Geoscience Reference
In-Depth Information
variables, the GCM stores information on the global pattern of state variables at
each level in the atmosphere in the form of a Fourier series. In this series, each
term has a wavelength an integral multiple of which corresponds to the distance
around the Earth. By expressing the longitudinal distribution of state variables in
this way between time steps, it is possible to capture most (but not all) of the spatial
variability using less computer storage by truncating the Fourier series after a
specified number of wavelengths. The number of wavelengths before truncation
varies with the application of the GCMs, longer climate prediction runs having
truncation earlier than shorter NWP runs. When using a spectral grid, movement
within each vertical layer of the atmosphere can be calculated in spectral space and
this computation can be efficient. However, vertical movement and calculating the
physics for each atmospheric column requires transposition of the state variables
into coordinate space. Using a spectral grid is most effective when describing state
variables that vary smoothly across the globe. Truncating the Fourier series in a
spectral description can give rise to artificial wavelike features and unrealistic
divergences and, as computer memory becomes more available, the technical
advantages of using a spectral description of state variables become less
significant.
Calculating the physics
Calculation of the physics is made for each atmospheric column extending from
the surface upward to a defined level, which is regarded as being the top of the
atmosphere. The GCM code contains subroutines that compute the divergence of
energy, momentum and the mass of atmospheric constituents, the surface
exchanges of these variables, and buoyant exchanges between cells at different
levels in this column. Typically GCMs will include at least the following
subroutines.
Radiation scheme
: The radiation transfer scheme calculates the propagation
and reflection of shortwave and longwave radiation through each layer in the
atmospheric column from the modeled profiles of temperature, humidity,
cloud amount and the concentration of aerosols, ozone, and radiatively
active gases such as water vapor and carbon dioxide.
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Boundary-layer scheme
: The boundary-layer scheme uses first order, height-
integrated representations of the surface energy and momentum exchanges
between the lowest model level represented in the GCM and the ground,
based on the modeled vertical gradients of the variables which control these
exchanges.
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Surface-parameterization scheme
: The surface-parameterization scheme pro-
vides a description of shortwave and longwave exchange, momentum cap-
ture, and how the available energy at the surface is shared as heat fluxes
for sea, sea-ice, land-ice, snow-covered land, and several snow-free
land-cover types.
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