Geoscience Reference
In-Depth Information
Figure 9.29
Schematic evolution of
three convective modes on the US
Great Plains showing several scales of
cloud development (shading).
Source
: Blanchard (1990, p. 996, fig. 2),
by
Initiation
Development
Maturity
LINEAR CONVECTIVE
SYSTEMS
permission
of
the
American
Meteorological Society.
OCCLUDING CONVECTIVE
SYSTEMS
CHAOTIC CONVECTIVE
SYSTEMS
During their life cycle, systems may travel from the
Colorado-Kansas border to the Mississippi River or
the Great Lakes, or from the Missouri-Mississippi river
valley to the east coast. A MCC usually decays when
synoptic-scale features inhibit its self-propagation. The
production of cold air is shut off when new convection
ceases, weakening the meso-high and -low, and the
rainfall becomes light and sporadic, eventually stopping
altogether.
Particularly severe thunderstorms are associated
with great potential vertical instability (e.g. hot, moist
air underlying dryer air, with colder air aloft). This was
the case with a severe storm in the vicinity of Sydney,
Australia, on 21 January 1991 (Figure 9.30). The storm
formed in a hot, moist, low-level airstream flowing
northeast on the eastern side of the Blue Mountains
escarpment. This flow was overlain by a hot, dry
northerly airstream at an elevation of 1500 to 6000
metres, which, in turn, was capped by cold air associated
with a nearby cold front. Five to seven such severe
thunderstorms occurred annually in the vicinity of
Sydney during 1950 to 1989.
On occasion, so-called
super-cell thunderstorms
may develop as new cells forming downstream are
swept up by the movement of an older cell (Figure 9.31).
These are about the same size as thunder cell clusters
but are dominated by one giant updraft and localized
strong downdrafts (Figure 9.32). They may give rise
to large hailstones and tornadoes, although some give
only moderate rainfall amounts. A useful measure of
