Climate modelling on an equal-area global grid

Dr John McGregor1

1CSIRO Oceans and Atmosphere, Aspendale, Australia


The Conformal-Cubic Atmospheric Model (CCAM) has been developed at CSIRO for about 20 years. CCAM is formulated on the conformal-cubic grid and employs 2-time-level semi-Lagrangian semi-implicit numerics. It has mainly been used in dynamical downscaling studies of climate change, but it is also used for specialized numerical weather prediction applications. CCAM uniquely employs reversible staggering for the wind components, producing good wave dispersion behaviour and this also leads to good behaviour for the kinetic energy spectra. In order to handle very fine resolution, CCAM utilizes the highly efficient non-hydrostatic equations of Miller and White. CCAM includes a comprehensive set of physical parameterizations, many of which are designed to be “scale-aware” in order to handle variable-resolution applications of the model.

Recently the CCAM code has been generalized to utilize a Uniform Jacobian (UJ) version of the cubed-sphere grid, which has several advantages. This grid is derived from the conformal-cubic grid to provide equal area for every grid cell. Since the grid lines are no longer orthogonal, covariant and contravariant velocity components are required. Apart from the complications of the velocity components, most of the CCAM semi-Lagrangian approach may be used, including reversible staggering of the velocity components to switch between values at cell centres and cell edges. The multigrid solver for the Helmholtz equation is more complicated than for the conformal-cubic version, but the overall model obtains similar extremely high efficiency.


John McGregor obtained his Ph.D. in applied mathematics from Monash University in 1974. Since then he has been employed by CSIRO developing weather and climate models. Early in his career he reformulated the operational Australian ARPE weather forecasting model, and also developed several physical parameterizations for the global forecast model. Since the early 1990s he has been active in regional climate modelling, being a pioneer in that field with the one-way-nested downscaling approach of his DARLAM model. From the late 1990s John developed the Conformal-Cubic Atmospheric Model (CCAM), also used for many climate downscaling studies with its variable resolution and digital filter technique. Recently he has developed new versions of CCAM upon the equal-area cubed-sphere. John is presently an honorary fellow at CSIRO Oceans and Atmosphere.


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