Snow and Ice Far-Infrared Spectral Emissivity Retrievals from FORUM-like Measurements

In cold and dry conditions typical of high-latitude and high-altitude locations, a significant fraction of the far-infrared (FIR) emission from the surface can reach the top of atmosphere (TOA). Recent works examined the impact of FIR surface emissivity on climate model projections and highlighted the influence of surface spectral emissivity on the surface and TOA FIR radiation budgets of polar regions. As spectral measurements in the FIR are still scarce, the authors of these studies used theoretical modelling to generate representative surface spectral emissivities. FORUM (Far-infrared Outgoing Radiation Understanding and Monitoring) is expected to be launched in 2026, and its core instrument will measure across Earth's FIR part of the electromagnetic spectrum. One of the key objectives of the mission is to obtain an extensive database of surface emissivity in the FIR. During the mission phases A and B1, the Far-Infrared Radiation Mobile Observation System (FIRMOS), a Fourier transform spectrometer, was built as an instrument demonstrator. In the future FIRMOS may also contribute to FORUM validation from the ground and from stratospheric balloon platforms. In the end of 2018, FIRMOS was deployed for a two-month ground-based campaign on Mount Zugspitze (German Alps, 2962m a.s.l.) where it measured the downwelling atmospheric spectral radiance from 100 to 1000 cm -1 (10-100 µm) with a resolution of 0.3 cm -1 . During this campaign, upwelling snow surface emission spectra were also acquired with FIRMOS. A variety of 9 snow and ice samples were collected in the vicinity of the experiment site. The samples were characterised in terms of snow grain type, density (kg m -3 ), and specific surface area (SSA, m 2 kg -1 ). The radiance measurements of the samples and of the atmosphere were inverted to retrieve spectral emissivity by means of an algorithm based on optimal estimation. This presentation describes the modelling of the inverse problem, the retrieval method and the preliminary results obtained.