Geoscience Reference
In-Depth Information
small island locations in the western Pacific.
Even large islands may show this effect, as well as
the afternoon maximum associated with sea-
breeze convergence and convection. There are
several theories concerning the nocturnal rainfall
peak. Recent studies point to a radiative effect,
involving more effective nocturnal cooling of
cloud-free areas around the mesoscale cloud
systems. This favors subsidence, which, in turn,
enhances low-level convergence into the cloud
systems and strengthens the ascending air currents.
Strong cooling of cloud tops, relative to their
surroundings, may also produce localized destabil-
ization and encourage droplet growth by mixing
of droplets at different temperatures (see Chapter
5D). This effect would be at a maximum near
dawn. Another factor is that the sea-air tempera-
ture difference, and consequently the oceanic
heat supply to the atmosphere, is largest at about
03:00-06:00 hours. Yet a further hypothesis sug-
gests that the semi-diurnal pressure oscillation
encourages convergence and therefore convective
activity in the early morning and evening, but
divergence and suppression of convection around
midday.
A large-scale survey of the Tropical Rainfall
Measurement Mission (TRMM) satellite program
data for 1998-2006 identifies three diurnal
rainfall regimes. They are: (1) oceanic, with a
peak at 06:00-09:00 LST and moderate amplitude.
This is found mainly in the oceanic tropical
convergence zones; (2) continental, with a peak at
15:00-18:00 LST and large amplitude, occurring
in South America and Equatorial Africa; and
(3) coastal, with large amplitude and phase
propagation. This differs from the seaward side,
where there are peaks between 21:00 and 12:00
LST with offshore propagation, and the landward
side where the peaks are 12:00-21:00 LST. Pattern
(3) is prominent in the Maritime continent, the
Indian subcontinent, northern Australia, the west
coast of equatorial Africa, northeast Brazil, and the
coast from Mexico to Ecuador. Land breezes are
generally weak and so may not produce much
convergence. An alternative mechanism in this
case may be gravity waves.
The Malaysian Peninsula displays very varied
diurnal rainfall regimes in summer. The effects of
land and sea breezes, anabatic and katabatic winds
and topography greatly complicate the rainfall
pattern by their interactions with the low-level
southwesterly monsoon current. For example,
there is a nocturnal maximum in the Malacca
Straits region associated with the convection set
off by the convergence of land breezes from
Malaysia and Sumatra (cf.
p. 343
). However, on
the east coast of Malaysia the maximum occurs in
the late afternoon to early evening, when sea
breezes extend about 30km inland against the
monsoon southwesterlies, and convective cloud
develops in the deeper sea breeze current over
the coastal strip. On the interior mountains the
summer rains have an afternoon maximum
due to the unhindered convection process. In
northern Australia, the sea-breeze phenomenon
apparently extends up to 200km inland from the
Gulf of Carpentaria by late evening. During the
August to November dry season, this may create
suitable conditions for the bore-like 'Morning
Glory' - a linear cloud roll and squall-line that
propagates, usually from the northeast, on the
inversion created by the maritime air and
nocturnal cooling. Sea breezes are usually associ-
ated with a heavy buildup of cumulus cloud and
afternoon downpours.
I FORECASTING TROPICAL
WEATHER
In the past two decades, significant progress has
been achieved in tropical weather forecasting. This
has resulted from many of the advances in
observing technology and in global numerical
modeling discussed in Chapter 8. Of particular
importance in the tropics has been the availability
of geostationary satellite data on global cloud
conditions, wind vectors, sea surface tempera-
tures, and vertical profiles of temperature and
