http://grads.iges.org/home.html. ), but with substantial modifications in the treatment of vertical diffusion, radiation and cloud-radiative interactions, surface characteristics and fluxes, and land-surface processes. , with subsequent modifications described by Sato et al. (1989a ,b ), Xue et al. (1991) , and Hou (1991) . . January soil moisture and snow cover/depth are obtained from GFDL climatologies.  frequency filter. A time step of 12 minutes is used for dynamics and physics, except for full calculation of atmospheric radiation, which is done hourly for the shortwave fluxes and every 3 hours for the longwave fluxes. An implicit scheme with explicit coefficients also is used to eliminate numerical oscillation while integrating the coupled heat and mass exchanges between the surface and the atmospheric boundary layer (cf. Sato et al. 1989a) . Orography). Negative atmospheric moisture values arising from the model's spectral truncation are filled by resetting these to zero.
- Fourth-order (del^4) horizontal diffusion is applied to the vorticity, divergence, specific humidity, and virtual temperature. (The coefficient of diffusion for the divergence is 0.61 x 10^16 m^4/s, while it is 0.81 x 10^16 m^4/s for the other fields.) For the specific humidity and virtual temperature, the del^2 correction necessary to account for diffusion on constant pressure (rather than constant sigma) surfaces is also applied. This correction includes a priori specification of estimates for global-mean specific humidity and temperature.
- Stability-dependent vertical diffusion with Mellor and Yamada (1982)  level-2 turbulence closure is used in the planetary boundary layer and free atmosphere. To obtain the eddy diffusion coefficients, a prognostic equation is solved for the turbulent kinetic energy, with other second-order moments being calculated diagnostically.
- The radiation code follows Harshvardhan et al. (1987)
. The shortwave scheme is based on the method of Lacis and Hansen (1974)
. Six absorption bands are considered, one for ozone in the ultraviolet (wavelengths < 0.35 micron) and visible (wavelengths 0.50 to 0.70 micron) spectral ranges, and five near-infrared bands (wavelengths 0.70 to 4.0 microns). At the surface, solar radiative fluxes are separated into four components: the ultraviolet and visible direct and diffuse beams, and the near-infrared direct and diffuse beams (see Surface Characteristics). For clear-sky conditions, a combined surface-atmosphere treatment is employed for Rayleigh scattering. For cloudy skies, multiple scattering effects are treated by a delta-Eddington approach (cf. Joseph et al. 1976
). In this case optical depth is estimated from cloud temperature and pressure thickness after Harshvardhan et al. (1989)
, while the asymmetry factor and single-scattering albedo are prescribed.
- The longwave scheme follows the broadband transmission approach of Chou (1984)  for water vapor, that of Chou and Peng (1983)  for carbon dioxide, and that of Rodgers (1968)  for ozone. The treatment of continuum absorption by water vapor follows Roberts et al. (1976) . Water vapor absorption is calculated in spectral domains corresponding to two band centers (0-3.4 x 10^4 m^-1 and 1.38 x 10^5-1.90 x 10^5 m^-1) and the associated band wings (in the range 3.40 x 10^4-3.00 x 10^5 m^-1). Carbon dioxide absorption is treated similarly (with band-center domain 6.80 x 10^4-7.20 x 10^4 m^-1 and band-wing domains in the range 5.40 x 10^4-8.00 x 10^4 m^-1). Ozone absorption is calculated in the interval 9.80 x 10^4-1.10 x 10^5 m^-1. For cloudy-sky conditions, longwave emissivity is a function of the optical thickness of the cloud layer (see above). Cumuloform clouds are treated as fully overlapped in the vertical, and stratiform clouds as randomly overlapped (see Cloud Formation).
- Penetrative convection is simulated following Kuo (1965) with modifications as described by Sela (1980). Convection occurs in the presence of large-scale moisture convergence accompanied by a moist unstable lapse rate under moderately high relative humidity conditions. The vertical integral of the moisture convergence determines the total moisture available for moistening vs heating (through precipitation formation) the environment. If the moisture convergence in the first several lowest layers of a vertical column exceeds a critical threshold (2 x 10^-8/sec), a moist adiabat is computed assuming the bottom layer is saturated, and using the preliminary pressure and temperature prediction of the model.
- An unstable subcolumn is then defined which extends from the bottom layer to the first layer for which a moist adiabatically lifted air parcel is not warmer than the environment. Within this subcolumn, the departures of the temperature and specific humidity of a saturated parcel from the respective environmental profiles in each layer determine the fraction of the total available moisture contributed to latent heat release vs moistening of that layer; the temperature and humidity profiles are revised accordingly. In addition, if the revised temperature profile exceeds a dry adiabatic lapse rate, a dry convective adjustment is performed and the moisture in the column is vertically redistributed to reflect the adjusted temperature profile. Cf. Sela (1980)
 for further details.
- Following Tiedtke (1983) , simulation of shallow (nonprecipitating) convection is parameterized as an extension of the vertical diffusion scheme (see Diffusion).
- Cloud formation is simulated following the diagnostic method of Slingo (1987)
. Two basic cloud types--cumuloform and stratiform--are represented. The height of cumuloform cloud is determined by the level of nonbuoyancy for moist adiabatic ascent in the model's convective scheme (see Convection). The cumuloform cloud fraction (not exceeding 0.8) is estimated from the scaled 3-hour mean convective precipitation rate (see Precipitation). In the case of convection penetrating above the 400 hPa level, the cumuloform cloud is capped by a cirrus anvil.
- Up to three separated layers of stratiform cloud are allowed in predefined domains (high, middle, and low). Clouds associated with fronts/tropical disturbances have fractional extent determined by a quadratic function of the difference between the local relative humidity and a threshold value of 80 percent. In the low-cloud domain, the fractional cloudiness is reduced in regions of moist subsidence, while stratus cloud forms if a temperature inversion is present under dry layers.
- Precipitation is produced both from large-scale condensation and from the convective scheme (see Convection). The large-scale precipitation algorithm compares the predicted specific humidity with a modified saturation value that is a function of temperature and pressure of a vertical layer (cf. Sela 1980)
. If the predicted humidity exceeds this threshold value, condensation occurs and the predicted temperature field is adjusted to account for the associated latent heat release.
- To prevent convective precipitation when the environment is unstable but relatively dry, the falling condensed water evaporates as it acts to progressively saturate lower layers. Large-scale precipitation may similarly evaporate. In both cases all precipitation that penetrates the bottom atmospheric layer is allowed to fall to the surface. See also Snow Cover.
- Roughness lengths over oceans are determined from the surface wind stress after the method of Charnock (1955)
. The roughness length over sea ice is a uniform 1 x 10^-4 m. Over land, the 12 vegetation/surface types of the Simple Biosphere (SiB) model of Sellers et al. (1986)
 and associated monthly varying roughness lengths are specified from data of Dorman and Sellers (1989)
- Over oceans the surface albedo depends on zenith angle, but not spectral interval (cf. Payne 1972
). The albedo of sea ice is a constant 0.50. Surface albedos of vegetated land are prescribed after data of Dorman and Sellers (1989)
, and vary monthly according to seasonal changes in vegetation. The land albedo is specified separately for visible (0.0-0.70 micron) and near-infrared (0.70-4.0 microns) spectral intervals, and is also a function of solar zenith angle. The changes in land albedo associated with partial snow cover (including effects of multiple reflections between snow and the vegetation canopy) are parameterized as described by Xue et al. (1991)
- Surface longwave emissivity is everywhere prescribed to be unity (blackbody emission).
- Surface solar absorption is determined from the surface albedos, and longwave emission from the Planck equation with uniform emissivity of 1.0 (see Surface Characteristics).
- In the lowest atmospheric layer, surface turbulent eddy fluxes of momentum, heat, and moisture follow Monin-Obukhov similarity theory. To avoid an iterative solution for the surface fluxes, the associated drag and transfer coefficients are approximated as analytical functions of the surface characteristics and bulk Richardson number. Over the oceans, the equations formulated by Miyakoda and Sirutis (1986)
, expressed as bulk formulae, are used to compute surface fluxes. Over land, stability-dependent drag and transfer coefficients (expressed as aerodynamic and surface resistances) are determined after Xue et al. (1991)
- Surface evaporation is at the potential rate over oceans, snow, and ice. Over land, the surface moisture flux includes both evapotranspiration via vegetation root uptake (including the effects of bulk stomatal resistance) and direct evaporation from the vegetation canopy and from bare soil (see Land Surface Processes).
- Above the constant-flux surface layer, diffusion of momentum, heat, and moisture are predicted by the Mellor and Yamada (1982)  level-2 turbulence closure scheme (see Diffusion).
- Land-surface processes are simulated following the Xue et al. (1991)
 modification of the SiB model of Sellers et al. (1986)
. Within the single-story vegetation canopy, evapotranspiration from dry leaves includes detailed modeling of stomatal and canopy resistances; direct evaporation from the wet canopy and from bare soil is also treated (see Surface Fluxes). Precipitation interception by the canopy is simulated, and its infiltration into the ground is limited to less than the hydraulic conductivity of the soil.
- Soil temperature is determined in two layers by the force-restore method of Deardorff (1978) . Soil moisture, which is predicted from diffusion equations in three layers, is increased by infiltrated precipitation and snowmelt, and is depleted by evapotranspiration, direct evaporation, and drainage. Both surface runoff and deep runoff from gravitational drainage are simulated. See also Surface Characteristics and Surface Fluxes.
Last update April 19, 1996. For further information, contact: Tom Phillips ( email@example.com)