Slide 17: Contemporary GCMs: an outline
GCMs are involved incredibly complex calculations of atmospheric functions, and the best we can manage within the auspices of this course is a brief outline of their function. The most complex current models are known as coupled atmospheric ocean general circulation models (AOGCMs).
They have between 10 and 20 layers in the atmosphere, and as many as 30 layers in the ocean.
Contemporary AOGCMs have a horizontal resolution of between 250km and 600km.
For local planning, this is a very coarse scale, and the underlying topography is poorly represented. If necessary, results can be scaled down post hoc using a series of modelling functions (or by using regional modelling), but this is a complex and as yet, relatively inaccurate process.
However, taken over the whole globe, this resolution results in an extremely large number of individual cells. (1.5 million oceanic cells, and approximately 250 000 atmospheric cells).
For a given time step, calculations are carried out for each of these cells over the whole globe, including energy exchanges between each of the 26 adjacent cells.
Clearly this is very computationally intensive, and it is no surprise that atmospheric predictions have been at the forefront of computer development since the early 1950s.