Comparing the formulations of CCAM and VCAM and their performance as atmospheric GCMs
Seminar Room 1, Newton Institute
Two cube-based atmospheric GCMs have been developed at CSIRO, the Conformal-Cubic Atmospheric Model (CCAM) and, more recently, the Variable Cubic Atmospheric Model (VCAM). The formulations of the dynamical cores of both models will be described and compared. CCAM is formulated on the conformal-cubic grid, whereas VCAM is cast on the equiangular gnomonic-cubic grid. CCAM is a 2-time-level semi-Lagrangian semi-implicit Eulerian model, whereas VCAM employs a split-explicit flux-conserving approach. Both models use reversible staggering for the wind components (McGregor, MWR, 2005) to produce good wave dispersion behavior. CCAM employs several orographic treatments that are not available for use in the VCAM dynamical core, with the interesting consequence that only VCAM requires hybrid vertical coordinates.
Both models include the same efficient message-passing framework. Although VCAM avoids the message-passing overheads necessitated by the Helmholtz solver of CCAM, if does have some overheads from more frequent calls to the wind staggering/unstaggering routines.
Aspects of the climatologies of both models will be compared and their overall advantages and disadvantages discussed. VCAM is presently being coupled to the Parallel Cubic Ocean Model (PCOM) from JAMSTEC; both the atmosphere and ocean model employ identical grids, and progress on this activity will be briefly presented.