Terrestrial radiation transfer in GCMs

11.4.3.1 HadAM3 Scheme The longwave fluxes are calculated by a two-stream approximation, where the spectrum is divided into 8 bands with the following boundaries (in cm-1): 0-400; 400-550; 550-800 (excluding 590-750); 590-750; 800-1200 (excluding 990-1120); 990-1120; 1200-1500; and 1500-3000. Gaseous absorption data are again derived from HITRAN and continuum absorption by water vapour is treated using a continuum model.

11.4.3.2 NCAR CAM 3.0 Scheme Longwave absorption by ozone and carbon dioxide is treated by the broadband absorptance technique. The broadband approach assumes that the spectral range of absorption by a gas is limited to a relatively small range in wave number, and hence can be evaluated at the band center. This broadband formalism is employed for CO2, O3, CH4, N2O, and minor absorption bands of CO2, while for CFCs and stratospheric aerosols an exponential transmission approximation is employed. The longwave absorptance formulation includes Voigt line profile effects for CO2 and O3. For the mid-to-upper stratosphere spectral absorption lines are no longer Lorentzian in shape. Clouds are treated as grey bodies with emissivities that depend on cloud phase, condensed-water path, and the effective radius of ice particles.

11.4.3.3 ECHAM5 Scheme The scheme implemented in ECHAM5 is the so-called rapid radiative transfer model based on the correlated-k approximation. Absorption coefficients are derived from the LBLRTM line-by-line model and include the effect of the water-vapor continuum. The scheme computes 7 fluxes in the spectral range 10 cm-1 to 3000 cm-1 in 16 spectral bands and includes line absorption by H2O, CO2, O3, CH4, N2O, CFC-11, and CFC-12. For cloud droplets, the mass absorption coefficient is a function of the respective effective radius with coefficients independent of wave number obtained from a polynomial fit to the results of Mie calculations. For ice clouds, an inverse dependence of the mass absorption coefficient on ice-crystal effective radius is assumed, and the coefficients vary with wave number. Maximum cloud overlap is assumed for contiguous cloud layers, random overlap elsewhere. Aerosols are considered in all spectral bands with emissivities derived from a climatology that distinguishes time-independent spatial distributions of the optical thickness of sea, land, urban, and desert aerosols, and well-mixed tropospheric and stratospheric background aerosols.

11.4.4 Surface albedo and! emissivity in GCMs

11.4.4.1 HadAM3 Scheme The albedos of different types of surface are included explicitly, while their solar absorption and longwave emissivity assumes they are all blackbodies. The surface albedo of open ocean is a function of solar zenith angle. The albedo of sea ice varies between 0.60 and 0.85 as a linear function of the ice temperature above -5 °C, and is also modified by snow cover. Where there is partial coverage of a grid box by sea ice, the surface albedo is given by the fractionally weighted albedos of sea ice and open ocean. Albedos of snow-free and deeply snow-covered land are specified according to the different land-cover classes, where the deep-snow albedo decreases linearly with surface temperature above +2 °C. For intermediate snow depths, the land albedo approaches the deep-snow value exponentially, according to an e-folding depth of snow equivalent to 5 mm of water.

11.4.4.2 NCAR CAM 3.0 Scheme The surface albedo is specified in two wavebands (0.2-0.7 pm, and 0.7-5.0 pm) and distinguishes albedos for direct and diffuse incident radiation. Albedos for ocean surfaces, geographically varying land surfaces, and sea-ice surfaces are distinguished. Surface albedos depend on the solar zenith angle, the amount of leaf and stem material present, their optical properties, and the optical properties of snow and soil. For snow and ice the albedo depends upon the spectral band, snow thickness, ice thickness and surface temperature. The zenith-angle dependence of snow and ice is ignored. Ice thermal radiation emissivity is set to 0.95.

11.4.4.3 ECHAM5 S'cheme For snow-free land surfaces, an annual mean background albedo derived from satellite data is allocated to a high-resolution global distribution of major ecosystem complexes. For most surfaces, the albedo of snow and ice is assumed to be a linear function of surface temperature, ranging between a minimum value at the melting point and a maximum value for 'cold' temperatures. The minimum and maximum values of snow/ice albedos are assigned according to the underlying surface type: land 0.30-0.80; canopy 0.20; land ice 0.60-0.80; sea ice 0.50-0.75; lake ice 0.50-0.75; snow on lake ice 0.600.80. For water surfaces, such as lakes and ocean, the albedo is set to 0.07. The longwave emissivity is set to 0.996 for all surfaces and spectral intervals.

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