Geography Reference
In-Depth Information
The minimum prediction equations for a σ coordinate GCM are the horizontal
momentum equation (10.30), the mass continuity equation (10.34), the thermody-
namic energy equation (10.36), and a moisture continuity equation, which can be
expressed as
D
Dt
(q v )
=
P v
(10.79)
where q v is the water vapor mixing ratio and P v is the sum of all sources and sinks.
In addition we require the hydrostatic equation (10.35) to provide a diagnostic
relationship between the geopotential and temperature fields. Finally, a relationship
is needed to determine the evolution of the surface pressure p s (x,y,t). This is
given by integrating (10.34) vertically and using the boundary conditions
σ
˙
=
0
at σ
=
0 and 1 to obtain
1
∂p s
∂t =−
∇·
(p s V )dσ
(10.80)
0
Vertical variations are generally represented by dividing the atmosphere into
a number of levels and utilizing a finite difference grid. AGCMs typically have
prediction levels at 1-3-km intervals extending from the surface to about a 30-km
altitude. Some models, however, have many more levels extending nearly to the
mesopause. Horizontal resolution of global models varies widely, from an effective
grid size of several hundred kilometers, to less than 100 km.
10.8.3
Physical Processes and Parameterizations
The various types of surface and atmospheric processes represented in a typical
AGCM and the interactions among these processes are shown schematically in
Fig. 10.22. The most important classes of physical processes are (i) radiative, (ii)
cloud and precipitation, and (iii) turbulent mixing and exchange.
As pointed out in Section 10.1, the fundamental process that drives the circula-
tion of the atmosphere is the differential radiative heating that results from the fact
that solar absorption at the surface is greater than long-wave emission to space at
low latitudes, whereas long-wave emission dominates over solar absorption at high
latitudes, at least in the winter hemisphere. The general circulation of the atmo-
sphere and the oceans provides the meridional and vertical heat transfer required
for balance.
Most of the solar radiation absorbed at the surface is used to evaporate water,
and hence to moisten the atmosphere. Solar heating is realized in the atmosphere
primarily in the form of latent heat release associated with convective clouds. The
global distribution of evaporation clearly depends on the sea surface temperature,
which is itself dependent on the general circulation of the oceans as well as on
interactions with the atmosphere. That is why for detailed understanding of the
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