Far-Infrared Surface Emissivity: An Unconstrained Property that Impacts Atmospheric Radiation and Climate

Presently, there are no global measurement constraints on the surface emissivity at wavelengths longer than 15 μm, even though these surface characteristics in this far-infrared region have a direct impact on the outgoing longwave radiation (OLR) and infrared cooling rates where the column precipitable water vapor (PWV) is less than 1 cm. These conditions are common at high latitudes and high altitudes. The published literature contains several theoretical studies that state that common minerals can exhibit a far-infrared surface emissivity as low as 0.8 and that snowpack can exhibit emissivity as low as 0.91 but is a strong function of snow grain size. Sensitivity studies using the Community Earth System Model (CESM) find significant changes in many climate variables including OLR, surface temperature, and frozen surface spatial patterns when the model is subjected to lower values of far-infrared surface emissivity. Spectrally-resolved measurements of far-infrared microwindows, if implemented, could be used to determine the spatial and temporal patterns of this quantity.