The Climate System and Its Portrayal by Climate Models: A Review of Basic Principles

Abstract

The global climate system is reviewed in terms of the characteristic physical processes which operate within and between its atmospheric, oceanic, cryospheric and land surface components (part one of this paper). Together with conditions external to the system, these processes are responsible for both the maintenance and change of climate over time scales ranging from a season to geological epochs. The most critical aspect of the portrayal of the climate (part two of this paper) in terms of either simplified statistical-dynamical models or comprehensive general circulation models is identified as the parameterization of subgrid-scale processes, especially those involving convection, clouds and the surface boundary layer. The formulation and solution of general circulation models is reviewed, and the climate simulation capability of atmospheric general circulation models in particular is evaluated. In terms of the average sea-level pressure and precipitation such models simulate the observed statistics reasonably well, although a number of modeling problems remain; chief among these is the coupling of the atmosphere and ocean in order to examine the predictability of the climate on seasonal and interannual time scales.