http://crga.atmos.uiuc.edu/. . Subsequent modifications principally include an increase in vertical resolution from 2 to 7 layers, as well as substantial changes in the treatment of atmospheric radiation, convection, cloud/precipitation formation, and land surface processes.  The parameterizations of radiation, cloud formation, and related physics are discussed by Oh (1989)  and by Oh and Schlesinger (1991a , b , c ) , conserving total atmospheric mass, energy, and potential enstrophy. Orography). A longitudinal smoothing of the zonal pressure gradient and the zonal and meridional mass flux is performed at latitudes polewards of 38 degrees (cf. Ghan et al. 1982 ). It is unnecessary to fill spurious negative values of atmospheric moisture, since these are not generated by the numerical schemes. Cloud Formation).
- Horizontal diffusion is not modeled.
- Vertical diffusion of momentum, sensible heat, and moisture operates at all vertical levels. The diffusion depends on the vertical wind shear, but not on stability (cf. Oh 1989  and Oh and Schlesinger 1991a ).
- The spectral range of shortwave radiation is divided into three intervals: 0 to 0.44 micron, 0.44 to 0.69 micron, and 0.69 to 3.85 microns. The first two intervals are for the treatment of Rayleigh scattering (after Coakley et al. 1983
) and ozone and carbon dioxide absorption (after Lacis and Hansen 1974
 and Fouquart 1988
, respectively); the last interval (further subdivided into six subintervals) is for water vapor absorption. Scattering and absorption by both gases and cloud droplets are modeled following a two-stream approach with delta-Eddington approximation. The optical depth and single-scattering albedo for cloud droplets are determined following Stephens (1978)
 for non-ice clouds and Starr and Cox (1985)
 for cirrus clouds.
- The longwave flux calculations are based on a two-stream formulation with parameterized optical depths, but with scattering neglected. Longwave absorption is treated in four spectral bands, one each for carbon dioxide (5.4 x 10^4 to 8.0 x 10^4 m^-1) and ozone (9.8 x 10^4 to 1.1 x 10^5 m^-1), and the other two bands (with multiple subintervals between 0 and 3.0 x 10^5 m^-1) for the line centers, wings, and continuum of the water vapor absorption spectra. Pressure-broadening effects are included in all the absorption calculations, which follow Chou (1984)
 for water vapor, Kneizys et al. (1983)
 for ozone, Chou and Peng (1983)
 for carbon dioxide, Donner and Ramanathan (1980)
 for methane and nitrous oxide, and Ramanathan et al. (1985)
 for chlorofluorocarbon compounds CFC-11 and CFC-12. The absorption by trace gases (methane, nitrous oxide, CFC-11, and CFC-12) is normalized in each subinterval. Absorption by cloud droplets is treated by an emissivity formulation based on data by Stephens (1978)
 for non-ice clouds, and by Starr and Cox (1985)
 and Griffith et al. (1980)
 for extratropical and tropical cirrus clouds, respectively.
- The radiation parameterization includes cloud-cover feedback by calculating separately the radiative fluxes for the cloudy and clear portions of each grid box; it includes cloud optical-depth feedback by linking the radiative properties to the prognostic cloud water content (see Cloud Formation). Clouds are vertically distributed by groups that make up an ensemble of contiguous cloud layers, and which are separated from each other by at least one layer of clear air. Following Geleyn (1977) , the contiguous cloud layers within each group overlap one another fully in the vertical, while the noncontiguous cloud groups overlap randomly. Cf. Oh (1989)  and Oh and Schlesinger (1991c)  for further details.
- Penetrative convection is simulated by a modified Arakawa-Schubert (1974)
 scheme. The scheme predicts mass fluxes from mutually interacting cumulus subensembles which have different entrainment rates and levels of neutral buoyancy (depending on the properties of the large-scale environment) that define the tops of the clouds and their associated convective updrafts. In turn, the predicted convective mass fluxes feed back on the large-scale fields of temperature (through latent heating and compensating subsidence), moisture (through precipitation and detrainment), and momentum (through cumulus friction). The effects on convective cloud buoyancy of phase changes from water to ice, and the drying and cooling effects of convective-scale downdrafts on the environment are not explicitly included.
- The cloud-base mass flux for each cumulus sub-ensemble is determined following Arakawa (1969)  such that the convective instability for each subensemble is removed with an e-folding time of one hour (cf. Oh 1989).
- The model also simulates middle-level convection, defined by convective instability between any two adjacent layers, with the instability also being removed with an e-folding time of one hour. In addition, if the lapse rate becomes dry-convectively unstable anywhere within the model atmosphere, enthalpy is redistributed vertically in an energy-conserving manner.
- The cloud parameterization is formulated separately for stratiform and cumuloform clouds, as described by Oh (1989)
 and Oh and Schlesinger (1991b)
. For both cloud types, the liquid/ice water is computed prognostically, and the fractional cloud coverage of each grid box semiprognostically. The stratiform cloud fraction varies as the square root of the relative humidity. The cumuloform cloud fraction is determined as a function of the relative humidity and the convective mass flux (see Convection).
- Cloud in the PBL (see Planetary Boundary Layer) is diagnostically computed on the basis of a cloud-topped mixed layer model (cf. Lilly 1968  and Guinn and Schubert 1989 ).
- Precipitation forms via the simulated microphysical processes (autoconversion from cloud liquid/ice water) in the prognostic cloud scheme (cf. Oh 1989
 and Oh and Schlesinger 1991b
). The large-scale precipitation rate is an exponential function of the liquid water mixing ratio and the cloud water content. The difference of these quantities multiplied by the cumulus mass flux yields the convective precipitation rate (see Convection).
- The rate of evaporation of falling large-scale precipitation is proportional to the product of the rainfall rate, the relative humidity deficit from saturation, and the cloud-free fraction of the grid box. Evaporation of convective precipitation is proportional to the product of the relative humidity deficit and the cloud water content.
- Surface roughness is specified as in Hansen et al. (1983)
. Over land, the roughness length is a fit to the data of Fiedler and Panofsky (1972)
 as a function of the standard deviation of the orography. The maximum of this value and that of the roughness of the local vegetation (including a "zero plane displacement" value for tall vegetation types--cf. Monteith 1973
) determines the roughness length over land. Over sea ice, the roughness is a constant 4.3 x 10^-4 m after Doronin (1969)
. Over ocean, the roughness length is a function of the surface wind speed, following Garratt (1977)
- Snow-free surface albedo is updated monthly by interpolation using values for January, April, July, and October specified from data of Matthews (1983)
. The albedo of snow-covered surfaces is determined as a linear weighted (by snow depth) interpolation of snow-free and snow-covered values. The albedo of snow is a function of its temperature (cf. Manabe et al. 1991
); it also depends on solar zenith angle (cf. Briegleb and Ramanathan 1982
), but not on spectral interval.
- Longwave emissivity is specified to be unity (blackbody emission) for all surfaces.
- The absorbed surface solar flux is determined from the surface albedo, and surface longwave emission from the Planck function with constant surface emissivity of 1.0 (see Surface Characteristics).
- The turbulent surface fluxes of momentum, sensible heat, and moisture are parameterized as bulk formulae that include surface atmospheric values of winds, as well as differences between skin values of temperatures and specific humidities and their surface atmospheric values. Following Oh and Schlesinger (1990) , the surface wind is taken as a fraction (0.7 over water and 0.8 over land and ice) of the winds extrapolated from the lowest two model layers. The surface atmospheric values of temperatures and humidities are taken to be the same as those at the lowest atmospheric level (sigma = 0.990).The aerodynamic drag and transfer coefficients depend on vertical stability (bulk Richardson number) and surface roughness length (see Surface Characteristics), with the same transfer coefficient used for the fluxes of sensible heat and moisture. In addition, the surface moisture flux depends on an evapotranspiration efficiency beta that is taken as unity over snow, ice and water; over land, beta is a function of the fractional soil moisture (see Land Surface Processes).
- Following Priestly (1959)
 and Bhumralkar (1975)
, the average ground temperature over the diurnal skin depth is computed from a prognostic budget equation whose source/sink terms include the net surface radiative flux and the sensible and latent heat fluxes (see Surface Fluxes); the thermal conductivity, volumetric heat capacity, and bulk heat capacity of snow, ice, and land are also taken into account. If the predicted ground temperature for land ice is > 0 degrees C, the ice is implicitly assumed to melt, since the model does not include a budget equation for land ice. See also Snow Cover.
- Soil wetness is expressed as the ratio of soil moisture content to a field capacity that is specified as a function of soil texture and surface cover after data of Vinnikov and Yeserkepova (1991) . Soil wetness is determined from a prognostic budget equation that includes the rates of precipitation, snowmelt, surface evaporation, and runoff. The evapotranspiration efficiency beta over land (see Surface Fluxes) is assigned a value that is the lesser of 1.33 times the soil wetness fraction or unity. The runoff rate is a nonlinear function of the soil wetness and the combined rates of precipitation and snowmelt. If the predicted soil wetness fraction exceeds unity, the excess moisture is taken as additional runoff.
Last update April 19, 1996. For further information, contact: Tom Phillips ( email@example.com)