Two- and Four-Stream Combination Approximations for Computation of Diffuse Actinic Fluxes

The d-twoand four-stream combination approximations, which use a source function from the two-stream approximations and evaluate intensities in the four-stream directions, are formulated for the calculation of diffuse actinic fluxes. The accuracy and efficiency of the three computational techniques—the d-two-stream approximations, the d-twoand four-stream combination approximations based on various two-stream approaches, and the d-four-stream approximation—have been investigated. The diffuse actinic fluxes are examined by considering molecular, aerosol, haze, and cloud scattering over a wide range of solar zenith angles, optical depths, and surface albedos. In view of the overall accuracy and computational efficiency, the d-twoand four-stream combination method based on the quadrature scheme appears to be well suited to radiative transfer calculations involving photodissociation processes.