Retrieval of surface emissivity from FORUM-EE9 simulated measurements: optimization of constraints,

FORUM (Far-infrared Outgoing Radiation Understanding and Monitoring) is a satellite mission selected in 2019 as the ninth ESA (European Space Agency) Earth Explorer mission. FORUM will provide interferometric measurements in the spectral interval encompassing the Far-InfraRed (FIR) part of the spectrum, responsible for about 50% of the outgoing longwave flux lost by our planet into space. While existing instruments are unable to measure the Top of the Atmosphere (TOA) resolved spectrum in the FIR, more accurate measurements may reduce the uncertainties in estimating the Earth's warning. The observations will also improve the knowledge of several atmospheric variables, such as tropospheric water vapor, ice cloud properties and land emissivity in the FIR. In the early stages of the mission development, an End-to-End Simulator (E2ES) has been devised to demonstrate proof-of-concept and to evaluate the impact of instrument characteristics and scene conditions on the accuracy of the reconstructed atmospheric properties. The retrieval is obtained through inversion of the radiative transfer equation. In the retrieval step, performed in the E2ES by the CLAIM (CLouds and Atmosphere Inversion Module) code, the atmospheric state that best reconstructs the simulated measured spectrum is determined. The inversion is a severely ill-conditioned problem, and is solved using a Gauss-Newton (GN) method combined with the Optimal Estimation (OE) approach, a Tikhonov regularization scheme based on a standard Bayesian approach. The retrieved profiles may present unphysical oscillations, requiring the application of the Iterative Variable Strength regularization (IVS) after the convergence of the GN method. In this seminar, I will focus on the retrieval of the surface emissivity, in particular on the choice of the retrieval grid step and the IVS parameters, using the FORUM simulated measurements in different latitude bands.