Supercells - Severe Convective Storms and Tornadoes: Observations and Dynamics - page 251

Figure 4.61. Illustration of how the orientation of low-level vertical shear vector (vectors)
affects behavior along the leading edge of a cold pool (in between ''C'' and ''W''). (Top) Shear
vector points from cold to warm air: RKW effect is dominant. (Middle) Shear vector points
along the outflow boundary, with colder air lying to the left: a cyclonic vortex is produced by
tilting of environmental horizontal vorticity associated with the shear as air approaches the
cold pool from the right and is lifted up and over it (in the reference frame of the cold pool,
which moves to the right). (Bottom) Shear vector has components both from cold to warm air
and along the outflow boundary, with colder air lying to the left: both RKW effects and vortex
production may be significant.
air and decay. With the exception of a cell on the downshear side of the boundary,
right-moving, cyclonically rotating cells interact with surface cold pools from ad-
jacent cells. When the boundary is skewed at a 45 angle from mean vertical shear, it
is possible that all neighboring right-moving, cyclonically rotating cells and only left-
moving, antiyclonically rotating cells on the downshear end of the line do not
interact with their neighbors and thus behave like isolated supercells. So, outbreaks
of right-moving, cyclonically rotating supercells are most likely to occur when mean
vertical shear is oriented at a 45 angle from the line along which they have been
initiated.
The effects of the orientation of low-level shear on the behavior of the rear-
flank gust front must also be considered ( Figure 4.61 ). For example, according to
RKW theory, the potential for longer lasting cells along the flanking line is
increased when the shear vector is oriented normal to the gust front and pointing
from the cooler side to the warmer side ( Figures 4.61, top panel, and 3.40). The
cooler the air behind the gust front, the stronger the shear vector must be for new
convection to sustain itself. When there is little or no temperature gradient across
the rear-flank gust front, vertical shear is not of any such consequence; however,
shear along the rear-flank gust front in the absence of a temperature gradient
would act to tilt vertical circulation over and reduce the chances for regeneration
of new cells (as in the bottom left panel of Figure 3.40 ). On the other hand, when