Mesoscale convective systems - Severe Convective Storms and Tornadoes: Observations and Dynamics - page 289

side of a stationary front or warm front or outflow boundary, air flows from the
boundary layer up and over the front/outflow boundary, producing a convective
line, and an ascending rear-to-front flow branch carries cloud material and
precipitation to the front of the MCS, while a ''leading inflow jet'' enters the MCS
at mid-levels.
5.3 THE DYNAMICS AND THERMODYNAMICS OF MATURE MCS
SQUALL LINES
The time-dependent behavior of MCS squall lines can be understood through an
extension of RKW theory, discussed earlier (Section 3.2.2) in connection with the
behavior of gust fronts in the presence of vertical shear. As a brief review, the
two-dimensional aspects of the evolution of a convective line summarized by
Morris Weisman (cf. Figure 3.40 ) are considered.
First, before precipitation falls, when deep convection is initiated, the
convective cloud leans in the downshear direction: baroclinically generated hori-
zontal vorticity is produced along the edges of the cloud in response to the latent
heat release from condensation; this vorticity is augmented by the import of low-
level environmental horizontal vorticity associated with vertical shear on the
downshear side ( Figure 5.20a ). (In the case of supercell formation when vertical
shear is deep and very strong, the initial rate of production of horizontal vorticity
baroclinically is much less than the rate of vorticity advected in from the environ-
ment, so that the circulation of the cloud leans so far over that the magnitude of
the updraft is reduced significantly.)
Later, after precipitation has begun to fall (on the downshear side) and an
evaporatively produced cold pool is produced, the rate of generation of circulation
induced at the leading edge of the cold pool may become counterbalanced by the
rate of circulation produced by the import of environmental horizontal vorticity
(of opposite sign) associated with environmental vertical shear at low levels
( Figure 5.20b ). In this case, air feeding into the convective cloud at the leading
edge of the cold pool follows a completely vertical trajectory, thus maximizing the
chances for new-cell growth as air is lifted to its LFC. This is referred to as the
''optimal state''.
As the convective system evolves, the cold pool may build up in intensity and
deepen if more and more precipitation evaporates, so that the rate of generation
of horizontal vorticity baroclinically at the leading edge of the cold pool is no
longer balanced by the advection of environmental vorticity, but instead
overwhelms it. The resulting circulation produced at the leading edge of the con-
vection system now leans in the upshear direction and air flowing into the
convective system moves rearward with respect to the convective system, carrying
cloud particles and precipitation with it. Thus, a stratiform precipitation region
forms to the rear of the leading convective line ( Figure 5.20c ). This stage requires
that precipitation continue and that unsaturated air from the environment
continues to be advected underneath the rear of the convective storm.
The development of the transition zone just to the rear of the leading
convective line is likely a result of microphysical processes, as has been suggested