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Physical Processes
Some physical processes are too complicated or occur at such small
scales that they cannot be resolved by the coarse grid spacing of model and
must be approximated by what are called parameterizations. Cloud
processes, the absorption and radiating properties of the atmosphere and
clouds, and atmospheric interactions with vegetation and other surface
features must be described by parameterizations. Figure 2 shows
some of the factors that are taken into account in surface
parameterizations. Vegetation transpires water into the air and suspends
liquid water from precipitation or dew deposition.
The roots, stems, and leaves provide a pathway for subsurface water to be
delivered to the atmosphere. The surface parameterization must account for seasonal change
in vegetation and its change in transpiration, its change in reflective
properties, and its change in drag forces on the atmosphere. Evaporation,
infiltration of water to the deep soil, and surface runoff to streams and
lakes must be taken into account at each time step.
