulation is
enhanced or vorticity increased as air enters the mesocyclone.
6.5.6 Negative viscosity
The mechanisms hitherto considered involve mean azimuthal velocities. In
tornadoes that have eddies, such as secondary vortices or vortex Rossby waves
(these waves are analogous to Rossby waves in large-scale flow, except that the
radial gradient of the centrifugal force plays the same role as the latitudinal
gradient of Earth's vorticity) or Ekman instabilities, etc., it is possible that the