Σ-iasi-β: a Hyperfast Radiative Transfer Code to Retrieve Surface and Atmospheric Geophysical Parameters

This paper describes an improved, faster, implementation of the σ-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 10 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. The down-sampling of the look-up table is responsible for an additional speed-up which makes the code useful for almost real time retrieval applications and thus useful for operational purposes. This code is a powerful tool also to check the validity of the molecular spectroscopic parameters. It is an evolution of the well-known code σ-IASI. It has been developed in the context of the Infrared Atmospheric Sounding Interferometer (IASI) of the European Space Agency EUMETSAT, but it is well suited for every nadir viewing satellite, airplane sensor or ground-based sensor with a sampling rate in the range 0.1-2 cm.