Geoscience Reference
In-Depth Information
oceans. Their definition is rather arbitrary, but
they may extend over an area 2
differences between them probably result from
regional and seasonal variations in the structure of
the tropical atmosphere. Their wavelength is
about 2000-4000km, and they have a lifespan of
one to two weeks, travelling some 6-7
°
square. It is important to note that the peak
convective activity has passed when cloud cover is
most extensive through the spreading of cirrus
canopies. Clusters in the Atlantic, defined as more
than 50 percent cloud cover extending over an
area of 3° square, show maximum frequencies of
10 to 15 clusters per month near the ITC and also
at 15-20°N in the western Atlantic over zones of
high sea surface temperature. They consist of a
cluster of mesoscale convective cells with the
system having a deep layer of convergent airflow
(see
Figure 9.3
). Some persist for only one to two
days, but others develop within synoptic-scale
waves. Many aspects of their development and
role remain to be determined. While convection
has been stressed, studies in the western equatorial
Pacific 'warm pool' region indicate that large rain
areas in cloud clusters consist mainly of stratiform
precipitation. This accounts for over 75 percent of
the total rain area and for more than half of the
rain amount. Moreover, the cloud systems are
not 'warm clouds' (
p. 128
) but are made up of ice
particles.
The fourth category of tropical weather system
includes the synoptic-scale waves and cyclonic
vortices (discussed more fully below) and the fifth
group is represented by the planetary-scale waves.
The planetary waves (with a wavelength from
10,000 to 40,000km) need not concern us in detail.
Two types occur in the equatorial stratosphere
and another in the equatorial upper troposphere.
While they may interact with lower tropospheric
systems, they do not appear to be direct weather
mechanisms. The synoptic-scale systems that
determine much of the 'disturbed weather' of the
tropics are sufficiently important and varied to be
discussed under the headings of wave disturbances
and cyclonic storms.
°
square up to 12
°
longitude
per day.
The first wave type to be described in the
tropics was the easterly wave of the Caribbean
area. This system is quite unlike a mid-latitude
depression. There is a weak pressure trough,
which usually slopes eastward with height
(
Figure 11.4
). Typically the main development of
cumulonimbus cloud and thundery showers is
behind the trough line. This pattern is associated
with horizontal and vertical motion in the
H
0
km
500
0
miles
500
15
50
40
10
30
20
5
10
0
0
400
200
0
200
400
600
X
km
Y
Surface streams
200mb streamlines
Top of moist layer
Wave trough
Figure 11.4
A model of the areal (above) and
vertical (below) structure of an easterly wave.
Cloud is stippled and the precipitation area is
shown in the vertical section. The streamline
symbols refer to the areal structure, and the arrows
on the vertical section indicate the horizontal and
vertical motions.
Source: Partly after Malkus and Riehl (1964).
1 Wave disturbances
Several types of wave travel westward in the
equatorial and tropical tropospheric easterlies; the
