17 Mar 2020

17 Mar 2020

Review status: a revised version of this preprint was accepted for the journal TC and is expected to appear here in due course.

First spectral measurements of light attenuation in Greenland Ice Sheet bare ice suggest shallower subsurface radiative heating and ICESat-2 penetration depth in the ablation zone

Matthew G. Cooper1, Laurence C. Smith2,3, Asa K. Rennermalm4, Marco Tedesco5,6, Rohi Muthyala4, Sasha Z. Leidman4, Samiah E. Moustafa2, and Jessica V. Fayne1 Matthew G. Cooper et al.
  • 1Department of Geography, University of California, Los Angeles, Los Angeles, 90027, USA
  • 2Institute at Brown for Environment and Society, Brown University, Providence, 02912, USA
  • 3Department of Earth, Environmental and Planetary Sciences, Brown University, Providence, 02912, USA
  • 4Department of Geography, Rutgers University, Piscataway, 08854, USA
  • 5NASA Goddard Institute for Space Studies, New York,10025, USA
  • 6Lamont Doherty Earth Observatory, Columbia University, New York, 10964, USA

Abstract. Light transmission into bare glacial ice affects surface energy balance, bio-photochemical cycling, and light detection and ranging (LiDAR) laser elevation measurements but has not previously been reported for the Greenland Ice Sheet. We present in-ice solar irradiance measured over the spectral range 350–900 nm and 12–77 cm depth collected at a site in the western Greenland ablation zone. The acquired spectral irradiance measurements are used to calculate flux attenuation coefficients using an exponential decay Bouguer law model and are compared to values calculated from two-stream radiative transfer theory. Relative to asymptotic two-stream theory, our empirical attenuation coefficients are up to one order of magnitude larger in the range 350–530 nm, suggesting light absorbing particles embedded in ice enhance visible light absorption at our field site. The empirical coefficients accurately describe light attenuation in the ice interior but underestimate light attenuation near the ice surface. Consequently, Bouguer’s law overestimates transmitted flux by up to 50 % depending on wavelength. Refraction is unlikely to explain the discrepancy. Instead, vertical variation in the ice microstructure and the concentration of light absorbing particles appears to enhance near-surface attenuation at our field site. The magnitude of this near-surface attenuation implies that optical penetration depth is lower by up to 19 cm (28 %) at wavelengths relevant to visible-wavelength lidar altimetry of ice surface elevation (e.g. 532 nm for the Ice, Cloud, and Land Elevation Satellite-2) than is suggested by e-folding depths inferred from two stream theory for optically pure glacier ice. This enhanced near-surface attenuation implies shallower light transmission and therefore lower subsurface light availability for subsurface radiative heating and bio-photochemical cycling. We recommend radiative transfer models applied to bare ice in the Greenland Ice Sheet ablation zone account for vertical variation in light attenuation due to the vertical distribution of light absorbing particles and ice microstructure, and we provide new values of flux attenuation, absorption, and scattering coefficients to support model validation and parameterization.

Matthew G. Cooper et al.

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AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment
Printer-friendly Version - Printer-friendly version Supplement - Supplement

Matthew G. Cooper et al.

Data sets

Optical attenuation coefficients of glacier ice from 350-700 nm (West Greenland) M. G. Cooper, L. C. Smith, A. K. Rennermalm, M. Tedesco, R. Muthyala, S. Z. Leidman, S. E. Moustafa, J. V. Fayne

Matthew G. Cooper et al.


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Short summary
As sunlight shines on glacier ice some of its energy is absorbed below the surface. This causes ice to melt from within and disaggregate, creating porosity that stores meltwater. Most ice melt models assume ice is solid. Light that transmits into ice also affects satellite measurements of ice surfaces. We measured the intensity of light inside glacier ice i.e. how much light transmits into ice, so we can model meltwater generated inside glacier ice and its effect on satellite measurements.