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

The delta-two- and 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 delta-two-stream approximations, the delta-two- and four-stream combination approximations based on various two-stream approaches, and the delta-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 delta w-two- and four-stream combination method based on the quadrature scheme appears to be well suited to radiative transfer calculations involving photodissociation processes.