Retrieval of high latitude surface emissivity across the far and mid-infrared from a high-altitude aircraft flight

Abstract:

The far infrared (FIR) defined here as wavelengths, λ, > 15 μm contributes around 50 % to the total outgoing longwave radiation to space in the global mean. Typically, even in clear-sky conditions, strong absorption by water vapour means that at these wavelengths the surface is obscured when viewed from any significant altitude. However, the cold, dry conditions often encountered at high latitudes simultaneously shift the peak of the Planck function to longer wavelengths and see semi-transparent micro-windows in the FIR ‘open up’, allowing surface emission to propagate further through the atmosphere. Under these conditions, simulations imply it is possible to observe FIR surface emission from space. As such, FIR surface emissivity can, in principle, be inferred from satellite instrumentation observing across this spectral range. Indeed, this is one goal of the forthcoming Polar Radiant Energy in the Far InfraRed Experiment (PREFIRE) and Far infrared Outgoing Radiation Understanding and Monitoring (FORUM) missions.

In this study we present, to the best of our knowledge, the first retrievals of infrared spectral surface emissivities spanning the far and mid infrared from radiances measured at altitude. Specifically, we exploit observations from an aircraft 'flight of opportunity' over Greenland, at an altitude of 9.2 km above sea level. We describe the flight campaign, available measurements and the retrieval method. We find that the principal barriers to reducing uncertainty in the emissivity retrievals are instrumental noise and our ability to simultaneously retrieve the underlying surface temperature. However, our results indicate that, with the airborne instrumentation available to us, it is possible to retrieve emissivities with an uncertainty of ~ 0.02 or better across much of the infrared. Our results also show good consistency with retrievals from the same flight made from near-surface observations. This gives confidence in the methodology employed for this high altitude case, whose viewing configuration is more akin to PREFIRE and FORUM. Moreover, our retrievals confirm that the FIR emissivity of snow and ice surfaces has a distinct spectral shape and can depart substantially from unity, with values as low as 0.9 between 400-450 cm^-1.