Simultaneous retrieval from the full IASI spectrum of cloud and atmospheric parameters using the new all-sky forward model sigma-IASI/F2N: the first day-night infrared retrieval of the Antarctica ozone hole

We have developed a new forward model for all sky radiative transfer calculations in the spectral range 10 to 2760 cm−1 . This new code, which we call σ-IASI/F2N, allows us to calculate in all-sky based on an original parametrization of the optical depth of atmospheric gases and clouds. Clouds are represented through the atmospheric profiles of Liquid Water Content (LWC), Ice Water Content (IWC), and effective radii of both water droplets (re) and ice crystals (De). The cloud parametrization relies on suitable scaling laws, which make the radiative transfer equations for a cloudy atmosphere identical to those for a clear atmosphere. Therefore, the difficulties in applying a multiple-scattering algorithm to a partly cloudy atmosphere are avoided, and the computational efficiency is practically the same as that for a clear atmosphere. The new radiative transfer code has been coupled with an inverse scheme based on the Optimal Estimation methodology. The problem of dimensionality of the data and parameter space is handled by considering suitable transforms, which allow the representation of the radiances (data space) and the atmospheric state vector (parameter space) through a set of reduced components. The dimensionality is diminished through the random Projections transform for the radiance space, whereas we use the usual Principal Component Analysis for the parameter space. The scheme’s performance has been evaluated using the Infrared Atmospheric Sounder Interferometer (IASI) spectral radiances. The soundings are collocated with analyses from the European Centre for Medium-Range Forecasts (ECMWF) model. The ECMWF analyses provide the basic information, i.e., the first guess state vector to initialize the inverse scheme. The forward/inverse technique uses the whole IASI spectral coverage (645 to 2760 cm−1 ). As such, it is the first scheme for all sky using the full IASI spectrum to retrieve clouds and atmospheric parameters simultaneously. This new forward/inverse model is exemplified through the analysis of a set of IASI soundings over the Antarctica continent on 9 September 2021 at the onset of the ozone hole. We will show that infrared retrievals add information to assess ozone’s spatial extent and depletion.