A Fast Retrieval Model for Synergistic Inversion of Nadir / Zenith Spectral Radiance Measurements

Starting from 2019, the Italian Space Agency (ASI) is supporting dedicated projects for the development of new methods, tools and competences for the interpretation and the exploitation of the future measurements of the FORUM (Far-infrared Outgoing Radiation Understanding and Monitoring) experiment. FORUM will be the ninth Earth Explorer mission of the European Space Agency, scheduled for launch on a polar orbiting satellite in 2027. The core instrument of the mission will be a Fourier Transform Spectrometer with spectral range extending down to the Far-InfraRed (FIR), from 100 to 1600 cm-1 (from 100 to 6.25 microns in wavelength), thus covering the whole Earth's outgoing longwave radiation spectrum. FORUM will fly in loose formation with the MetOp-SG-A satellite, hosting the Infrared Atmospheric Sounding Interferometer - New Generation (IASI-NG). The Middle-InfraRed (MIR) range (645 to 2760 cm-1) of the upwelling atmospheric spectrum measured by IASI-NG will effectively complement the FORUM measurement. All together, the two missions will provide matching spectral radiance measurements with unprecedented coverage, from 100 to 2760 cm-1. While the FIR part of the spectrum (100-667 cm-1) measured by FORUM is the most sensitive to the water vapour content in the UTLS and to ice cloud properties, the atmospheric windows in the MIR are measured by IASI-NG with a very high signal-to-noise ratio, thus supplying very precise information on the surface temperature and on the temperature profile, which are essential to constrain the retrieval of the other geophysical parameters. To get ready for the exploitation of the synergistic FORUM and IASI-NG measurements, in the frame of the mentioned ASI projects a Bayesian retrieval algorithm with the capability to perform the simultaneous inversion of two different synergistic spectral radiance measurements was developed. The tool is named FAst Retrieval Model (FARM) as it is based on a fast monochromatic and parametrized radiative transfer model (?-IASI) which is also being further extended and refined within the same projects. FARM includes the capability to retrieve simultaneously both atmospheric and cloud parameters. Furthermore, the code can handle both air/space- borne nadir measurements and ground- based zenith measurements. In this work, we introduce the functionalities of the developed algorithm and present the results of the self-consistency and verification tests. The preliminary results of the inversion of some existing real measurements are also discussed.