Mesoscale convective systems - Severe Convective Storms and Tornadoes: Observations and Dynamics - page 292

tinuing evaporative cooling. So, in addition, the cooler the cold pool, the stronger
the rear-inflow jet. If the air becomes saturated beneath the stratiform cloud, then
no more evaporational cooling can occur. There may also be some cooling due to
melting of frozen precipitation and sublimation of ice crystals.
The most important consequence of the rear-inflow jet, however, is that there
is horizontal vorticity associated with it of opposite sign just above and below it.
Eventually, horizontal vorticity associated with vertical shear just underneath the
rear-inflow jet, which is the same sign as that of the low-level environmental shear
ahead of the system, is advected forward through the convective system until it
reaches the leading edge, where it now can re-establish a balance between the
baroclinically generated horizontal vorticity at the leading edge of the cold pool
and the horizontal vorticity advected into it; as a consequence, the rising branch
of the circulation at the leading edge is stronger, deeper, and more erect ( Figure
5.22, bottom panel). Thus, the dynamics of the convective system itself result in a
re-invigoration of the system and promote longevity. The rear-inflow jet of the
(a) Descending rear inflow
(b) Elevated rear inflow
Figure 5.22. A conceptual model of the mature structure of a long-lived squall line MCS for a
descending rear-inflow jet (top) and an elevated (bottom) rear-inflow jet. Vertical cross section
showing the updraft branch (thick, double-lined, storm-relative streamline), the rear-inflow jet/
current (dashed streamline). All other features as in Figure 5.20 (from Weisman, 1992).