Performance of Synergistic Retrieval and Complete Data Fusion methods applied to FORUM and IASI-NG synthetic measurements

The Far-infrared Outgoing Radiation Understanding and Monitoring (FORUM) experiment will be the ninth Earth Explorer mission of the European Space Agency (ESA). FORUM is scheduled for launch on a Polar orbiting satellite in 2027. The mission's core instrument will be a Fourier Transform Spectrometer covering both the Far- and the Middle- InfraRed (FIR & MIR) spectral regions. FORUM will measure the upwelling longwave spectral radiance emitted by the Earth, from 100 to 1600 cm-1 (from 100 to 6.25 microns in wavelength), with a spectral resolution of 0.5 cm-1. FORUM will fly in loose formation with the MetOp-SG-A satellite, hosting the Infrared Atmospheric Sounding Interferometer - New Generation (IASI-NG). In tight coincidence with FORUM, IASI-NG will measure the upwelling atmospheric spectrum in the MIR interval from 645 to 2760 cm-1, thus nicely complementing the FORUM FIR spectrum. Together, the two missions will provide an unprecedented spectral coverage, from 100 to 2760 cm-1. On the one hand, the FIR part of the spectrum (100-667 cm-1) measured by FORUM is the most sensitive to the water vapour content in the Upper Troposphere / Lower Stratosphere. On the other hand, 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, that are essential to constrain atmospheric, cloud and surface parameter retrievals. We compare two alternative techniques aiming to determine atmospheric and surface state parameters by exploiting simultaneously both FORUM and IASI-NG measurements. Specifically, we examine the synergistic retrieval of state parameters from the simultaneous inversion of the two measurements, and the Complete Data Fusion method, combining a posteriori the state parameters retrieved from the individual measurements. Test retrievals based on realistic clear-sky scenarios are presented. We assess the performance of synergistic and fused products by means of their errors evaluated both from the covariance matrix of the retrieval and from the statistics of the differences between retrieved and true state parameters. The retrieval products considered include the profiles of water vapour and of temperature, surface temperature and spectral emissivity in the range from 100 to 2300 cm-1. We find that IASI-NG measurements constrain and improve significantly the quality of FORUM retrieval products. The improvements, however, are less important when the spatial and temporal mismatch between the measurements becomes too large. Synergistic retrieval and Complete Data Fusion provide almost identical results for perfectly matching measurements. In the presence of realistic time- and / or space- mismatches between the measurements, the size of the differences between the two methods is comparable to the noise error applicable to the retrieved parameters. Applicability, advantages and disadvantages of the two methods are discussed.