Design and description of the MUSICA IASI full retrieval product

IASI (Infrared Atmospheric Sounding Interferometer) is the core instrument of the currently three Metop (Meteorological operational) satellites of EUMETSAT (European Organization for the Exploitation of Meteorological Satellites). The MUSICA IASI processing has been developed in the framework of the European Research Council project MUSICA (MUlti-platform remote Sensing of Isotopologues for investigating the Cycle of Atmospheric water). The processor performs an optimal estimation of the vertical distributions of water vapour (H$_{2}$O), the ratio between two water vapour isotopologues (the HDO/H$_{2}$O ratio), nitrous oxide (N$_{2}$O), methane (CH$_{4}$), and nitric acid (HNO$_{3}$) and works with IASI radiances measured under cloud-free conditions in the spectral window between 1190 and 1400 cm$^{-1}$. The retrieval of the trace gas profiles is performed on a logarithmic scale, which allows the constraint and the analytic treatment of ln [HDO]−ln [H$_{2}$O] as a proxy for the HDO/H$_{2}$O ratio. Currently, the MUSICA IASI processing has been applied to all IASI measurements available between October 2014 and June 2021 and about two billion individual retrievals have been performed.