Geoscience Reference
In-Depth Information
is now recognized as an important discontinuity
and zone of maximum cloudiness. It extends from
the eastern tip of Papua New Guinea to about
30
than the long-recognized tropical cyclone. Our
view of tropical weather systems was radically
revised following the advent of operational
meteorological satellites in the 1960s. Special
programs of meteorological measurements at the
surface and in the upper air, together with aircraft
and ship observations, have been carried out in the
Pacific and Indian Oceans, the Caribbean Sea and
the tropical eastern Atlantic.
Five categories of weather system can be
distinguished according to their space and
timescales (see
Figure 11.3
). The smallest, with a
lifespan of a few hours, is the individual cumulus,
1-10km in diameter, which is generated by
dynamically induced convergence in the Trade
Wind boundary layer. In fair weather, cumulus
clouds are generally aligned in 'cloud streets',
more or less parallel to the wind direction, or form
polygonal honeycomb-pattern cells, rather than
scattered at random. This seems to be related to
the boundary-layer structure and wind speed (see
p. 120
). There is little interaction between the air
layers above and below the cloud base under these
conditions, but in disturbed weather conditions
updrafts and downdrafts cause interaction
between the two layers, which intensifies the
convection. Individual cumulus towers, associated
with violent thunderstorms, develop particularly
in the Intertropical Convergence Zone, sometimes
reaching above 20km in height and having
updrafts of 10-14m s
-1
. In this way, the smallest
scale of system can aid the development of larger
disturbances. Convection is most active over sea
surfaces with temperatures exceeding 27°C, but
above 32
W. At sea level, moist northeasterlies,
west of the South Pacific subtropical anticy-
clone, converge with southeasterlies ahead of high
pressure systems moving eastward from Australia/
New Zealand. The low-latitude section west of
180
°
S, 120
°
longitude is part of the ITCZ system, related
to warm surface waters. However, the maximum
precipitation is south of the axis of maximum sea
surface temperature, and the surface converg-
ence is south of the precipitation maximum in
the central South Pacific. The southeastward
orientation of the SPCZ is caused by interactions
with the mid-latitude westerlies. Its southeastern
end is associated with wave disturbances and jet
stream clouds on the South Pacific polar front.
The link across the subtropics appears to reflect
upper-level tropical mid-latitude transfers of
moisture and energy, especially during subtropical
storm situations. Hence, the SPCZ shows sub-
stantial short-term and interannual variability in
its location and development. The interannual
variability is strongly associated with the phase of
the Southern Oscillation (see
p. 378
). During the
northern summer, the SPCZ is poorly developed,
whereas the ITCZ is strong all across the Pacific.
During the southern summer, the SPCZ is well
developed, with a weak ITCZ over the western
tropical Pacific. After April, the ITCZ strengthens
over the western Pacific and the SPCZ weakens as
it moves westward and equatorward. In the
Atlantic, the ITCZ normally begins its northward
movement in April to May, when South Atlantic
sea surface temperatures start to fall and both the
subtropical high pressure cell and the southeast
trades intensify. In cold, dry years this movement
may begin as early as February and in warm, wet
years as late as June.
°
C convection ceases to increase, due to
feedbacks that are not fully understood.
The second category of system develops
through cumulus clouds becoming grouped into
mesoscale convective areas (MCAs) up to 100km
across (see
Figure 11.3
). In turn, several MCAs
may comprise a
cloud cluster
100-1000km in
diameter. These subsynoptic-scale systems were
initially identified from satellite images as
amorphous cloud areas; they have been studied
primarily from satellite data over the tropical
°
B TROPICAL DISTURBANCES
It was not until the 1940s that detailed accounts
were given of types of tropical disturbances other
