A linearized discrete ordinate radiative transfer model for atmospheric remote-sensing retrieval

The radiative transfer forward model simulation of intensities and associat ed parameter derivatives (weighting functions) is a vital part of the retri eval of earth atmospheric constituent information from measurements of back scattered light. The discrete ordinate method is the most commonly used app roach for the determination of solutions to the radiative transfer equation . In this paper, we carry out an internal perturbation analysis of the comp lete discrete ordinate solution in a plane parallel multi-layered multiple- scattering atmosphere. Perturbations in layer atmospheric quantities will t ranslate into small changes in the single-scatter albedos and optical depth values. In addition, we consider perturbations in layer thermal emission s ource terms and in the surface albedo. It is shown that the solution of the boundary value problem for the perturbed intensity field leads in a natura l way to the weighting function associated with the parameter causing the p erturbation. We have developed a numerical model LIDORT (linearized discret e ordinate radiative transfer) for the simultaneous generation of backscatt er intensities and weighting function output at arbitrary elevation angles, for a user-defined set of atmospheric variations. Results for a 5-layer te st atmosphere with two scatterers and thermal emission terms are shown. Int ensities are validated against benchmark discrete ordinate results, while w eighting functions are checked for consistency against finite difference re sults based on external perturbations. A second example is presented for a 60-layer terrestrial atmosphere with molecular and aerosol scattering and o zone trace gas absorption in the UV spectral range; weighting functions are shown to correspond closely with results derived from another radiative tr ansfer model. Published by Elsevier Science Ltd.