Climate and Climate-System Modelling - Environmental Modelling: Finding Simplicity in Complexity - page 149

processes, as illustrated in Figure 9.1. AOGCMs compute
the transfer of solar and infrared radiation through the
atmosphere, and so automatically compute the radia-
tive forcing associated with changes in greenhouse-gas or
aerosol concentrations, as well as the fast feedback pro-
cesses that collectively determine the climate sensitivity.
Atmospheric and oceanic general circulation models also
compute the absorption of heat by the oceans, which
delays and distorts the surface temperature response to
increasing greenhouse-gas concentrations. McGuffie and
Henderson-Sellers (2005: Chapter 5) provide a compact
introduction toAOGCMs, while their strengths andweak-
nesses in simulating the present climate are discussed in
Randall et al . (2007) and Reichler and Kim (2008).
The early AOGCMs computed the transfer of heat
and moisture between the land surface and atmosphere
using prescribed vegetation types (such as rainforest,
grassland or savannah) with fixed properties for each
vegetation type. A key parameter controlling the rate of
evapotranspiration is the stomatal resistance r s ,andin

early AOCGMs r s was computed as some simple function
of the amount of moisture in the model soil.

9.2.3 Addingthe terrestrial biospherecomponent
of thecarboncycle toAOGCMs

The most recent models compute the distribution of ten
or so plant functional types (PFTs), with some allowing
only one PFT in a given grid cell and others allowing for a
mixture of PFTs in each grid cell. Some recent models also
include a simple representation of the terrestrial carbon
cycle in each grid cell, as illustrated in Figure 9.2. Car-
bon fluxes (photosynthesis, respiration and transfers) are
computed several times per hour, then summed annually
and used to compute the change fromone year to the next
in the amount of carbon as leafy andwoody plant biomass,
as detritus, and as soil carbon. The photosynthetic carbon
flux depends in part on the stomatal resistance, with
the stomatal resistance now dependent not only on soil
moisture but also on atmospheric CO 2 concentration

Sun

Incoming
solar radiation

.009
.074
.189

Outgoing
infrared radiation

Atmospheric GCM (R15, ∼ 7.5 ° × 4.5 ° )

Cirrus clouds

Stratus clouds

.336

Atmosphere

.500 Cumulus

coluds

Stratus clouds

Precipitation
evaporation

Sublimation

Sensible and latent
heat fluxes
(bulk aerodynamic)

.664

Precipitation
evaporation

Runoff

.811
.826
.991
50 m
450 m

Snow cover
Soil moisture (15 cm bucket)

Win d str ess

Land surface processes
(snow cover, soil moisture, orography)
Realistic
geography

Sea ice

Ocean
prediction of currents,
temperature and salinity

Salinity
flux

Convective
overturning

1500 m

Ocean
bottom topography

Ocean GCM
(5 ° × 5 ° GRID)

2000 m

Figure 9.1 Illustration of the major processes occurring inside a typical horizontal grid cell of an atmosphere-ocean general
circulation model (AOGCM). Reproduced fromWashington and Meehl (1989), where a full explanation can be found. 'Sigma level'
refers to the ratio of pressure at a give height to the surface pressure.