Far-infrared surface emissivity and climate

Presently, there are no global measurement constraints on the surface emissivity at wavelengths longer than 15 μm, even though this surface property in this far-IR region has a direct impact on the outgoing longwave radiation (OLR) and infrared cooling rates where the column precipitable water vapor (PWV) is less than 1 mm. Such dry conditions are common for high-altitude and highlatitude locations, with the potential for modeled climate to be impacted by uncertain surface characteristics. This paper explores the sensitivity of instantaneous OLR and cooling rates to changes in far-IR surface emissivity and how this unconstrained property impacts climate model projections. At high latitudes and altitudes, a 0.05 change in emissivity due to mineralogy and snow grain size can cause a 1.8–2.0 W m−2 difference in the instantaneous cleartemperature, water vapor, and cloud profiles. Moreover, there is evidence that cloud optical properties in this spectral region are difficult to characterize without direct measurements (13, 14). However, the role of far-IR surface emissivity on the radiation budget, including surface emission, OLR, cooling rates, and ultimately climate variables, has not been explored as much as for clouds and water vapor.