A Fast Retrieval Model for Synergistic Inversion of FORUM and IASI-NG Measurements

In 2019, the Italian Space Agency (ASI) started a new project entitled FORUM - scienza (FORUM science). One of the project objectives is to improve and share, within the interested National scientific community, the models needed for interpretation and analysis of the measurements of the forthcoming FORUM (Far-infrared Outgoing Radiation Understanding and Monitoring) experiment. FORUM will be the ninth Earth Explorer mission of the European Space Agency (ESA), scheduled for launch on a polar satellite in 2026. The core instrument will be a Fourier Transform Spectrometer covering the spectral region from 100 to 1600 cm-1 (from 100 to 6.25 microns in wavelength), with a full-width-half-maximum spectral resolution of 0.5 cm-1. The spectrometer will measure the Earth's upwelling spectrum in the nadir-looking geometry. FORUM will fly in loose formation with the MetOp-SG-A satellite, hosting the Infrared Atmospheric Sounding Interferometer - New Generation (IASI-NG). The MIR (645 to 2760 cm-1) upwelling atmospheric spectrum measured by IASI-NG will complement the FORUM FIR spectrum, providing an unprecedented spectral 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, 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 parameters. Among the models improved in the frame of the FORUM - scienza project, ?-FORUM is a fast parametric radiative transfer code with the capability of simulating the upwelling spectrum, from 50 to 2810 cm-1, with sufficiently high spectral resolution (0.01 cm-1), in all sky conditions. Along with the spectral radiance, the model computes its Jacobians with respect to relevant atmospheric, cloud and surface parameters. The characteristics and the accuracy figures of the ?-FORUM model are presented by Masiello et al., within this conference. Based on ?-FORUM, we have built a fast and flexible Bayesian inversion model. This model includes the possibility of performing the synergistic inversion of both FORUM and IASI-NG measurements and to apply various types of constraints in the inversion, such as Optimal Estimation and / or Tikhonov regularization. Profile retrieval grids, together with the set of parameters to be retrieved are selected via input files. Currently, the following sets of parameters can be retrieved: temperature, water vapour and ozone profiles, surface temperature and surface spectral emissivity (at a selectable set of wavenumbers). The program also offers the possibility to minimize the effect of possible frequency scale errors of the spectrum by retrieving a frequency scale factor. The possibility to retrieve profiles of ice- and liquid- water contents and of droplet- and ice- effective dimensions will be implemented in the near future. In this work we illustrate the structure of the developed retrieval model, its performance, and the results of the first tests we made to optimize the retrieval setup in clear-sky conditions.