The Third Generation Atmospheric General Circulation Model
Canadian Centre for Climate Modelling and Analysis
The Third Generation Atmospheric General Circulation Model (AGCM3) (McFarlane et al. 2005, Scinocca et al. 2008) shares many basic features with The Second Generation Atmospheric General Circulation Model (AGCM2) (McFarlane et al. 1992). As in AGCM2, the spectral transform method is used 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 AGCM3 corresponds to a higher horizontal resolution than that used in AGCM2, being comprised of a 47 wave triangularly truncated (T47) spherical harmonic expansion. The vertical domain of AGCM3 is deeper than in AGCM2 and the vertical resolution is also higher. The third-generation model domain extends from the surface to the stratopause region (1hPa, approximately 50km above the surface).This region is spanned by 32 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.
The treatment of many of the parameterized physical processes in the third-generation model is qualitatively similar to AGCM2. However, the are some key features that are new to the third generation model. These include the introduction of CLASS, a new module for treatment of the land surface processes (Verseghy et al, 1992). This new land surface scheme is considerably more comprehensive than the simple single soil layer scheme used in AGCM2. In particular, the new scheme includes 3 soil layers, a snow layer where applicable, and a vegetative canopy treatment. Both liquid and frozen forms of soil moisture are carried as prognostic variables. Soil surface properties such as surface roughness heights for heat and momentum (which differ from each other in general), and surface albedos are taken to be functions of the soil and vegetation types and soil moisture conditions within a given grid volume.
Surface exchanges of heat moisture and momentum in AGCM3 are formulated following Abdella and McFarlane (1996). The treatment of turbulent transfer within the planetary boundary layer is similar in form to that in AGCM2 except that an additional direct non-local mixing of heat and moisture is carried out in circumstances where the buoyancy flux at the surface is upward. In these cases it is assumed that the boundary layer will tend to become well mixed to a depth that is determined to be such that the virtual potential temperature of the mixed layer does not exceed that of the layer directly above it. In practice this mixed layer is always comprised of the lowest model layer and an integral number of adjacent layers, rarely more than one or two.
In the AGCM3 the moist convective adjustment algorithm that was used in AGCM2 has been replaced by the cumulus parameterization of Zhang and McFarlane (1995). This parameterization is a bulk mass flux scheme which includes a representation of convective scale updrafts and downdrafts and is designed to account for the effects of penetrative convection in a manner that is more physically sound than is the moist convective adjustment scheme.
Solar radiative heating in AGCM3 is computed using an improved version of the scheme used in AGCM2. This newer version employs (4) bands in the visible and near infrared region. Cloud cover is specified as a function of the local relative humidity and the liquid water content of the clouds is taken to be proportional to the adiabatic value obtained by lifting a parcel through a specified depth, as in AGCM2. The optical properties of the clouds are parameterized in terms of the liquid water/ice content. The treatment of terrestrial radiation is similar to that used in AGCM2 but with improved treatment of broad band emissivities and the water vapour continuum.
A significant difference between the second and third generation models is in the treatment of water vapour transport. While the spectral transport algorithm is retained as in the earlier generation models, the quantity that is transported is the hybrid moisture variable proposed by Boer (1995). Use of this variable alleviates to a considerable extent the undesirable effects that accompany the development of unphysical negative values in association with spectral transport of specific humidity. A semi-Lagrangian transport algorithm also exists in AGCM3 as an option for transport of moisture and other trace constituents.
Since the current version of the CMAM does not have an active chemical source of water vapour, moisture (in terms of the hybrid variable) is not evaluated prognostically above the 25hPa level. For the purpose of evaluating radiative heating rates in the region of the model that is above this level the water vapour content is specified.
Abdella, K. , and N.McFarlane, 1996, Parameterization of the Surface-Layer Exchange Coefficients for Atmospheric Models., B. Layer Met., 80, 223-248.
Boer, G.J., 1995: A hybrid moisture variable suitable for spectral GCMs. Research Activities in Atmospheric and Oceanic Modelling. Report No. 21, WMO/TD-No. 665, World Meteorological Organization, Geneva.
Laprise, R. and C. Girard, 1990: A spectral general circulation model using a piecewise-constant finite element representation on a hybrid vertical coordinate system. J. Climate, 3, 32-52
Scinocca, J. F., N. A. McFarlane, M. Lazare, J. Li, and D. Plummer, 2008: The CCCma third generation AGCM and its extension into the middle atmosphere. Atmos. Chem. and Phys., 8, 7055-7074.
Verseghy, D.L., N.A. McFarlane, and M. Lazare, 1993: A Canadian Land Surface Scheme for GCMs:II. Vegetation model and coupled runs. Int. J. Climatol., 13, 347-370.
Zhang, G.J. and N. A. McFarlane, 1995: Sensitivity of climate simulations to the parameterization of cumulus convection in the CCC GCM. Atmos.-Ocean, 33, 407-446.
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