sigma-IASI-beta: a hyperfast radiative transfer code to retrieve surface and atmospheric geophysical parameters

This paper describes an improved, faster, implementation of the sigma-IASI model, with a new parameterization of radiative transfer in cloudy atmosphere. The model can compute up and/or downwelling spectral radiances, emitted from the Earth's system and their analytical Jacobians with respect to a set of geophysical parameters and the water vapour and carbon dioxide continua absorbing coefficients. The paper presents also its software implementation and a retrieval exercise of the tropospheric content of CO2, CO, N2O and CH4 on the Mediterranean Sea. The content of the gases is compared with the ground-based measurements of the Global Atmosphere Watch network. The innovation introduced in the model is the down-sampling of the look-up table by means of a spectral averaging of the layer optical depths on bins of 0.01 /cm width before they are parameterized as a low order polynomial of temperature and, only for water vapour, of water vapour concentration itself to take into account the self-broadening effect.