Absorption approximation with scattering effect for infrared radiation

A scheme that can handle cloud infrared scattering based on the absorption approximation is developed. In a two-stream mode, the new scheme produces m ore accurate results than those from the modified two-stream discrete ordin ate method. For low and middle clouds, the two-stream version of the scheme produces a flux error less than I W m(-2) and a heating rate error less th an 0.5 K day(-1). With high clouds, the errors in calculated fluxes and hea ting rates are less than 1.4 W m(-2) and 1.5 K day(-1), respectively. The f our-stream version of the proposed scheme is slightly inferior to the four- stream discrete ordinate method. However, as opposed to the discrete ordina te technique, this scheme treats cloud-free layers the same as the absorpti on approximation. Therefore, numerically it is much more efficient. Conside ring the radiative transfer module only, in a two-stream mode, the new sche me, which considers multiple scattering, uses only about 50% more CPU time than the absorption approximation method for a 100-layer column atmosphere with 20 cloudy layers.