Tornadoes - Severe Convective Storms and Tornadoes: Observations and Dynamics - page 356

that interaction with a gust front from a neighboring convective storm might play
an important role.
6.5.9 Counter-rotating tornado pairs
It was noted earlier that while most tornadoes in supercells rotate cyclonically,
there are a number of well-documented cases of anticyclonic tornadoes forming in
cyclonically rotating, right-moving (RM) supercells. Anticyclonic tornadoes have
been documented extremely rarely in anticyclonically rotating, left-moving super-
cells ( Figure 6.38 ). When anticyclonically rotating tornadoes are observed in
cyclonically rotating supercells, though, they are found near or along the edge of
the rear-flank gust front, 5-10 km from a surface mesocyclone or cyclonic
tornado (i.e., anticyclonic tornadoes in supercells are found paired with nearby
cyclonic tornadoes or the remnants of them; Figures 6.8 and 6.9 ). Anticyclonic
vortices of larger scale and weaker vorticity than that associated with tornadoes
are sometimes observed in the region where anticyclonic tornadoes have been
found ( Figure 4.24 ). These anticyclonic vortices may be associated with the tilting
of baroclinically generated horizontal vorticity along the edge of the rear-flank
gust front at the end of the updraft associated with the flanking line and the main
updraft or the tilting of environmental horizontal vorticity (associated with ver-
tical shear) along the edge of the rear-flank downdraft ( Figures 6.39 ). (Vertical
vorticity is thus produced in a manner similar to that of bookend vortices in
MCSs; cf. Figure 5.26 . ) Perhaps tornadoes can be formed when low-level con-
vergence in the vicinity of such an anticyclonic vortex appears, in response to a
rapidly growing updraft along the flanking line, above the rear-flank gust front,
as for landspouts. It is not known whether anticyclonic tornadoes are related
dynamically to the cyclonic member of the pair of vortices, or whether they are
independent of it.
Figure 6.38. Left-moving, anticyclonically rotating supercell containing a meso-anticyclone
and an anticyclonic tornado. WSR-88D radar imagery from KTLX on May 10, 2010, south-
west of Oklahoma City, OK. Range rings are shown every 10 km. (Left) Radar reflectivity
factor in dBZ; (right) Doppler velocity in m s 1 .