Mesoscale convective systems - Severe Convective Storms and Tornadoes: Observations and Dynamics - page 266

tion, one sees cumulus or cumulus congestus as a precursor to cumulonimbus
clouds. On the other hand, elevated convection is seen initially as altocumulus
castellanus clouds ( Figure 3.2b ), which may be visible only from above if there is
an intervening layer of clouds below.
Stable lift above an outflow boundary, particularly enhanced by a low-level jet
(LLJ, see textbooks on mesoscale meteorology; Figure 5.6 ) is a mesoscale mechan-
ism for triggering elevated convection, particularly during the evening when the
LLJ is strongest and after earlier convection has produced an outflow boundary.
The nature of this lift is similar to that of the lift produced above a density
current.
Weaker quasi-geostrophic lift as a result of warm advection or vorticity
advection becoming more cyclonic with height may also lift a layer of air covering
a broad area, particularly on the cold side of a warm front or stationary front or
outflow boundary, or in advance of a baroclinic wave. Moist symmetric instability
(see textbooks on mesoscale meteorology) is another possible formation

mechanism.
Mesoscale or synoptic-scale lift, which is not driven by buoyancy, acting on
an unsaturated, conditionally unstable air mass could lead to a saturated unstable
layer, sometimes over 100 hPa deep, called a ''MAUL (moist absolutely unstable
layer)'' ( Figure 5.7 ). MAULs may be 100 km or more wide and persist on time
scales longer than that of cumulus clouds (i.e., for as long as 30min or longer).
MAULs first identified on soundings were rather curious since it is usually
assumed that they should immediately trigger convection that destroys them, and
it was therefore thought that they might be specious or not representative, particu-
larly when a radiosonde leaves the top of a saturated region and evaporation cools
the temperature sensor. George Bryan and Mike Fritsch in 2000 argued that many
are real and that they represent ''slabs of saturated, turbulent flow rather than a
collection of discrete cumulonimbus clouds separated by sub-saturated areas.''
They postulated that the speed of dynamically driven rising air on the mesoscale
(e.g., via frontal or other baroclinically driven circulations) can become greater
than the speed of buoyancy-induced vertical parcel excursions, so that the rate at
which the saturated absolutely unstable layer is maintained by mesoscale ascent is
greater than the rate at which it is being removed by cumulus-scale turbulent
mixing. They also suggested, in agreement with speculation about there being an
analogy between the dry boundary layer and the moist layer in MCSs by Ed
Zipser and Peggy LeMone 20 years earlier, that the depth of a MAUL might
dictate the size/horizontal scale of moist eddies, just as the depth of a dry, surface
mixed layer might dictate the size/horizontal scale of eddies in the dry boundary
layer.
Upward forcing of potentially unstable moist air along a line or through the
back-building process is not the only way to form an MCS that has a line
configuration. Numerically simulated lines can also evolve from initially isolated
cells that trigger secondary cells along the gust front of the original cell. As the
gust front spreads out, Morris Weisman and Joe Klemp have shown new
convective cells can break out along the arc of the outward-expanding cold pool
( Figure 5.8 ). Such a process is a good example of the ''upscale growth'' of