Abstract
The Programme for Monitoring of the Greenland Ice Sheet (PROMICE) provides surface meteorological and glaciological measurements from widespread on-ice automatic weather stations since mid-2007. In this study, we use 105 PROMICE ice-ablation time series to identify the timing of seasonal bare-ice onset preceded by snow cover conditions. From this collection, we find a bare-ice albedo at ice-ablation onset (here called bare-ice-onset albedo) of 0.565 ± 0.109 that has no apparent spatial dependence among 20 sites across Greenland. We then apply this snow-to-ice albedo transition value to measure the variations in daily Greenland bare-ice area in Sentinel-3 optical satellite imagery covering the extremely low and high respective melt years of 2018 and 2019. Daily Greenland bare-ice area peaked at 153 489 km² in 2019, 1.9 times larger than in 2018 (80 220 km²), equating to 9.0% (in 2019) and 4.7% (in 2018) of the ice sheet area.
Highlights
The recent net loss of Greenland land ice is among the largest contributors to global sea-level rise (Box & Sharp 2017)
By applying the 6 cm threshold to identify the date of bare-ice onset for each station year in a semi-automatic procedure, we determined a bareice-onset albedo of 0.565 ± 0.109 for the Greenland ice sheet
After bare-ice onset, we found a further albedo decrease of 0.111, which may be the result of ice-algal growth
Summary
The recent net loss of Greenland land ice is among the largest contributors to global sea-level rise (Box & Sharp 2017). While warm air advection produces the highest daily ice-ablation observations (Fausto et al 2016), absorbed sunlight is the largest melt energy source over seasonal time scales (van den Broeke et al 2008; Box et al 2012; Fausto et al 2016). Absorbed sunlight increases during the melt season as surface conditions shift from a highly reflective, dry snow cover, to lower albedo wet snow with larger grains (Wiscombe & Warren 1980; Brun 1989), and yet lower albedo across the ablation area. We study the surface climate conditions spanning the melt season transition from dry snow to bare ice using PROMICE ground measurements. The acquisition and/or computation of each variable is described
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