Geoscience Reference
In-Depth Information
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
convergence, localized convection and orographic
uplift.
usually downwind of orographic features that
favor the formation of low-level jets.
Mesoscale convective systems fall into two
categories: non-squall- and squall-line. The
former contain one or more mesoscale precipita-
tion 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), cumulonimbus
cells give precipitation. The cells are linked by an
upper-level cloud shield, which persists when the
convection dies about 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 percent of the total in
large precipitation areas (boxes 240
240km),
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. 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 Malaysia 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 south-
westerly monsoon air is overrun by dry, warm
Saharan air. The meridional air-mass contrast
helps to set up the lower tropospheric African
Easterly Jet (AEJ) (see
Figure 11.38
). The convec-
tive DLs are transported across West Africa by
African easterly waves that are steered by the AEJ
at around 600mb. The waves recur with a four- to
eight-day period during the wet season (May
to October). Disturbance lines tend to form when
×
3 Tropical cloud clusters
Mesoscale convective systems (MCSs) are wide-
spread in tropical and subtropical latitudes. The
mid-latitude mesoscale convective complexes
discussed in Chapter 9I are an especially severe
category of MCS. Satellite studies of cold (high)
cloud-top signatures show that tropical systems
typically extend over a 3000-6000km
2
area. They
are common over tropical South America and the
maritime continent of Indonesia-Malaysia and
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 percent over
the continents and 35 percent 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
