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Model description

The Canadian Middle Atmosphere Model (CMAM) [2] is a vertically extended and modified version of the general circulation model (GCM) of the Canadian Centre for Climate Modelling and Analysis (CCCma) [3]. It is a spectral model with a hybrid vertical coordinate which is terrain following near the surface and changes to a pressure coordinate throughout the middle atmosphere. This version of the model is run with 50 levels and T32 spectral resolution in the horizontal and uses a 15-min time step. In the middle atmosphere the vertical resolution is ~3 km, and the model top is at p= 0.000637 hPa or about 95 km. This model version uses a parameterization scheme to describe the momentum deposition by a broad spectrum of subgrid-scale gravity waves [4].

The model includes a photochemical module to allow for the use of a prognostic and radiatively active representation of ozone [5]. The photochemistry module contains 44 species including odd-hydrogen, odd-nitrogen, odd-chlorine, and odd-bromine families, N₂O, CFC-11, and CFC-12, CH₃Br, CH₄, and its oxidation products including CO [6]. It has 127 photochemical reactions, including 34 photolysis reactions. Transport is active throughout the domain, while chemistry is solved on-line from ~6 km to the top of the model. For this model version a spectral advection scheme is used for the transport of moisture and other chemical constituents. A full diurnal cycle is simulated with photolysis rates provided by a lookup table. The chemistry solver is a mass-conserving fully implicit backward difference scheme.