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The Fourth Generation Coupled Global Climate Model
Canadian Centre for Climate Modelling and Analysis
The atmospheric component of CGCM4/CanCM4 is The Fourth Generation Atmospheric General Circulation Model and its oceanic component is the Fourth Generation Ocean General Circulation Model (OGCM4/CanOM4).
OGCM4 uses a z-level vertical coordinate, with horizontal differencings formulated on an Arakawa B-Grid. It was developed from the NCAR CSM Ocean Model (NCOM). There are 40 vertical levels with spacings ranging from 10m near the surface (there are 16 levels in the upper 200m) to nearly 400m in the deep ocean. Horizontal coordinates are spherical with grid spacings approximately 1.41 degrees in longitude and 0.94 degrees in latitude. Computational instabilities due to convergence of meridians near the North Pole are suppressed via Fourier filtering, and there is a column of special tracer grid cells centred on the North Pole as described by Gent at al. (1998).
The OGCM4 grid and associated coastlines are congruent with that of the overlying AGCM, with six OGCM grid cells (two in longitude vs three in latitude) situated beneath each AGCM cell. Coupling is once per day, although in the ESM version of the model a simulated position-dependent diurnal cycle in incident shortwave radiation is employed.
Vertical mixing is via the K-profile parameterization (KPP) scheme of Large et al. (1994), to which is added an energetically constrained, bottom intensified vertical tracer diffusivity to represent effects of tidally-induced mixing; the latter is computed in a manner similar to Simmons et al. (2004). Horizontal friction is treated using the anisotropic viscosity parameterization of Large et al. (2001). Isoneutral mixing is according to the parameterization of Gent and McWilliams as described in Gent et al. (1995), with layer thickness diffusion coefficients optionally determined according to the formulation in Gnanadesikan et al. (2006). The model incorporates the equation of state for seawater of McDougall et al. (2003).
In treating shortwave penetration, a fraction 0.45 of the incident radiation is photosynthetically active (Baker and Frouin (1987)) and penetrates according to an attenuation coefficient which is the sum of a clear-water term and a term that varies linearly with chlorophyll concentration as in Lima and Doney (2004). Chlorophyll concentrations in the physical version of the model are specified from a seasonally varying climatology constructed from SeaWiFS observations, and in the ESM version are a prognostic variable. The remaining (red) incident shortwave flux is absorbed in the topmost layer.
The Strait of Gibraltar, Hudson Strait and pathways through the Canadian Archipelago that are unresolved are treated by mixing water properties instantaneously between the nearest ocean cells bordering intervening land. To prevent excessive accumulation of freshwater adjacent to river mouths, half of runoff from each modelled river is distributed across the AGCM cell (encompassing six OGCM cells) into which the river drains, with the remaining half distributed among all adjoining ocean-coupled AGCM cells.
Gnanadesikan, A., K.W. Dixon, S.M. Griffies, V. Balaji, M. Barreiro, J.A. Beesley, W.F. Cooke, T.L. Delworth, R. Gerdes, M.J. Harrison, I.M. Held, W.J. Hurlin, H.C. Lee, Z. Liang, G. Nong, R.C. Pacanowski, A. Rosati, J. Russell, B.L. Samuels, Q. Song, M.J. Spelman, R.J. Stouffer, C.O. Sweeney, G. Vecchi, M. Winton, A.T. Wittenberg, F. Zeng, R. Zhang, and J.P. Dunne, 2006: GFDL's CM2 Global Coupled Climate Models. Part II: The Baseline Ocean Simulation. J. Climate, 19, 675-697.
Large, W. G., G. Danabasoglu, J. C. McWilliams, P. R. Gent, and F. O. Bryan, 2001: Equatorial circulation of a global ocean climate model with anisotropic horizontal viscosity. J. Phys. Oceanogr., 31, 518-536.
McDougall, T.J., D.R. Jackett, D.G. Wright, and R. Feistel, 2003: Accurate and Computationally Efficient Algorithms for Potential Temperature and Density of Seawater. J. Atmos. Oceanic Technol., 20, 730-741.
Simmons, H, L. St. Laurent, S. Jayne and A. Weaver, 2004: Tidally driven mixing in a numerical model of the ocean general circulation. Ocean Modelling, doi:10.1016/S1463-5003(03)00011-8.
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