Environmental Engineering Reference
In-Depth Information
terrestrial radiation, and they can be used to
estimate such elements as temperature changes
initiated by changing atmospheric aerosol
levels. They treat the earth as a uniform surface
with no geography and no seasons. As a result
they are inadequate to deal with the uneven
surface energy distribution associated with the
differences in heat capacity between land and
ocean. One dimensional models have been used
frequently to estimate the impact of volcanic
eruptions on climate, and a 1-D radiative
convective model was used to establish the
TTAPS scenario of nuclear winter (Turco et al.
1983). At best, 1-D models are useful for the
preliminary investigation of global scale
radiative and convective processes at different
levels in the atmosphere. However, they cannot
deal with seasonal or regional scale features,
and require so many assumptions that their
ability to provide accurate predictions is
limited.
Two-dimensional (2-D) models add a
meridional or latitudinal component to the
altitudinal component of the 1-D models. They
can consider variations in climate along a
vertical cross-section from pole to pole, for
example, or along a specific line of latitude. This
allows the horizontal redistribution of such
elements as energy or particulate matter to be
examined. Two-dimensional models can
therefore include consideration of differences in
heat capacity between land and ocean, but their
ability to deal with the evolving dynamics of the
atmosphere once change has been initiated
remains limited. Like the 1-D models, they
contributed to the early development of the
concept of nuclear winter, but they were quickly
superseded by more sophisticated three-
dimensional models.
Three-dimensional (3-D) models provide full
spatial analysis of the atmosphere. They
incorporate major atmospheric processes plus
local climate features predicted through the
process of parameterization. The simulations of
current and future climates provided by these
models require powerful computers capable of
processing as many as 200,000 equations at tens
of thousands of points in a three-dimensional
grid covering the earth's surface, and reaching
through two to fifteen levels as high as 30 km
into the atmosphere (Hengeveld 1991). In
addition to these grid-point models, spectral
models have been developed. In these, the
emphasis is on the representation of
atmospheric disturbances or waves by a finite
number of mathematical functions. Many of the
more advanced models incorporate this
approach (Cubasch and Cess 1990).
Climate models of this type are known as
general circulation models (GCMs). They can be
programmed to recognize the role of land and
sea in the development of global climates. Their
complex representation of atmospheric processes
allows the inclusion of the important feedback
mechanisms missing from 1-D and 2-D models,
and they can deal with the progressive change
set in motion when one or more of the
components of the atmosphere is altered. With
typical integration times for GCMs ranging from
several decades to 100 years, this ability to deal
with the evolving dynamics of the atmosphere is
important.
In an attempt to emulate the integrated
nature of the earth/atmosphere system,
atmospheric GCMs have been coupled with
other environmental models (see Figure 2.14).
Recognizing the major contribution of the
oceans to world climatology, the most common
coupling is with ocean models. In theory, such
models combining the atmospheric and oceanic
circulations should provide a more accurate
representation of the earth's climate. This is
not always the case, however. The coupling of
the models leads to the coupling of any errors
included in the individual models. The so-
called 'model drift' which occurs can be
treated, but it remains a constraint for coupled
models (Cubasch and Cess 1990). Another
major problem is the difference in time scales
over which atmospheric and oceanic
phenomena develop and respond to change.
The atmosphere generally responds within
days, weeks or months, while parts of the
oceans—the ocean deeps, for example—may
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