chapter - Atmospheric and Oceanic Science

Global climate models
Table 11.1. Boundary data sets required for an AGCM. Source: CIER Magazine N° 43
Boundary Data Parameterization Dimensions
Sea Surface Temperature Radiation and PBL Lat, Lon and Time
Surface type (land, ocean, …) Surface temperature Lat, Lon
Surface roughness length Surface Lat, Lon
Land hydrology Surface Lat, Lon
Surface albedos Radiation Lat, Lon
Ozone mixing ration Radiation Lat, Lon
Orography Dynamics Lat, Lon
Subgrid variance of orography Gravity wave drag Lat, Lon
One of the most challenging aspects of modeling the climate system is the
treatment of nonresolvable physical processes, otherwise known as parameterization.
Atmospheric processes operate over a very wide range of time and space
scales. Because of computational cost, however, numerical integrations of the governing
meteorological equations explicitly resolve only the primary energetic and
phenomenological scales of motion. Nevertheless, there are significant interactions
between the explicitly resolved large-scale flow and the truncated scales of motion
which must be accounted for in some way. Parameterization techniques seek to
express the statistical contribution of these nonresolvable processes to the time evolution
of the explicitly resolved motions. At the same time, the contributions of
these nonresolvable motions are diagnosed as functions of the large-scale fields. As
observed from the table 11.1, there are many physical parameterizations employed
by AGCMs and the continued success of atmospheric modeling depends on
improving these various parameterizations.
The treatment of the water budget is the most difficult component of the parameterization
problem. Convective scale motions (e.g., motions on the order of several
km) are responsible for most of the phase change and associated precipitation
occurring in the atmosphere. These processes occur well below the resolvable scales
of motion in a GCM, but represent a very large, and often dominant, local energy
source/sink in the climate system. The effect of convective motions on the evolution
of the thermal, moisture and even dynamical properties in the model atmosphere
must be parameterized. Since the meteorological term for convective clouds is
cumulus, in the atmospheric modeling community it is also called cumulus parameterization.
These parameterizations are, in general, for deep convective clouds that
occur in regions of moisture convergence. Over the years, several deep convective
parameterizations have been developed for AGCMs. Like many parameterizations,
the unresolved properties must be linked to the large scale model variables, see for
instance Kiehl (1993) for a detailed description of the method and examples.
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