RRTM is a rapid radiative transfer model which utilizes the correlated-k approach to calculate fluxes and heating rates efficiently and accurately.

Key features of RRTM_SW are:

• k-distributions are obtained directly from a line-by-line radiative transfer code, LBLRTM , which has been extensively validated against observations, principally at the ARM SGP site
• fluxes and cooling  rates are calculated over fourteen contiguous bands in the  shortwave (820-50000 cm^-1)
• modeled sources of extinction are water vapor, carbon dioxide, ozone, methane, oxygen, aerosols, and Rayleigh scattering
• DISORT is used to perform scattering calculations
• agreement with line-by-line calculations: 1 W/m² for direct irradiance, 2 W/m² for diffuse irradiance
• water clouds:
• The optical properties of water clouds are calculated for each spectral band from the Hu and Stamnes parameterization.  The optical depth,  single-scattering albedo, and asymmetry parameter are parameterized as a function of cloud equivalent radius and liquid water path.
Reference: Hu, Y. X., and K. Stamnes, An accurate parameterization of the radiative properties of water clouds suitable for use in climate models.  J. Climate, Vol. 6, 728-742, 1993.
• ice clouds
• The optical properties of ice clouds are calculated for each spectral band from the Fu parameterization, which assumes the ice crystals are hexagonal and randomly-oriented in space.  The optical depth,  single-scattering albedo, and asymmetry parameter are parameterized as a function of the generalized effective size of the ice crystals and the ice water content.  (It is important to note that the generalized effective size is not equivalent to the effective mean size.)
Reference: Fu, Q., An accurate parameterization of the solar radiative properties of cirrus clouds for climate models.  J. Climate, 9, 2058-2082, 1996.

Atmospheric and Environmental Research