Comparing two Methods to exploit the Synergy between FORUM and IASI-NG

In recent years, the availability of a large amount of data has stimulated the use of synergistic approaches to gain the largest amount of information from measurements provided by a great number of satellite missions. Two classes of strategies are widely used to combine multiple sets of independent retrievals of the same atmospheric target: the synergistic retrieval (SR) and the a posteriori combination of the retrieved products. The Complete Data Fusion (CDF) algorithm was presented as a new a posteriori method to combine independent measurements of the same vertical profile of an atmospheric parameter into a single estimate for a concise and complete characterization of the atmospheric state. The CDF is able to reduce the complexity of managing high volumes of data and to improve their quality with respect to the operational outcome of individual instruments. FORUM (Far-infrared Outgoing Radiation Understanding and Monitoring) experiment has been selected to be the ninth Earth Explorer mission of the European Space Agency (ESA) and it is scheduled for launch on a polar orbiting satellite in 2027. The core instrument will be a Far-InfraRed (FIR) spectrometer measuring, at nadir, the upwelling spectral radiance emitted by the Earth across the most relevant infrared part of the electromagnetic spectrum, 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 satellite, hosting the Infrared Atmospheric Sounding Interferometer - New Generation (IASI-NG). Since IASI-NG will measure the Middle-InfraRed (MIR) part of the upwelling atmospheric spectrum (from 645 to 2760 cm-1), the matching FORUM FIR spectra can be effectively used to synergistically complement the IASI-NG measurements. The Level 2 products of FORUM include the water vapour profile, surface spectral emissivity in dry regions, cloud parameters and spectral fluxes. In this work, we present the results obtained from the application of two synergistic approaches, the SR and the CDF, to IASI-NG and FORUM synthetic measurements. We carried out two sets of test retrievals, based on synthetic clear-sky measurements, emulating an idealized situation in which both FORUM and IASI-NG measure, either with perfect matching or with a realistic mismatch, for 900 times, the same area over the Antarctic Plateau surface covered by coarse snow. The retrieval products considered in this study include the profiles of water vapour and temperature, surface temperature and spectral emissivity from 100 to 2300 cm-1. To characterize the quality of synergistic and fused products we evaluate their number of degrees of freedom as well as their total errors from the statistics of the differences between retrieved and true state parameters. Finally we compare the results obtained by the two methods by considering the statistics of the differences between synergistic and fused products. We find that in case of perfectly matching measurements, SR and CDF provide results that differ by less than 1/10 of their associated noise retrieval error. If a realistic mismatch (both in time and space) between the measurements is considered, the two methods provide more different results but with differences that are still within the retrieval error. The results obtained from the two methods are compared and, for both of them, the effect of the mismatch on the synergistic products is quantified.