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Discussion papers
https://doi.org/10.5194/tc-2019-131
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/tc-2019-131
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 15 Jul 2019

Submitted as: research article | 15 Jul 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal The Cryosphere (TC).

Algal growth and weathering crust structure drive variability in Greenland Ice Sheet ice albedo

Andrew J. Tedstone1, Joseph M. Cook2, Christopher J. Williamson1, Stefan Hofer1, Jenine McCutcheon3, Tristram Irvine-Fynn4, Thomas Gribbin1, and Martyn Tranter1 Andrew J. Tedstone et al.
  • 1Bristol Glaciology Centre, School of Geographical Sciences, University of Bristol, Bristol, UK
  • 2Department of Geography, University of Sheffield, Sheffield, UK
  • 3School of Earth and Environment, University of Leeds, Leeds, UK
  • 4Department of Geography and Earth Sciences, Aberystwyth University, Aberystwyth, UK

Abstract. One of the primary controls upon the melting of the Greenland Ice Sheet (GrIS) is albedo. There is a major difference in the albedo of snow-covered versus bare-ice surfaces, but observations also show that there is substantial spatio-temporal variability of up to ~ 0.4 in bare-ice albedo. Variability in bare ice albedo has been attributed to a number of processes including the accumulation of Light Absorbing Impurities (LAIs) and the changing physical properties of the near-surface ice. However, the combined impact of these processes upon albedo remains poorly constrained. Here we use field observations to show that among LAIs, pigmented glacier algae are ubiquitous and cause surface darkening both within and outside the south-west GrIS dark zone, but that other factors including modification of underlying ice properties by algal bloom presence, surface topography and weathering crust development are also important in determining patterns of daily albedo variability. We further use unmanned aerial system observations to examine the scale gap in albedo between ground versus remotely-sensed measurements made by Sentinel-2 (S-2) and MODIS. S-2 observations provide a highly conservative estimate of algal bloom presence because algal blooms occur in patches much smaller than the ground resolution of S-2 data. Nevertheless, the bare-ice albedo distribution at the scale of 20 × 20 m S-2 pixels is generally unimodal and unskewed. Conversely, bare ice surfaces have a left-skewed albedo distribution at MODIS MOD10A1 scales. Thus, when MOD10A1 observations are used as input to energy balance modelling then meltwater production can be under-estimated by ~ 2 %. Our study highlights that (1) the impact of physical ice surface processes is of similar importance to the direct darkening role of light-absorbing impurities upon ice albedo and (2) there is a spatial scale dependency in albedo measurement which reduces detection of real changes at coarser resolutions.

Andrew J. Tedstone et al.
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Andrew J. Tedstone et al.
Data sets

Multi-spectral unmanned aerial system imagery, S6, south-west Greenland, July 2017: Level 1 (unmosaiced radiance measurements) A. Tedstone, J. Cook, and M. Tranter https://doi.org/10.5285/0579d4a8-e315-41d7-af43-25fb50c7d3da

Multi-spectral unmanned aerial system imagery, UPE_U, north-west Greenland, July 2018: Level 1 (unmosaiced radiance measurements) A. Tedstone, J. Cook, and M. Tranter https://doi.org/10.5285/a87b7897-354c-4435-a1bc-e6053e7569e0

Model code and software

Code for Tedstone et al. (2019) "Algal growth and weathering crust structure drive variability in Greenland Ice Sheet ice albedo", Cryosphere Discussions A. Tedstone https://doi.org/10.5281/zenodo.3335925

Andrew J. Tedstone et al.
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Short summary
Albedo describes how much light that hits a surface is reflected without being absorbed. Low-albedo ice surfaces melt more quickly. There are large differences in the albedo of bare-ice areas of the Greenland Ice Sheet. They are caused both by dark glacier algae and by what the underlying ice looks like. Changes occur over centimetres to metres, so satellites struggle to detect real albedo changes. Estimates of melt made using satellite measurements therefore tend to be under-estimates.
Albedo describes how much light that hits a surface is reflected without being absorbed....
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