Geoscience Reference
In-Depth Information
LAND SURFACE MODELS
Land surface models
(LSMs) are coupled to AGCMs to provide a more com-
plete treatment of land surface/atmosphere interactions. These models have
become increasingly sophisticated in recent years, and they replace the simple
surface heat balance equation (Eq. 5.22) and so-called
bucket model
for surface
hydrology. In the bucket model, a unit area of land is visualized as a bucket
with a certain depth, for example, 15 cm, that represents the water-holding
capacity of soil. As the GCM is integrated over time, the bucket model keeps
track of the net delivery of water to the surface (precipitation minus evapora-
tion). If that value exceeds the depth of the bucket, the water is assumed to run
off to the sea.
Contemporary LSMs, such as the model sketched in
Figure 12.1a,
govern
the exchanges of water (precipitation, evapotranspiration, and sublimation)
and energy (latent and sensible heat fluxes, interactions with longwave and
shortwave radiation) between the atmosphere and the land surface. Momen-
tum exchanges through the treatment of surface friction are also included in
detail. The surface may be covered with natural vegetation, managed vegeta-
tion, or buildings and cement as in an urban environment.
In the bucket model, no accounting is made of river routing or the pos-
sible reabsorption of runoff by surrounding land regions—the water is simply
deposited into the ocean. The more complex LSMs represent the land surface
with multiple soil layers through which heat diffuses and water infiltrates (
Fig.
12.1b
). Surface runoff is accounted for and may be organized into streams and
rivers to allow for reabsorption of the water before it reaches the ocean, and
flooding is simulated. The vegetation type is calculated as a function of the
modeled climate state in
dynamic vegetation models
(DVMs).
ICE MODELS
The formation of ice and snow at the surface is important for simulating cli-
mate at high latitudes and high altitudes. A calculation for snow and glaciers is
included in LSMs, and a treatment of sea ice is coupled to OGCMs.
Information about atmospheric temperature and precipitation can be com-
bined to simulate snowfall rates within the AGCM and pass them into the
LSM. The surface temperature calculation is used to determine a rate of snow-
melt, which adds to surface moisture, and snow sitting on the surface is “aged”
in the sense that its initially bright albedo of about 0.8 is decreased to an old-
snow albedo of 0.6.
Land-based glaciers are another important component of the cryosphere
(see section 2.4). These slowly varying components of the climate system may
be prescribed in shorter simulations, or their observed distributions may be
prescribed as initial conditions since snowfall over many thousands of years is
needed in some regions (e.g., Antarctica) to build glaciers. Glacier change may
be parameterized as a function of surface and atmospheric temperatures and
