Environmental Engineering Reference
In-Depth Information
allowing the mid-latitude westerlies and their
associated travelling highs and lows to extend
their influence into lower latitudes.
In the southern hemisphere at this time, the
sub-tropical high pressure systems expand, and
extend polewards over the oceans. The increased
solar radiation of the summer season contributes
to the formation of thermal lows over South
America, Africa and Australia. The absence of
major landmasses polewards of 45-50°S allows
the westerly wind belt to stretch as a continuous
band around the earth.
The patterns change during the northern
summer, as the ITCZ moves northwards again,
and the other elements of the system respond to
changing regional energy budgets. Although such
changes are repeated year after year, the
movements of the systems are not completely
reliable. In the tropics, for example, the ITCZ
migrates at different rates and over different
distances from one year to the next. This inherent
variability contributes to the problem of drought.
It also adds complexity to the impact of
environmental problems—such as the enhanced
greenhouse effect or atmospheric turbidity—
which cause changes in the earth's energy budget,
and therefore have the potential to alter
circulation patterns.
(Trewartha and Horn 1980), and, in combination
with feedback mechanisms, they may augment
or dampen the effects of a particular change in
the system. Lower temperatures at the earth's
surface, for example, would allow the persistence
of snow cover beyond the normal season. This,
in turn, would increase the amount of solar
radiation reflected back into space, causing
surface temperatures to fall even more, and
encourage the snow to remain even longer. Such
a progression, in which the original impact is
magnified, illustrates the concept of positive
feedback. Rising temperatures may initiate
negative feedback. One of the initial effects of
higher temperatures would be an increase in the
rate of evaporation from the earth's surface.
Subsequent condensation of the water vapour in
the atmosphere, would increase cloudiness, and
reduce the amount of solar radiation reaching
the surface. As a result, temperatures would fall
again, and the initial impact of rising
temperatures would be diminished. Ultimately,
these changes in the earth's energy budget would
be reflected in the general circulation of the
atmosphere. Many current global issues—such
as the intensification of the greenhouse effect,
increased atmospheric turbidity and
desertification—involve autovariations, and
include positive or negative feedback in their
development.
AUTOVARIATIONS AND FEEDBACK
It is convenient to consider the various elements
of the earth/atmosphere system as separate
entities, as has been done here. They are, however,
quite intimately linked, and understanding the
nature of these links is important—not only in
the pure scientific study of the atmosphere itself,
but also in the applied, interdisciplinary approach
required for the study of modern global
environmental problems. The elements and
processes incorporated in the earth's energy
budget and the atmospheric circulation, for
example, are parts of a dynamic system, in which
the components are sufficiently integrated that
one change will automatically produce others.
Such changes, produced through the activities of
internal processes are called autovariations
MODELLING THE ATMOSPHERE
Atmospheric modelling has a long tradition in
climatology, stretching back to Hadley and his
basic representation of the earth's wind and
pressure belts. That classic model of the
atmospheric circulation, along with the many
variants which were subsequently developed, was
a representative model. It showed the
characteristic distribution of wind and pressure
across the earth's surface in a simplified form,
with no attempt to predict change in the system.
Change was first dealt with at the local or
regional level when attempts were made to
forecast future weather conditions, and it is to
weather forecasting that most modern predictive
