The Fourth Generation Atmospheric General Circulation Model

Canadian Centre for Climate Modelling and Analysis

As in The Third Generation Atmospheric General Circulation Model (AGCM3) (McFarlane et al., 2005; Scinocca et al., 2008), the spectral transform method is used in the Fourth Generation Atmospheric General Circulation Model (AGCM4) to represent the horizontal spatial structure of the main prognostic variables while the vertical representation is in terms of rectangular finite elements defined for a hybrid vertical coordinate as described by Laprise and Girard (1990).

The spectral representation currently used in AGCM4 corresponds to a higher horizontal resolution than that used in AGCM3, being comprised of a 63 wave triangularly truncated (T63) spherical harmonic expansion. As in AGCM3, the vertical domain of AGCM4 extends from the surface to the stratopause region (1hPa, approximately 50km above the surface), but the vertical resolution is slightly higher in AGCM4 with more uniform resolution across the tropopause. In the vertical the domain is spanned by 35 layers. The mid point of the lowest layer is approximately 50 meters above the surface at sea level. Layer depths increase monotonically with height from approximately 100 meters at the surface to 3km in the lower stratosphere.

There have been substantial changes to physical parameterizations in AGCM4. The radiative transfer scheme in AGCM3 has been replaced by a new scheme that includes a new correlated-k distribution model (Li, 2002; Li and Barker, 2002; Li and Barker, 2005) and a more general treatment of radiative transfer in cloudy atmospheres using the McICA methodology (Pincus et al., 2003; Barker et al., 2008). In conjunction with other parameterizations, the radiative transfer scheme accounts for the direct and indirect radiative effects of aerosols. A prognostic bulk aerosol scheme with a full sulphur cycle, along with organic and black carbon, mineral dust, and sea salt has been added (Lohmann et al., 1999; von Salzen et al., 2000; Croft et al., 2005). A fully prognostic single-moment cloud microphysics scheme is now used in the model, based on the work of Lohmann and Roeckner (1996), Rotstayn (1997), and Khairoutdinov and Kogan (2000). A statistical approach is used for macrophysical properties of layer clouds (Chaboureau and Bechtold, 2005). A new shallow convection scheme has been added to the model (von Salzen et al., 2005). Approaches to local and non-local turbulent mixing by Abdella and McFarlane (1996) and McFarlane et al. (2005) were improved for AGCM4.

AGCM4 has two strategies to deal with the artifacts of spectral advection (Gibbs effect). In the first, the hybrid variable approach adopted for the moisture variable in AGCM3 has been generalized and extended to tracers in AGCM4 (see Section 2.2 of Scinocca et al. (2008)). The second strategy follows an approach first suggested by Lander and Hoskins (1997) where both input fields and the output tendencies from the physics package are spatially filtered (further details may be found in Scinocca et al. (2008).


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