Mesoscale convective systems - Severe Convective Storms and Tornadoes: Observations and Dynamics - page 291

Figure 5.21. Conceptual model of precipitation particle trajectories and mean vertical motions
in a trailing stratiform MCS (from Biggerstaff and Houze, 1993).
by Bob Houze and his students Steve Rutledge, Mike Biggerstaff, and Scott
Braun: they have presented evidence that there is less aggregation (see texts on
cloud microphysics) above the melting layer just to the rear of the leading convec-
tive line than farther to the rear of the convective system at the same altitude,
because mid-level subsidence just to the rear of the leading convective line reduces
the availability of small ice crystals; at the same time a mesoscale updraft farther
to the rear enhances the growth of particles there (a mesoscale downdraft is found
at lower levels) ( Figure 5.21 ). In addition, precipitation particles falling out just to
the rear of the leading convective line originate at low levels and thus have less
time to grow than particles originating at higher levels, which travel farther back
and have a much longer period of time over which to grow. The largest particles
fall out quickly near the leading convective line, while the smaller particles with
slower fall speeds are advected farther rearward before they fall out and are
available for precipitation formation.
At the rear edge of the convective system, baroclinically generated horizontal
vorticity is produced aloft as a result of latent heat release in the cloud, and below
as a result of the cold pool. The horizontal vorticity produced aloft is opposite in
direction to that produced below, so that a rear-inflow jet develops ( Figure 5.20c )
in between the vorticity couplet. The strength of the rear-inflow jet is proportional
to CAPE, since the higher the CAPE, the greater the temperature excess in the
cloud above and the lower the hydrostatic pressure deficit underneath the cloud;
the stronger the low-pressure deficit under the cloud, the greater the rear-to-front
pressure gradient force. The rear-inflow jet advects unsaturated environmental air
into the convective system, thus enhancing the cold pool even more through con-