Multiple scattering lowtran and fascode models

This paper describes the implementation of an efficient and accurate multiple scattering parameterization within the LOWTRAN and FASCODE transmittance/radiance models. The parameterization is based on a stream approximation in which the local radiance field needed to evaluate the multiple scattering source function is estimated from the local radiation fluxes. The latter are calculated based on a parameterized two-flux for individual layers and an adding method for combining layers. Because of the line-by-line nature of FASCODE, it is straightforward to implement the multiple scattering treatment. For LOWTRAN, an interface scheme was developed using the k-distribution method to match the multiple scattering approach to the band model calculation of gas absorption. The interface scheme represents the LOWTRAN band model by a sum of pseudomonochromatic calculations. The approach is valid for any band model for which k-distribution parameters can be evaluated. The accuracy of the multiple scattering parameterization has been demonstrated by comparing it with more detailed calculations for a variety of atmospheric conditions. RMS errors in radiance considering all possible viewing angles are <20%. In addition, to insure consistency between models, overlapping LOWTRAN and FASCODE spectral regions are compared. Finally, it is demonstrated that the implemented multiple scattering parameterization corrects LOWTRAN's previous underestimation of path radiance for long horizon paths where multiple scattering is significant.