Mesoscale convective systems - Severe Convective Storms and Tornadoes: Observations and Dynamics - page 263

of precipitation too small to be considered MCSs. Supercells may be embedded
within MCSs and also coexist with multicells. Portions of some MCSs during
parts of their life may be composed of both a broken line or solid line ( Figure
5.1 ) or a solid line ( Figure 5.2 ) of convective cells and (a broader region of ) strati-
form precipitation. In parts of the U. S., a relatively large fraction of the annual
precipitation falls in MCSs. While MCSs occur in both mid-latitudes and the
tropics, the focus of this discussion will be on MCSs in mid-latitudes. The former
tend to occur over the land in the presence of vertical shear, mainly westerly.
Since Doppler radar observations and three-dimensional non-hydrostatic
numerical cloud models have become available, the major aspects of MCSs have
been explained at least qualitatively. The following is a discussion of how
MCSs form and a discussion of their two-dimensional and three-dimensional wind
structure and thermodynamic structure.
5.1 FORMATION
MCSs frequently begin as squall lines, which at least in the central U. S. form in
four major ways ( Figure 5.3 ). (a) When convective cells break out along a line, but
the cells are initially discrete and then eventually the spaces between adjacent cells
fill in with heavy precipitation, the process is referred to as a ''broken line'' forma-
tion. Such a process often occurs along surface boundaries such as fronts and
outflow boundaries, and sometimes along the dryline (a surface boundary separat-
ing relatively cool, moist, marine air from relatively, warm, dry, continental air; in
the U. S., the marine air is from the Gulf of Mexico and the dry air is from the
elevated terrain of the southwest U. S. and Mexico). Less frequently, lines may
form in response to boundary-layer rolls that trigger deep convection. (b) When
one convective cell forms, and subsequent cells form just upstream with respect to
storm motion, the process is called ''back-building''; eventually, a line forms as
newer and newer cells extend the length of the line. The back-building process
may be a result of new convection triggered at a cool/cold outflow boundary on
the rear flank of the existing cell, or by upward-directed perturbation pressure
gradients on the rear flank. (c) When a region of convective cells conglomerates
into a convective line the process is called ''broken areal'' formation. (d) When a
convective line appears within an area of stratiform precipitation, the process is
called ''embedded areal'' formation.
The most common types of squall line formation are probably the broken line
and back-building processes. In these situations, narrow lines of deep convection
are produced during the early stages of squall line formation. These narrow lines
usually form in air masses that are potentially unstable with respect to air based in
the boundary layer. The original spacing of the first cells might be a result of the
enhanced lift along the rising branch of boundary-layer rolls that intersect the
boundary along which the storms are triggered ( Figure 2.20 ), or gravity waves, or
some other scale-selecting process. In some instances, a bore (see textbooks on
mesoscale meteorology) might play a role in triggering convection ( Figure 5.4 ), or
even internal gravity waves, particularly out ahead of a pre-existing squall line