Mesoscale convective systems - Severe Convective Storms and Tornadoes: Observations and Dynamics - page 277

It was found in the 1980s from Doppler radar studies that a jet of unsaturated
air from the environment enters the stratiform precipitation region from the rear
side of the MCS. Brad Smull and Bob Houze in the late 1980s named this feature
the ''rear-inflow'' jet ( Figure 5.10, top panel); it aids in the production of a cold
pool at the surface as stratiform precipitation falls into the unsaturated air and
cools evaporatively. However, it will be noted in the next section that the very
presence of a cold pool aids in the production of the rear-inflow jet, so there is a
positive feedback mechanism between the rear-inflow jet and the cold pool.
Ascending front-to-rear air motion is found above the rear-inflow jet, in the
anvil cloud region ( Figure 5.10 ), which is composed of condensed/frozen water
substance formed in the leading convective line.
New convective cells form ahead of the leading convective line and eventually
become the leading convective line, while the dissipating rear edge of the leading
convective line is absorbed into the trailing stratiform precipitation area. A ver-
tical cross section normal to an MCS composed of a leading convective line with
a trailing stratiform precipitation area ( Figure 5.10 ) is like looking at the temporal
history of air parcels in the MCS, and is perhaps similar to tree rings, which
depict spatially the annual growth cycle in trees. An area of enhanced radar reflec-
tivity is observed in the stratiform precipitation area at the freezing level; this
bright band is a result of ice particles coated with water, which have higher radar
reflectivity than the snow above and the rain below.
The classic MCS squall line with a leading convective line and stratiform
precipitation area, symmetric and asymmetric, may take on various appearances.
While the archetype structure is that of a linear leading convective line, sometimes
the leading convective line has the appearance more of blobs of intense echo cores
( Figure 5.11a, bottom panel) or individual cells that are canted with respect to the
leading line ( Figure 5.12 ) . The leading line itself may contain wiggles and bow-
shaped line segments ( Figure 5.13 ).
The pressure field underneath MCS squall lines is characterized by a mesohigh
underneath the main area of the leading convection and a ''wake low'' to the rear,
just at the rear edge of the enhanced stratiform precipitation area ( Figure 5.14 ).
These mesoscale, surface pressure features were first mentioned in the literature in
the mid-1950s by Ted Fujita and were discovered through mesoanalysis of a
mesoscale network of observations then and until a decade or two ago, during
special field programs, but now routinely observed in some states such as
Oklahoma and Texas (in a subsection of the state) which maintain operational
mesonetworks. Mesohighs were first seen in data from the Thunderstorm Project.
Dick Johnson at CSU has found that when the rearward-directed pressure
gradient force is relatively weak the rear-inflow jet continues toward the leading
convective line; when the rearward-directed pressure gradient force is relatively
strong the rear-inflow jet may be blocked ( Figure 5.15 ). A much weaker ''pre-
squall low'' may also be found just ahead of the leading gust front ( Figure 5.14 ).
In asymmetric MCSs, the mesohigh is still found underneath the stratiform

precipitation region ( Figure 5.16 ).
A mesohigh is produced in large part by the hydrostatic pressure excess of the