CCM/MP-2D Performance Studies

Performance Studies using

CCM/MP-2D

Brief Description of CCM3

CCM3 is the latest version of the community climate model. The vertical and temporal aspects of the model are represented by finite difference approximations. The spherical harmonic transform (spectral transform) method is employed to compute the dry dynamics. This method computes the spherical harmonic function coefficient representation of the atmospheric state variables by first transforming them from the physical domain (longitude-latitude-vertical) to the Fourier domain (wavenumber-latitude-vertical) using fast Fourier transforms (FFTs) in the longitude direction. Then the state variables are transformed from the Fourier to the spectral domains (spectral coefficients - vertical) using Legendre transforms (LT) in the latitude direction. The set of spectral coefficients is typically truncated in some fashion to avoid aliasing. Horizontal derivatives and linear terms involving these variables are calculated and combined to form the dynamical right hand sides in spectral space. These spectral space computations are independent between spectral coordinates. The results are transformed back into the physical domain where they are used to update the model variables.

The calculation of nonlinear terms in the equations of motion are carried out on a grid in the physical domain. A tensor product grid is used, where the latitude coordinates are chosen to be Gauss quadrature points, as used to approximate the Lengendre transform, and the longitude points are equispaced. The "physics" computations involve only the vertical column above each longitude-latitude grid point and are thus numerically independent of each other in the horizontal direction. Trace gases, including water vapor, are transported by the wind fields using a shape perserving semi-Lagrangian scheme on the physical grid.

For spectral global circulation models such as CCM3 it is canonical to denote the resolution by the truncation wave number and the number of vertical levels in the model discretization. For example, a spectral model that uses a 128 longitude by 64 latitude grid and 18 vertical levels and a triangular truncation of the spherical harmonic coefficients is called a T42L18 model. The ``T'' indicates triangular truncation, 42 indicates the maximum Fourier wave number, and the ``L'' denotes the number of vertical levels. Resolution T170L18 has 512 longitude by 256 latitude grid and 18 levels.

Patrick H. Worley / ( worleyph@ornl.gov)
Last Modified Monday, 15-Jul-2002 09:58:42 EDT.
5108 accesses since 1/2/96.