Mesoscale convective systems - Severe Convective Storms and Tornadoes: Observations and Dynamics - page 298

Figure 5.26. Idealized illustration of how (a) upward tilting of vorticity in the y-direction
associated with easterly vertical shear produces an anticyclonic vortex to the south and cyclonic
vortex to the north and how (b) downward tilting of vorticity in the þ y-direction associated
with westerly shear produces the same kind of vortex couplet. In (a) easterly shear is produced
baroclinically at the leading edge of the cold pool while in (b) westerly shear is associated with
environmental vertical shear (from Weisman and Davis, 1998).

the leading edge of the cold pool to overwhelm the rate at which environmental
horizontal vorticity of the opposite sign (associated with low-level shear) is

advected to the leading edge of the cold pool.
When bookend vortices are relatively close together, the rear-inflow jet is
enhanced as rear-to-front flow is strengthened, which can increase the intensity of
the updraft at the leading edge of the cold pool; as a consequence, the tilting of
system-generated horizontal vorticity (baroclinically generated along the leading
edge of the cold pool) is stronger. In effect, the shorter the length of the convective
system, the less time it takes for bookend vortices to form.
Late in the evolution of MCSs, small-scale vortices (both cyclonic and anti-
cyclonic vortex pairs) within the MCS, not just at the ends, may be produced by
the tilting of baroclinic vorticity produced at the leading edge of the gust front by
either localized updrafts or downdrafts ( Figure 5.27 ).
Since vortices are associated with dynamically induced regions of relatively
low pressure, there is a downward-directed pressure gradient force that tends to