Geoscience Reference
In-Depth Information
b Other tropical disturbances
adjacent western equatorial Pacific Ocean warm pool.
Other land areas include Australia, India and Central
America, in their respective summer seasons. As a result
of the diurnal regimes of convective activity, MCSs are
more frequent at sunset compared with sunrise by 60 per
cent over the continents and 35 per cent more frequent
at sunrise than sunset over the oceans. Most of the intense
systems (MCCs) occur over land, particularly where
there is abundant moisture and usually downwind of
orographic features that favour the formation of low-
level jets.
Mesoscale convective systems fall into two cate-
gories: non-squall and squall line. The former contain
one or more mesoscale precipitation areas. They occur
diurnally, for example, off the north coast of Borneo in
winter, where they are initiated by convergence of a
nocturnal land breeze and the northeast monsoon flow
(Figure 11.11). By morning (08:00 LST), cumulonim-
bus cells give precipitation. The cells are linked by an
upper-level cloud shield, which persists when the
convection dies at around noon as a sea breeze system
replaces the nocturnal convergent flow. Recent studies
over the western equatorial Pacific warm pool indicate
that convective cloud systems account for <50 per cent
of the total in large precipitation areas (boxes 240
240
km), while stratiform precipitation is more widespread
and yields over half of the total precipitation.
Tropical squall line systems (Figure 11.12) form the
leading edge of a line of cumulonimbus cells (Plate 29).
The squall line and gust front advance within the low-
level flow and by forming new cells. These mature and
eventually dissipate to the rear of the main line. The
process is analogous to that of mid-latitude squall lines
(see Figure 9.28) but the tropical cells are weaker. Squall
line systems, known as sumatras, cross Malaya from the
west during the southwest monsoon season giving
heavy rain and often thunder. They appear to be initiated
by the convergent effects of land breezes in the Malacca
Straits.
In West Africa, systems known as disturbance
lines (DLs) are an important feature of the climate in
the summer half-year, when low-level southwesterly
monsoon air is overrun by dry, warm Saharan air. The
meridional airmass contrast helps to set up the lower-
tropospheric African Easterly Jet (AEJ) (see Figure
11.40). The convective DLs are transported across West
Africa by African easterly waves that are steered by
the AEJ at around 600 mb. The waves recur with a four
to eight-day period during the wet season (May to
Not all low-pressure systems in the tropics are of the
intense tropical cyclone variety. There are two other
major types of cyclonic vortex. One is the monsoon
depression that affects South Asia during the summer.
This disturbance is somewhat unusual in that the flow
is westerly at low levels and easterly in the upper
troposphere (see Figure 11.27). It is described more
fully in C.4, this chapter.
The second type is usually relatively weak near the
surface, but well developed in the middle troposphere.
In the eastern North Pacific and northern Indian Ocean,
such lows are referred to as subtropical cyclones. Some
develop from the cutting off in low latitudes of a cold
upper-level wave in the westerlies (cf. Chapter 9H.4).
They possess a broad eye, 150 km in radius with little
cloud, surrounded by a belt of cloud and precipitation
about 300 km wide. In late winter and spring, a few such
storms make a major contribution to the rainfall of the
Hawaiian Islands. These cyclones are very persistent
and tend eventually to be reabsorbed by a trough in the
upper westerlies. Other subtropical cyclones occur over
the Arabian Sea making a major contribution to summer
('monsoon') rains in northwest India. These systems
show upward motion mainly in the upper troposphere.
Their development may be linked to export at upper
levels of cyclonic vorticity from the persistent heat low
over Arabia.
An infrequent and distinctly different weather
system, known as a
temporal
, occurs along the Pacific
coasts of Central America in autumn and early summer.
Its main feature is an extensive layer of altostratus fed
by individual convective cells, producing sustained
moderate rainfall. These systems originate in the ITCZ
over the eastern tropical North Pacific Ocean and are
maintained by large-scale lower tropospheric conver-
gence, localized convection and orographic uplift.
3 Tropical cloud clusters
Mesoscale convective systems (MCSs) are widespread
in tropical and subtropical latitudes. The mid-latitude
mesoscale convective complexes discussed in Chapter
9.I are an especially severe category of MCS. Satellite
studies of cold (high) cloud-top signatures show that
tropical systems typically extend over a 3000 to 6000
km
2
area. They are common over tropical South America
and the maritime continent of Indonesia-Malaysia and
