Geoscience Reference
In-Depth Information
moisture. Over the past few years there have been
significant improvements in ocean-observing
systems. Notably, there are now over 3,300 ARGO
floats operating in the world's oceans recording
temperature, salinity and velocity to 200m depth
and providing the data to operational worldwide
in real time. Weather radar installations are also
available at major centers in India, Central
America and the Far East; and at some locations
in Africa and the southwest Pacific; but up until
now there are few in South America. A major
difficulty in tropical forecasting is created by the
dominant energy source of latent heat released by
precipitation in convective cloud systems. This
process is not easily simulated, however, due to the
small-scale processes involved in cloud dynamics.
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
features. Examples include cold fronts moving
southward into Central America and the Caribbean,
or northward from Argentina into Brazil. The
motion of such systems can be anticipated from
numerical model forecasts prepared at major
centers such as NCEP and ECMWF.
2 Long-range forecasts
In the 15-90-day time range, it has been found
that numerical models are strongly dependent on
the initial conditions for several weeks. This is
determined by the intra-seasonal (30-60-day)
Madden-Julian Oscillation (MJO) and the slow
response of the atmosphere to a change in
boundary conditions. However, MJO variability is
largely removed by taking monthly averages.
Boundary forcing is the main determinant of
forecast skill for up to one season.
For longer timescales, three areas of advance
deserve attention. Predictions of the number of
Atlantic tropical storms and hurricanes and of the
number of days on which these occur have been
developed from statistical relations with the El
Niño state, mean April to May sea-level pressure
over the Caribbean and the easterly or westerly
phase of the stratospheric tropical winds at 30mb
(see pp. 342 ). Cyclones in the following summer
season are more numerous when during the
spring season 30 and 50mb zonal winds are
westerly and increasing, ENSO is in the La Niña
(cold) mode and there is below-normal pressure
in the Caribbean. Wet conditions in the Sahel
appear to favor the development of disturbances
in the eastern and central Atlantic. An initial
forecast is made in November for the following
season (based on stratospheric wind phase and
August to November rainfall in the western Sahel)
and a second forecast using information on nine
predictors through July of the current year.
At least five forecast models have been
developed to predict ENSO fluctuations with a
lead time of up to 12 months; three involve
1 Short- and extended-range
forecasts
The evolution and motion of tropical weather
systems are primarily connected 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 persist-
ence beyond 24 hours. They are also subject to
strong diurnal variations and orographic influ-
ences, 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 6-12-hour
forecasts, extrapolations can be made from the
smoothed track over the preceding 12-24 hours.
The accuracy of landfall location forecasts for the
storm center is typically within about 150km.
There are specialized centers for such regional
forecasts and warnings in Miami, Guam, Darwin,
Hong Kong, New Delhi and Tokyo. Forecasts for
 
 
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