In-situ Measurements of the Emissivity of Ice and Snow surfaces in the Mid- and Far-infrared

Knowledge of the emissivity of the Earth’s surface is vital for understanding the Earth’s radiation budget. However, there is a lack of emissivity measurements in the far-infrared (wavenumbers less than 667 cm^-1 or wavelengths greater than 15 μm) despite studies showing that the surface emissivity in these regions can have a discernible impact on the outgoing longwave radiation. In-situ measurements of surface emissivity in the far-infrared are also needed to support the upcoming far-infrared satellite missions; the Polar Radiant Energy in the Far-InfraRed Experiment (PREFIRE) developed by NASA and due to launch in spring 2024, and the European Space Agency’s Far-infrared Outgoing Radiation Understanding and Monitoring (FORUM) mission due to launch in 2027.

To fill this observational gap, the Far INfrarEd Spectrometer for Surface Emissivity (FINESSE) was developed at Imperial College London. FINESSE has a spectral range of 400 to 1600 cm^-1 (6.25 to 25μm) and is designed for in-situ measurements of surface emissivity, particularly in cold climates.

In this presentation we present observations from the first deployment of FINESSE to the ALOMAR observatory in Northern Norway (69°16' N, 16° 0' E). We describe the campaign, the radiance measurements made by FINESSE and the auxiliary data taken. We then outline the method used to determine the surface temperature and emissivity and finally present the retrieved emissivity of the ice and snow surfaces.