New secondary-scattering correction in DISORT with increased efficiency for forward scattering

We present an alternative method to calculate the directional distribution after secondary scattering of light in an atmosphere, and apply it to the correction developed by Nakajima and Tanaka (1988) 1] as implemented in the DISORT radiative transfer solver. This method employs the scattering phase functions directly, instead of expanding over their Legendre moments as in the original formulation, and hence is not compromised in cases where a prohibitive number of moments is required to maintain accuracy. The new approach is designed to be particularly efficient in the strongly forward-scattering case, which arises for example in problems involving cloud-ice or dust particles. We have implemented this in a newly rewritten C-code version of DISORT that provides additional computational efficiencies via dynamic and cache-aware memory allocation. The new version uses less memory and runs considerably faster than the original, while producing results with equal or greater accuracy.