Geoscience Reference
In-Depth Information
Figure 11.11
Schematic develop-
ment of a non-squall cloud cluster off
the north coast of Borneo: large
arrows indicate the major circulation;
small arrows, the local circulation;
vertical shading, the zones of rain;
stars, ice crystals; and circles, melted
raindrops.
Source
: After Houze
et al
. (1981),
by
permission
of
the
American
Meteorological Society.
Figure 11.12
Cross-section of a
tropical squall line cloud cluster
showing locations of precipitation and
ice particle melting. Dashed arrows
show the air motion generated by the
squall line convection and the broad
arrows the mesoscale circulation.
Source
: After Houze; from Houze and
Hobbs (1982)
October). Disturbance lines tend to form when there
is divergence in the upper troposphere north of the
Tropical Easterly Jet (see also Figure 11.40A). They are
several hundred kilometres long and travel westward at
about 50 km hr
-1
giving squalls and thunder showers
before dissipating over cold-water areas of the North
Atlantic. Spring and autumn rainfall in West Africa
is derived in large part from these disturbances. In
wet years, when the AEJ is further north, the wave
season is prolonged and the waves are stronger. Figure
11.13 for Kortright (Freetown), Sierra Leone illustrates
the daily rainfall amounts in 1960 to 1961 associated
with disturbance lines at 8°N. Here the summer mon-
soon rains make up the greater part of the total, but their
contribution diminishes northward.
