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Abstract:

The UCLA atmospheric general circulation model (AGCM) was originally developed by Arakawa [1,2] of UCLA. It has been extended by C. Roberto Mechoso and collaborators at UCLA and is being used by them in comprehensive studies on the dynamics of the atmosphere and of the coupled atmosphere-ocean system. The AGCM has two principal parts, one for dynamics and the other for physics. The first part solves the equations of hydrodynamics derived from the Navier-Stokes equations suitably adapted to the regimes encountered by the atmosphere. The equations are implemented based on a grid point finite difference model, with a horizontal Arakawa C-mesh in latitude/longitude and vertical staggering of velocity and thermodynamic variables. The vertical differencing scheme uses the hydrostatic approximation to reduce the three-dimensional equations to sets of coupled two-dimensional equations. Time differencing is explicit, with time steps small enough to satisfy the Courant-FriedrichLevy (CFL) stability condition at low latitudes. The step size can violate the CFL condition near polar cells through the use of a Fourier filtering scheme that dampens unstable modes in such regions. The second part of the AGCM consists of a collection of parameterized physical process models collectively referred to as the "column physics " modules, as they are typically computed for a vertical column over a given horizontal mesh cell. These include energy, momentum, and moisture transport (long and short wave radiation, cumulus convection, surface fluxes), cloud instability, simplified ozone photochemistry, and ground temperature evolution calculations, among others. The column physics time step is determined essentially by the vertical structure of the model and does not need to be decreased with increased horizontal resolution. Parallel Decomposition Parallelization of the UCLA AGCM has been a collaborative effort of researchers at the

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