A general circulation model or global climate model (these are synonymous interpretations of the acronym GCM) is a climate model that tries to incorporate all relevant processes on Earth in a three dimensional model. Such models are the most complex ones today.
The following graphic is an illustration of the components of a GCM. It was supposedly created in the 1980’s, and is provided by the climate modelling group of the University of Bremen, Germany.
The original can be downloaded here.
Usually the horizontal grid spacing is not the same as the vertical grid spacing. The GCMs mentioned in the last IPCC report had a horizontal grid spacing of 100-300 km. CGM currently under development aim at 50 km and less.
Earth has a surface of 510,072,000 , therefore a homogeneous grid of 50 km horizontal spacing corresponds to ca. 200 000 horizontal grid cells. If there are 25 vertical grid cells, we get ca. 5 000 000 grid cells. If we assume a memory capacity of 1 GB for holding the grid, we have ca. 2 kB per grid cell.
The NASA/GISS Atmosphere-Ocean Model? or AOM was a continually evolving computer program written in Fortran. The website for this project contains files that describe the model, both in English and in algorithm notation, together with the Fortran source code:
The AOM was NASA’s previous-generation GCM. As of 2010, the current generation is ModelE?:
This involves a complete rewrite of the underlying physics, combined with greater flexibility and better representations of the stratosphere, tracer components and various ocean models.
The frozen version used for IPCC AR4 simulations is called ModelE1 (internal version number 3.0, dated Feb. 1, 2004). This code can be freely downloaded (as a 1.2 MB gzip-ed tar file). There are nightly snapshots of the current code repository available also, but users should be aware that these snapshots are presented ‘as is’ and are not neccessarily suitable for publication-quality experiments. The frozen AR5 version will be posted soon.
Another GCM with publicly available source code is:
The geophysical fluid dynamics laboratory at Princeton has produced several GCMs, an introductory webpage can be found here:
Educational Global Climate Modeling, or EdGCM, is a relatively simple AOGCM based on ModelE (see above). Quoting the EdGCM web site:
EdGCM provides a research-grade Global Climate Model (GCM) with a user-friendly interface that can be run on a desktop computer. For the first time, students can explore the subject of climate change in the same way that actual research scientists do. In the process of using EdGCM, students will become knowledgeable about a topic that will surely affect their lives, and we will better prepare the next generation of scientists who will grapple with a myriad of complex climate issues.
Important numerical techniques include solvers for the Navier-Stokes equations including large eddy simulations to include turbulence. For models that combine the atmosphere and the oceans, see atmospheric and oceanic fluid dynamics.
Global climate model, Wikipedia.
Warren M. Washington, Claire L. Parkinson: Introduction To Three-dimensional Climate Modeling (University Science Books; 2nd edition (May 16, 2005))
Thomas Tomkins Warner: Numerical Weather and Climate Prediction (Cambridge University Press, January 17, 2011)
Geoffrey K. Vallis: Atmospheric and Oceanic Fluid Dynamics: Fundamentals and Large-scale Circulation (Cambridge University Press 2006)
For a historical overview of the models, see: