Geoscience Reference
In-Depth Information
12.3 GENERAL CIRCULATION MODELS
To account for feedback processes, nonlinearities, and climate subsystem inter-
actions, computer models of the global climate known as
general circulation
models
, or
GCMs, are constructed. GCMs are mathematical compilations of
our understanding about how the climate system works, and they are our most
sophisticated tool for predicting future climate.
Given a set of governing equations, accompanied by physical parameteriza-
tions for processes that cannot be explicitly calculated, GCMs produce solu-
tions for the climate state that are consistent with a set of climate forcing
factors. Climate prediction is fundamentally different from weather prediction,
which uses computer models initialized with the observed state of the atmo-
sphere to advance the initial condition through time to produce predictions for
a few days in the future. Climate simulations, in contrast, should not be highly
dependent on the initial state.
GCMs can be categorized according to how many and which subsystems of
the full climate system (see
chapter 1)
are included.
ATMOSPHERIC GCMS
The governing equations of an
atmospheric GCM
, or AGCM, are listed in Box
12.1. They may be written in a different form, for example, using different co-
ordinate systems, for different models, but the underlying physics is the same.
Box 12.1 Governing Equations of an Atmospheric GCM
2
u
uv
tan
φ
uw
1
2
p
v
=−
v
$
d
u
+
−
−
+ −
fv
2
Ω
w
cos
φ
+
λ
F
.
(12.5)
2
t
a
a
ρφλ
a
cos
2
u
2
tan
φ
2
p
2
v
vw
1
v
=−
v
$
d
v
−
− −−+
φ
fu
F
.
(12.6)
2
t
a
a
ρφ
a
2
2
w
uv
2
+
−−+
2
1
2
p
v
=−
v
$
d
w
+
g
2
Ω
u
cos
φ
+
F
.
(12.7)
2
t
a
ρ
2
z
z
Q
2
T
ω
ρ
.
v
=− ++
v
$
d
T
cc
(12.8)
2
t
p
p
d
ρ
+=
1
v
d
$
v
0.
(12.9)
ρ
dt
(12.10)
p T
=
ρ
.
2
()
ρ
q
v
=− +−
v
$
d
() .
ρ
qEP
(12.11)
2
t
