Retrievals of High‐Latitude Surface Emissivity Across the Infrared From High‐Altitude Aircraft Flights

Jonathan E Murray,Xiuhong Chen,Christophe Bellisario,Xianglei Huang,Cathryn Fox,Doug Anderson,Chawn Harlow,Richard Bantges,Juliet C Pickering,Maureen Smith,Alan Last,Helen E Brindley,Stuart Fox

doi:10.1029/2020jd033672

Abstract

AbstractWe present retrievals of infrared spectral surface emissivities spanning the far infrared and mid‐infrared from aircraft observations over Greenland, taken at an altitude of 9.2 km above sea level. We describe the flight campaign, available measurements, and the retrieval method. The principal barriers to reducing uncertainty in the emissivity retrievals are found to be instrumental noise and our ability to simultaneously retrieve the underlying surface temperature. However, our results indicate that using the instrumentation available to us it is possible to retrieve emissivities from altitude with an uncertainty of ~0.02 or better across much of the infrared. They confirm that the far‐infrared emissivity of snow and ice surfaces can depart substantially from unity, reaching values as low as 0.9 between 400 and 450 cm−1. They also show good consistency with retrievals from the same flight made from near‐surface observations giving confidence in the methodology used and the results obtained for this more challenging viewing configuration. To the best of our knowledge, this is the first time that far‐infrared surface emissivity has been retrieved from altitude and demonstrates that the methodology has the potential to be extended to planned satellite far‐infrared missions.

Highlights

The far infrared (FIR) defined here as wavelengths, λ > 15 μm, or equivalently wavenumbers, ν < 667 cm−1, contributes around 50% to the total outgoing longwave radiation to space in the global mean
Superposed on the spectrum is the atmospheric transmission, simulated using the associated dropsonde profile for the cycle, as well as surface emissivity estimates for snow and ice taken from the University of California Santa Barbara (UCSB) data base (Wan et al, 1994) which indicate a peak in emissivity at about 960 cm−1
The aim of this study was to investigate the feasibility of retrieving FIR surface emissivity from remotely sensed nadir radiance observations made from an aircraft flight over the Greenland Plateau

Summary

Introduction

The far infrared (FIR) defined here as wavelengths, λ > 15 μm, or equivalently wavenumbers, ν < 667 cm−1, contributes around 50% to the total outgoing longwave (infrared) radiation to space in the global mean. The cold, dry conditions often encountered at high latitudes simultaneously shift the peak of the Planck function to longer wavelengths and see semitransparent microwindows in the FIR open up, allowing surface emission to propagate further through the atmosphere. Under these conditions, it is possible to observe FIR surface emission from space. Feldman et al (2014) further illustrates the importance of the FIR surface emissivity in the Community Earth System Model (CESM) and introduces the concept of an “ice‐emissivity” feedback, whereby the emissivity changes induced by melting sea ice accelerate subsequent surface warming. The inhibiting factor is a lack of suitable spectrally resolved observations across the FIR from which the surface emissivity can be inferred

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Conclusion