Geoscience Reference
In-Depth Information
form through one of two mechanisms: (1) a pressure
jump that propagates as a bore; (2) the leading edge of
a cold front aloft (CFA) acting on instability present to
the east of an orographic lee trough. In frontal cyclones,
cold air in the rear of the depression may overrun air in
the warm sector. The intrusion of this nose of cold air
sets up great instability, and the subsiding cold wedge
tends to act as a scoop forcing up the slower-moving
warm air (see Plate 11).
Figure 9.28 demonstrates that the
relative
motion of
the warm air is towards the squall line. Such conditions
generate severe frontal thunderstorms such as that which
struck Wokingham, England, in September 1959. This
moved from the southwest at about 20 m s
-1
, steered by
strong southwesterly flow aloft. The cold air subsided
from high levels as a violent squall, and the updraft
ahead of this produced an intense hailstorm. Hailstones
grow by accretion in the upper part of the updraft, where
speeds in excess of 50 m s
-1
are not uncommon, are
blown ahead of the storm by strong upper winds, and
begin to fall. This causes surface melting, but the stone
is caught up again by the advancing squall line and re-
ascends. The melted surface freezes, giving glazed ice
as the stone is carried above the freezing level, and
further growth occurs by the collection of supercooled
droplets (see also Chapter 5, pp. 100 and 107).
Various types of MCS occur over the central United
States in spring and summer (see Figure 9.29), bringing
widespread severe weather. They may be small con-
vective cells organized linearly, or as a large amorphous
cell known as a
mesoscale convective complex
(MCC).
This develops from initially isolated cumulonimbus
cells. As rain falls from the thunderstorm clouds,
evaporative cooling of the air beneath the cloud bases
sets up cold downdrafts, and when these become
sufficiently extensive they create a local high pressure
of a few millibars' intensity. The downdrafts trigger the
ascent of displaced warm air, and a general warming of
the middle troposphere results from latent heat release.
Inflow develops towards this warm region, above the
cold outflow, causing additional convergence of moist,
unstable air. In some cases a low-level jet provides
this inflow. As individual cells become organized in a
cluster along the leading edge of the surface high, new
cells tend to form on the right flank (in the northern
hemisphere) through interaction of cold downdrafts
with the surrounding air. Through this process and
the decay of older cells on the left flank, the storm
system tends to move 10 to 20° to the right of the mid-
tropospheric wind direction. As the thunderstorm high
intensifies, a 'wake low', associated with clearing
weather forms to the rear of it. The system is now
producing violent winds, and intense downpours of rain
and hail accompanied by thunder. During the triggering
of new cells, tornadoes may form (discussed below).
As the MCC reaches maturity, during the evening and
night hours over the Great Plains, the mesoscale
circulation is capped by an extensive (>100,000 km
2
)
cold upper-cloud shield, readily identified on infra-red
satellite images. Statistics for forty-three systems over
the Great Plains in 1978 showed that the systems lasted
on average twelve hours, with initial mesoscale
organization occurring in the early evening (18:00 to
19:00 LST) and maximum extent seven hours later.
Figure 9.28
Thunder cell structure
with hail and tornado formation.
Source
: After Hindley (1977)
