Effects of aerosols on the wavelength dependence of atmospheric transmission in the ultraviolet and visible. 1. A 'single-scattering-separate' delta-Eddington model

A modification to the conventional delta-Eddington radiative transfer scheme was implemented in order to create a simple and efficient model particularly suitable for handling the highly anisotropic scattering properties exhibited by cloud drops and aerosol particles in the atmosphere. The modification includes separating out the singly scattered radiation and applying the conventional Eddington approximation to the multiply scattered radiation alone. The singly scattered irradiance is computed analytically without approximation, preserving more of the angular dependence in the radiance than conventional two-stream style solutions. The single-scattering-separate delta-Eddington model was found to handle strong absorption in the stratosphere and the anisotropic scattering within optically thin cloud layers better than both the simple two-stream and conventional Eddington models, and to produce results similar to a 22-stream discrete ordinates model.