http://www.bom.gov.au/bmrc/climdyn/staff/bma/bma.shtml , and McAvaney et al. 1978 ). , Hart et al. (1988 , 1990 ), Colman and McAvaney (1991) , McAvaney et al. (1991) , and Rikus (1991) . The model configuration for the AMIP experiment is described by McAvaney and Colman (1993) . . Snow cover is initialized from the albedo data of Hummel and Reck (1979) : albedos greater than 40 percent define areas of seasonal snow with initial depth of 5 m; in areas of permanent snow (i.e., Antarctica and Greenland) the initial depth is set to 250 m.  frequency filter is combined with a split implicit scheme for the vertical diffusion component of the model physics. A time step of 15 minutes is used for both dynamics and physics, except that full calculations of radiative fluxes and heating rates are done once every 3 hours. Orography). Filling of negative atmospheric moisture values is performed by a combination of local horizontal and vertical borrowing, and global borrowing following the method of Royer (1986) . A mass adjustment scheme is also used to prevent a slow drift in surface pressure during long integrations. Cf. McAvaney et al. (1991)  for further details.
- Linear second-order (del-squared) horizontal diffusion is applied for wave numbers n > 31 in the upper part of the spectral rhomboid, with a first-order sigma coordinate correction applied near topography.
- Stability dependent vertical diffusion after Louis (1979)  is only applied for sigma levels > 0.5 in stable layers, but it operates in all unstable layers with no separate removal of dry superadiabats, and with a minimum wind speed difference of 1 m/s assumed between model levels.
- Shortwave Rayleigh scattering and absorption in ultraviolet (< 0.35 micron) and visible (0.5-0.7 micron) spectral bands by ozone, and in the near-infrared (0.7-4.0 microns) by water vapor and carbon dioxide follow the method of Lacis and Hansen (1974)
. Pressure corrections and multiple reflections between clouds are treated. The radiative effects of aerosols are not included directly.
- Longwave radiation follows the simplified exchange method of Fels and Schwarzkopf (1975)
 and Schwarzkopf and Fels (1991)
, with slight modifications. (The parent code is compared against benchmark computations by Fels et al. 1991
.) Longwave calculations follow the broad-band emissivity approximation in 8 spectral intervals (with wavenumber boundaries at 0, 1.6 x 10^4, 5.6 x 10^4, 8.0 x 10^4, 9.0 x 10^4, 9.9 x 10^4, 1.07 x 10^5, 1.20 x 10^5, and 2.20 x 10^5 m^-1). Another 14 bands are accounted for in the cooling-to-space corrections. Included in the calculations are Fels and Schwarzkopf (1981)
 transmission coefficients for carbon dioxide, the water vapor continuum of Roberts et al. (1976)
, and the effects of water-carbon dioxide overlap and of a Voigt line-shape correction. , and the effects of water-carbon dioxide overlap and of a Voigt line-shape correction.
- The treatment of cloud-radiative interactions is as described by Rikus (1991)  and McAvaney et al. (1991) . Shortwave cloud reflectivity/absorptivity is prescribed for ultraviolet-visible and near-infrared spectral bands and depends only on the height class of the cloud (see Cloud Formation). In the longwave, all clouds are assumed to behave as blackbodies (emissivity of 1). For purposes of the radiation calculations, all clouds are assumed to be randomly overlapped in the vertical.
- Deep convection is simulated by a variation of the method of Kuo (1974)
 that includes modifications of Anthes (1977)
. Penetrative convection is assumed to occur only in the presence of conditionally unstable layers in the vertical and large-scale net moisture convergence. The convective cloud base is assumed to be at the first level (maximum sigma = 0.926) above the planetary boundary layer (PBL) which is conditionally unstable. The convective cloud is assumed to dissolve instantaneously through lateral mixing, thereby imparting heat and moisture to the environment. In a vertical column the total moisture available from convergence is divided between a fraction b that moistens the environment and the remainder (1 - b) that contributes to the latent heating (rainfall) rate. In the Anthes modification of the Kuo scheme, the moistening parameter b is determined as a cubic function of the ratio of the mean relative humidity of the cloud layer to a prescribed critical relative humidity threshold value; if the cloud relative humidity is less than the threshold, b is set to 1 (no heating of the environment).
- Simulation of shallow convection is parameterized in terms of the model's vertical diffusion scheme, following the method of Tiedtke (1983 , 1988 ).
- Distinguished surface types include ocean, land, land ice, and sea ice, and the presence of snow cover is also accounted for on the latter three surfaces. Soil or vegetation types are not distinguished.
- The roughness length over oceans is determined from the surface wind stress, following Charnock (1955)
, with a coefficient of 0.0185 assigned after Wu (1982)
; the ocean roughness is constrained to a minimum value of 1.5 x 10^-5 m. Roughness lengths are prescribed uniform values over sea ice (0.001 m) and land surfaces (0.168 m), but the presence of snow cover changes the roughness to a new (fixed) value.
- Over oceans, the surface albedo depends on solar zenith angle, following Payne (1972). Seasonal climatological surface albedos of Hummel and Reck (1979)
 are prescribed over land. The surface albedos of sea ice and snow-covered land follow the temperature-ramp formulation of Petzold (1977)
, with different values of albedo limits and a lower temperature range for sea ice and snow, as described by Colman and McAvaney (1992).
- Longwave emissivity is set to unity for all surfaces (i.e., blackbody emission is assumed).
- Surface solar absorption is determined from surface albedos, and longwave emission from the Planck equation with prescribed constant surface emissivity of 1.0 (see Surface Characteristics).
- The surface turbulent eddy fluxes of momentum, heat, and moisture follow Monin-Obukhov similarity theory, and are formulated in terms of bulk formulae with stability-dependent drag/transfer coefficients determined as in Louis (1979)
. The momentum flux is given by the product of the air density, a neutral drag coefficient, wind speed and wind vector at the lowest prognostic level (sigma = 0.991), and a transfer function that depends on roughness length (see Surface Characteristics) and stability (bulk Richardson number). Surface wind speed is constrained to a minimum of 1 m/s. The flux of sensible heat is given by a product of a neutral exchange coefficient, the wind speed at the lowest prognostic level, the difference in temperatures between the ground and the first prognostic atmospheric level, and a modified form of the transfer function for unstable conditions (cf. Louis 1979)
- The flux of surface moisture is given by a product of the same transfer coefficient and stability function as for sensible heat, an evapotranspiration efficiency beta, and the difference between the specific humidity at the first prognostic level and the saturation specific humidity at the surface temperature and pressure. For calm conditions over the oceans, evaporation also is enhanced following the approximation of Miller et al. (1992)  for the transfer coefficient. Over oceans, sea ice, and snow, beta is prescribed to be unity; over land, beta is a function of the ratio of soil moisture to a constant field capacity (see Land Surface Processes).
- Soil temperature is computed from heat storage in two layers with a climatological temperature specified in a deeper layer. The upper boundary condition is the surface energy balance (see Surface Fluxes). The heat conductivity of soil is fixed under all conditions.
- Prognostic soil moisture is represented by a single-layer "bucket" model with uniform field capacity of 0.15 m after Manabe and Holloway (1975). . Both precipitation and snowmelt contribute to soil moisture. The evapotranspiration efficiency beta (see Surface Fluxes) is a function of the ratio of soil moisture to the field capacity. Runoff occurs implicitly if this ratio exceeds unity.
Last update August 23, 1996. For further information, contact: Tom Phillips (firstname.lastname@example.org)