Geoscience Reference
In-Depth Information
downwind so that descending air may be located over
the centre of the island.
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 tech-
nology and in global numerical modelling 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
temperatures, and vertical profiles of temperature and
moisture. Weather radar installations are also available
at major centres in India, Central America and the Far
East, and at some locations in Africa and the southwest
Pacific; but up to now there are few in South America.
1 Short- and extended-range forecasts
Figure 11.57
Diurnal variation of rainfall intensity for tropical
islands in the Pacific. (A) Large and small islands in the western
Pacific. (B) Wet and dry seasons for Nandi (Fiji) in the southwest
Pacific (percentage deviation from the daily average).
Sources
: (A) After Gray and Jacobson (1977). (B) After Finkelstein, in
Hutchings (1964).
The evolution and motion of tropical weather systems
are connected primarily with areas of wind speed
convergence and horizontal wind shear as identified on
low-level kinematic analyses depicting streamlines and
isotachs and associated cloud systems, and their changes
can be identified from half-hourly geostationary satellite
images and weather radars; these are useful for 'now-
casting' and warnings. However, cloud clusters are
known to be highly irregular in their persistence beyond
twenty-four hours. They are also subject to strong
diurnal variations and orographic influences, which
need to be evaluated. Analysis of diurnal variations in
temperature with differing cloud states for wet and dry
seasons can be a useful aid to local forecasting. Giving
equal weight to persistence and climatology produces
good results for low-level winds, for example. The
forecasting of tropical storm movement also relies
mainly on satellite imagery and radar data. For six to
twelve-hour forecasts, extrapolations can be made from
the smoothed track over the preceding twelve to twenty-
four hours. The accuracy of landfall location forecasts
for the storm centre is typically within about 150 km.
There are specialized centres for such regional forecasts
and warnings in Miami, Guam, Darwin, Hong Kong,
New Delhi and Tokyo. Forecasts for periods of two to
five days have received limited attention. In the winter
months, the tropical margins, especially of the northern
hemisphere, may be affected by mid-latitude circulation
convection set off by the convergence of land breezes
from Malaya and Sumatra (cf. p. 274). However, on
the east coast of Malaya the maximum occurs in the late
afternoon to early evening, when sea breezes extend
about 30 km inland against the monsoon south-
westerlies, and convective cloud develops in the deeper
sea breeze current over the coastal strip. On the interior
mountains the summer rains have an afternoon max-
imum due to the unhindered convection process.
In northern Australia, sea breeze phenomenon appar-
ently extends up to 200 km inland from the Gulf of
Carpentaria by late evening. During the August to
November dry season, this may create suitable condi-
tions 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
associated with a heavy buildup of cumulus cloud and
afternoon downpours. On large islands under calm
conditions the sea breezes converge towards the centre
so that an afternoon maximum of rainfall is observed.
Under steady trade winds, the pattern is displaced
