Parameterizations are used in a
General circulation model (GCM) to represent processes that are not directly simulated, either because the corresponding
equations are not included or because the processes are too small for the model
resolution. For instance, cloud formation or precipitation are calculated from quantities like
radiation,
water vapor, and vertical wind speed, depending on the exact paramterization scheme employed.
Much of the differences between GCM experiments to predict the rate of global warming can be attributed to different parameteriztions in the models. However, the most important design goal in choosing parameterizations for a GCM is not so much picking the "best" one than finding a set of parameterizations for the model that is consistent.