TAPIO SCHNEIDER

GCMs: General Circulation Models

We use a variety of GCMs for our research, from dry atmosphere models to relatively complex coupled atmosphere-ocean models. The GCMs we use are built using the Flexible Modeling System (FMS) of NOAA's Geophysical Fluid Dynamics Laboratory. For idealized GCMs, we use the FMS dynamical core (that is, the basic numerical schemes FMS provides for the hydrostatic primitive equations), with various idealizations for the lower boundary conditions, for radiative transfer, and for moist or dry convection.

Several dry and moist idealized GCMs are available in a source tree here. A README file describes the structure of the tree and contains instructions on how to run the models; a CHANGES file describes recent changes. The entire tree is also available as zip archive.

The models were developed with support by the U.S. National Science Foundation (NSF) and the National Aeronautics and Space Administration (NASA).

The dry idealized GCM has a spherical model surface that is spatially uniform and thermally insulating. Radiative heating and cooling are represented by Newtonian relaxation of temperatures toward prescribed radiative-equilibrium states. Because the radiative-equilibrium states are statically unstable, it is necessary to include a representation of convection. (The GCM can be run without a convection scheme, but the results become strongly resolution-dependent.) In the GCM, if a layer is statically unstable relative to a specified convective temperature lapse rate, a convection scheme relaxes temperatures in that layer toward an enthalpy-conserving profile with the convective lapse rate. The convective lapse rate can be specified to be smaller than the dry-adiabatic lapse rate, in which case the convection schemes mimics the stabilizing effect latent heat release has in moist convection. This makes it possible to obtain in this dry GCM mean circulations that resemble those of Earth's atmosphere.

The GCM is described in

Schneider, T., and C. Walker, 2006: Self-organization of atmospheric macroturbulence into critical states of weak nonlinear eddy-eddy interactions. J. Atmos. Sci., 63, 1569-1586.

The radiative-equilibrium states toward which temperatures are relaxed are those of a semi-gray atmosphere, described in

Schneider, T., 2004: The tropopause and the thermal stratification in the extratropics of a dry atmosphere. J. Atmos. Sci., 61,1317-1340.

The moist idealized GCM has a spherical model surface that is entirely water covered (an "aquaplanet"), but the water does not move (a "slab ocean"). Radiative heating and cooling are represented by a two-stream radiative transfer scheme for a gray atmosphere, in which absorption and emission of solar and of thermal radiation do not depend on wavelength. The surface has a closed energy budget, with heating driving sensible heat fluxes and evaporation. Atmospheric water vapor is advected by the flow; it condenses and precipitates when it reaches saturation on the grid-scale of the model. The thermodynamics of water are simplified in that only the vapor-liquid phase transition is taken into account, and the latent heat of vaporization is taken to be constant, as in Frierson et al. (2006). Moist convection is represented by a simplified Betts-Miller convection scheme that relaxes temperatures toward a moist adiabat and specific humidities toward a profile with a prescribed relative humidity. (The convection scheme is a slight variant of that described in Frierson (2007).)

The GCM is described in

O'Gorman, P. A., and T. Schneider, 2008: The hydrological cycle over a wide range of climates simulated with an idealized GCM. J. Climate, 21, 3815-3832.

The tidally locked aquaplanet GCM is a variant of the moist idealized GCM. Like the latter, it has a spherical model surface that is entirely water covered (an "aquaplanet"), but the water does not move (a "slab ocean"). The insolation is that of a tidally locked planet, with the sun perpetually in zenith above a subsolar point. Radiative heating and cooling are represented by a two-stream radiative transfer scheme for a gray atmosphere. But unlike in the moist idealized GCM, the radiative transfer scheme includes a simple representation of water vapor feedback. This GCM is suitable for the study of tidally locked terrestrial exoplanets.

The GCM is described in

Merlis, T. M., and T. Schneider, 2010: Atmospheric dynamics of Earth-like tidally locked aquaplanets. Journal of Advances in Modeling Earth Systems, 2, Art. #13, 17 pp.
Zonal wind animations: [Slow rotation] [Rapid rotation]

The idealized GCMs described above are flexible and general enough that it is relatively straightforward to convert them into GCMs for other planets, for example, with different planetary rotation rates and radii and with different thermodynamic properties of the atmosphere, condensable species, and surface. GCM code for other planets that we have described in published work is available from us upon request.