Fast radiative transfer algorithms for atmospheric radiance computation in the presence of scattering

This thesis explores the implementation of fast scaling methodologies for the computation of mid and far-infrared spectrally resolved radiances in the presence of scattering. The study focuses on their application in operational satellite radiance data assimilation and remote sensing retrievals. In the first part of the work, two important scaling methodologies, Chou’s scaling and the similarity principle, initially developed for fluxes calculation, are considered. The assessment of their accuracy in reproducing spectrally resolved radiances in the mid and far-infrared is performed using a reference code in a nadir-looking geometry representative of the FORUM instrument (100-1600 cm−1). The results indicate good performance of the Chou method in reproducing radiances in the mid-infrared for water and ice clouds. However, limitations arise for ice clouds in the far-infrared, leading to overestimation of upwelling radiances. To address computational errors in the basic scaling method, a correction term is introduced based on the solution proposed by Tang et al. (2018). This correction term, originally derived for fluxes computation, is extended to spectrally resolved radiances through coefficient optimization. The application of this routine produces reduction in radiance residuals for various cloudy cases, especially for thin cirrus clouds targeted by the FORUM mission. However, challenges persist for medium-large optical depths and small effective radii. The radiative parameters needed for the Chou and Tang schemes are parametrized and integrated into the sigma-IASI/F2N code, a forward model for fast radiance and derivative calculations with respect to atmospheric and spectroscopic parameters. Finally, this thesis presents an improved approach for solving the radiative transfer equation efficiently. This new solution can be interpreted as an asymmetric adjusted scaling, and it excels in simulating spectrally resolved upwelling radiances in the presence of atmospheric diffusive layers, particularly for optically thin scattering layers like cirrus clouds.