MULTIPLE-SCATTERING PARAMETERIZATION IN THERMAL INFRARED RADIATIVE-TRANSFER

A systematic formulation of various radiative transfer parameterizatio ns is presented, including the absorption approximation (AA), delta-tw o-stream approximation (D2S), delta-four-stream approximation (D4S), a nd delta-two- and four-stream combination approximation (D2/4S), in a consistent manner for thermal infrared flux calculations. The D2/4S sc heme uses a source function from the delta-two-stream approximation an d evaluates intensities in the four-stream directions. A wide range of accuracy checks for monochromatic emissivity of a homogeneous layer a nd broadband heating rates and fluxes in nonhomogeneous atmospheres is performed with respect to the ''exact'' results computed from the del ta-128-stream scheme for radiative transfer. The computer time require d for the calculations using different radiative transfer parameteriza tions is compared. The results pertaining to the accuracy and efficien cy of various radiative transfer approximations can be utilized to dec ide which approximate method is most appropriate for a particular appl ication. In view of its overall high accuracy and computational econom y, it is recommended that the D2/4S scheme is well suited for GCM and climate modeling applications.