Radiation

The radiation model has explicit multiple scattering calculations for solar radiation [shortwave (SW)] and explicit integrations over both the SW and thermal [longwave (LW)] spectral regions. Gaseous absorbers of SW radiation are H2O, CO2, O3, O2, and NO2. Size-dependent scattering properties of clouds and aerosols are computed from Mie scattering, ray tracing, and T- matrix theory (Mishchenko et al. 1996) to include non-spherical cirrus and dust particles. The k-distribution approach (Lacis and Oinas 1991) utilizes 15 noncontiguous spectral intervals to model overlapping cloud–aerosol and gaseous absorption. The surface albedo utilizes six spectral intervals and is solar zenith angle dependent for ocean, snow, and ice surfaces. The spectral albedo of vegetation is seasonally dependent. The radiation model generates spectrally dependent direct/ diffuse flux ratios for use in biosphere feedback interactions.

Longwave calculations for H2O, CO2, and O3 use the correlated k distribution with 33 intervals (Lacis and Oinas 1991; Oinas et al. 2001), designed to match line-by-line computed fluxes and cooling rates throughout the atmosphere to within about 1%. Weaker bands of H2O, CO2, and O3, as well as absorption by CH4, N2O, CFC-11, and CFC-12 are included in an approximate fashion as overlapping absorbers, but with coefficients tuned to reproduce line-by-line radiative forcing over a broad range of absorber amounts. The vertical profiles and latitudinal gradients of CH4, N2O, and CFCs are from Minschwaner et al. (1998). Longwave forcing by aerosols is also included (Tegen et al. 2000).