Emissivity retrievals with FORUM's end-to-end simulator:challenges and recommendations

Spectral emissivity is a key property of the Earth's surface, of which only very few measurements exist so farin the far-infrared (FIR) spectral region, even though recent work has shown that the FIR is important for accurate mod-elling of the global climate. The European Space Agency's9th Earth Explorer, FORUM (Far-infrared Outgoing Radi-ation Understanding and Monitoring) will provide the firstglobal spectrally resolved measurements of the Earth's top-of-the-atmosphere (TOA) spectrum in the FIR. In clear-skyconditions with low water vapour content, these measurements will provide a unique opportunity to retrieve spectrally resolved FIR surface emissivity. In preparation for the FORUM mission with an expected launch in 2027, this study takes the first steps towards the development of an opera-tional emissivity retrieval for FORUM by investigating the sensitivity of the emissivity product of a full spectrum optimal estimation retrieval method to different physical andoperational parameters. The tool used for the sensitivity testsis the FORUM mission's end-to-end simulator. These tests show that the spectral emissivity of most surface types canbe retrieved for dry scenes in the 350-600 cm-1 region, with an absolute uncertainty ranging from 0.005 to 0.01. In addition, the quality of the retrieval is quantified with respectto the precipitable water vapour content of the scene, and the uncertainty caused by the correlation of emissivity withsurface temperature is investigated. Based on these investigations, a road map is recommended for the development of the operational emissivity product.