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Atmospheric General Circulation Model

The NSIPP-1 AGCM uses a finite-difference C-grid on latitude-longitude coordinates in the horizontal. The vertical coordinate is a standard σ-coordinate (σ = pressure/surface pressure) and vertical differencing follows Arakawa and Suarez (1983). The momentum equations use a 4th-order version of the enstrophy conserving scheme of Sadourny (1975). The horizontal advection schemes for potential temperature, moisture, and other tracers are 4th-order (Takacs and Suarez, 1996). An eighth-order Shapiro filter is used to dissipate small horizontal scales in all prognostic fields except surface pressure. A polar Fourier filter is applied poleward of 45° latitude to the time tendencies of all prognostic variables.

The parameterizations of solar and infrared radiative heating rates are those of Chou and Suarez (1994 and 1999). From the moist physics parameterization, the GCM estimates a cloud fraction at each level. For the solar radiation calculation, the GCM levels are then grouped into three regions which are identified with high (σ < 0.56), middle (0.56 < σ < 0.77), and low (σ > 0.77) clouds. Within each of these regions, clouds are assumed to be maximally overlapped and the cloud fractions are scaled using a scheme that depends on solar zenith angle and optical thickness.

The boundary layer scheme in the model is a simple K-scheme that calculates turbulent diffusivities for heat and momentum based on Monin-Obukhov similarity theory (Louis et al., 1982). Gravity-wave drag is parameterized according to Zhou et al. (1996).

Penetrative convection originating in the boundary layer is parameterized using the Relaxed Arakawa-Schubert (RAS) scheme (Moorthi and Suarez, 1992). RAS acts as a parameterization of both deep and shallow convection. Re-evaporation of convective rainfall is included and is based on the formulation of Sud and Molod (1988).

Large-scale cloudiness is determined using a simple relative humidity-based diagnostic scheme such as that of Slingo (1987). However, even with a high-threshold relative humidity, the scheme produces excessive cloudiness over subtropical oceans. Thus, a second “destruction” step is invoked that uses the magnitude of subsidence drying produced by RAS to destroy a fraction of the large-scale clouds produced by the relative humidity diagnostic.

The AGCM performance is documented in the Technical Memoranda of Suarez and Takacs (1995), Bacmeister et al. (2000) and Pegion et al. (2000). Further details and performance of the model used in the CGCMv1 are given Bacmeister and Suarez (2002).