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The climate models that GISS has developed and operates include several atmospheric general circulation models (AGCMs). Such computer models numerically solve fundamental equations describing the conservation of mass, energy, momentum, etc. for each atmospheric gridbox, while taking into account the transfer of those quantities between gridboxes. They also consider, often in parameterized form, the physical processes within the boxes, including sources and sinks of these quantities. Examples of GCMs in current use at GISS are the GISS Model E (see Schmidt et al 2006); GISS Model II (Hansen et al. 1983, 1984); the GISS Middle Atmosphere Model for use in studying stratospheric processes (Rind et al. 1987a, 1987b); and the GISS Atmosphere-Ocean Model (GR) (Russell et al. 1995), amongst others.
The GCMs are all Cartesian grid-point models which can be run at a variety of horizontal and vertical resolutions; e.g., 8°×10°, 4°×5°, or 2°×2.5° in the horizontal (latitude × longitude) and nine-layer, eighteen-layer, 23-layer, and 31-layer in the vertical. The dynamics are based on the "Arakawa B" grid scheme, except for an alternate version that uses scheme C. Scheme B uses no horizontal viscosity and is particularly suitable for coarse resolution models. Model II used a second-order scheme for the advection of momentum, temperature, and humidity, but a higher-order numerical scheme for advection of temperature and specific humidity has been incorporated into Model II' ("two-prime"). The scheme, called "second-order moments" (Prather 1986), is an extension of the linear upstream scheme developed by Russell and Lerner (1981). By creating sharper temperature gradients, this scheme strengthens both eddies and eddy transports.
The newest GISS GCM is ModelE, which was officially released in early 2004. This was the culmination of a multi-year developmental process and has been extensively tested during recent months. The new model includes the option of coupling to a variety of ocean models, up to and including fully dynamic three-dimensional (3D) ocean general circulation models (OGCMs) and the full incoporation into the model of a number of tracer subsystems including atmospheric chemistry and aerosol transport.
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