**2.2.1 General circulation models**

Global Circulation Models (GCM) solve the geophysical fluid dynamics of the atmosphere. They have the same general structure as the numerical meteorological models used for weather prediction. The main difference is that weather prediction models are run for several days or a week, while GCMs are run years even centuries. Therefore GCMs have to be developed using energy conserving algorithms. Another difference is the spatial and temporal discretization. The weather prediction models are run on a horizontal resolution of several ten kilometres, whereas GCMs have a horizontal resolution of several degrees of longitude and latitudes.

Climate change related studies are conducted using global Atmosphere-Ocean General Circulation Models (AOGCM). AOGCMs provide results that give general information on global scale and boundary forcing for higher resolution regional climate change models. Seveal well known AOGCMs are briefly described in following paragraphs.

Hadley Centre Coupled Model, version 3 (HadCM3) is a coupled atmosphere-ocean general circulation model (AOGCM) developed at the Hadley Centre in the United Kingdom (Gordon et al., 2000; Pope et al., 2000; Collins et al., 2001). It was one of the major models used in the IPCC Third Assessment Report in 2001. HadCM3 includes two components; the atmospheric model HadAM3 and the ocean model that includes a sea ice model. Simulations often use a 360-day calendar, where each month is 30 days. HadAM3 has a horizontal resolution of 3.75×2.5 degrees in longitude × latitude. This gives 96×73 grid points on the scalar (pressure, temperature and moisture) grid; the vector (wind velocity) grid is offset by 1/2 of a grid box resulting in a resolution of approximately 300 km. The timestep is 30 minutes (with three sub-timesteps per timestep in the dynamics).

The coupled global model ECHAM4/OPYC3 was developed in co-operation between the Max-Planck-Institute for Meteorology (MPI) and Deutsches Klimarechenzentrum (DKRZ) in Hamburg, Germany. The ECHAM model is an atmospheric circulation model. The reference horizontal resolution is 300 km , but the model is set up to use finer and coraser resolutions. The time step at reference horizontal resolution is 40 minutes. ECHAM4 is coupled with ocean circulation model OPYC3.
