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

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Type Article
Original languageEnglish
JournalCryosphere Discussions
Publication statusPublished - 2019
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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