Mesoscale convective systems - Severe Convective Storms and Tornadoes: Observations and Dynamics - page 284

Figure 5.17. Linear MCS archetypes—(a) trailing stratiform (TS), (b) leading stratiform (LS),
(c) parallel stratiform (PS)—from initiation to maturity. Low-level horizontal radar reflecivity
factor levels shaded at approximately 20 (no shading), 40, and 50 dBZ. The approximate time
intervals between phases are 3-4 h for the TS, 2-3 h for the LS, and 2-3 h for the PS archetypes
(from Parker and Johnson, 2000).
At mid-levels, air enters the MCS underneath the cloud and reverses direction,
flowing rearward, and out of the MCS near the surface. At mid-levels, air
approaches the MCS, but is reversed aloft. The last branch is not always found,
especially if deep-layer shear is not too strong. An example of a snapshot of the
flow, derived from a multi-Doppler wind analysis, in a vertical plane cutting
across an MCS, is shown in Figure 5.10, bottom panel; this real-life example
resembles the idealized model depicted in Figure 5.10, top panel. A simplified
model is therefore one in which air approaches at low levels, rises along the
leading convective line, and then flows toward the rear in the stratiform precipita-
tion area ( Figure 5.19 ). A second branch of airflow enters from the rear at
mid-levels, the rear-inflow jet, and then reverses direction near the surface ( Figure
5.19 ). These are the two fundamental branches of airflow in a typical trailing
stratiform linear MCS. This type of analysis is similar to that in synoptic
meteorology of the conveyor belt in idealized models of extratropical cyclones.
In this simple, idealized conceptual model of a trailing stratiform linear MCS
(squall line) over a depth H, described by Kerry Emanuel in his 1994 textbook on
convection (the basic technique was first used by Mitch Moncrieff and collabora-
tors in the 1970s, with inspiration from the earlier observational work done