Comparison of Radiative Transfer Models for Limb-Viewing Scattered Sunlight Measurements

The intensity of solar ultraviolet, visible, or near-infrared radiation back scattered from the Earth's atmosphere carries information on several trace gases and aerosol. A number of new satellite instruments will measure atmospheric constituents by looking at scattered radiation in limb-viewing geometry. The OSIRIS instrument flying on the Odin satellite since Feb. 2001 and the SOLSE and LORE instruments flown on the Shuttle flight STS-87 use this technique. SCIAMACHY and GOMOS on Envisat and SAGE III on Meteor-3M will carry out limb scatter measurements in addition to nadir-viewing measurements of scattered sunlight and limb-viewing occultation measurements. Inversion methods for retrieveing constituent profiles from limb intensity measurements are under active development. A central part of data inversion is a radiative transfer model. Accurate radiative transfer modeling for limb-viewing measurements is complicated, because the spherical shape of the atmosphere has to be taken into account. Multiple scatterings in the atmosphere as well as light reflected from Earth's surface significantly contribute to the intensity. We have compared limb intensities from six different models, which use different numerical methods to solve the radiative transfer problem. The objective is to validate the models aginst each other and to get an idea on their computational efficiency. Four of the models take fully into account the spericality of the atmosphere: a model which uses a Gauss-Seidel iteration scheme (GSS), a model called "Combining Differential-Integral approach involving the Picard Iterative approximation" (CDIPI), and two Monte Carlo models (a model called "Siro" and a model developed at the Russian Academy of Sciences). The two other models, a model based on the Combined Differential-Integral approach (CDI) and a model called "LIMBTRAN", involve some approximation in spherical geometry but are computationally much faster than the fully spherical models. In the first comparisons of limb intensity from a realistic atmosphere the fully spherical models differed at most by 5%, often less. The difference between fully spherical models and CDI and LIMBTRAN increased with tangent altitude, in the first comparisons differences were at most 8%.