Abstract
Abstract. Surface lakes on the Greenland Ice Sheet play a key role in its surface mass balance, hydrology and biogeochemistry. They often drain rapidly in the summer via hydrofracture, which delivers lake water to the ice sheet base over timescales of hours to days and then can allow meltwater to reach the base for the rest of the summer. Rapid lake drainage, therefore, influences subglacial drainage evolution; water pressures; ice flow; biogeochemical activity; and ultimately the delivery of water, sediments and nutrients to the ocean. It has generally been assumed that rapid lake drainage events are confined to the summer, as this is typically when observations are made using satellite optical imagery. Here we develop a method to quantify backscatter changes in satellite radar imagery, which we use to document the drainage of six different lakes during three winters (2014/15, 2015/16 and 2016/17) in fast-flowing parts of the Greenland Ice Sheet. Analysis of optical imagery from before and after the three winters supports the radar-based evidence for winter lake drainage events and also provides estimates of lake drainage volumes, which range between 0.000046 ± 0.000017 and 0.0200 ± 0.002817 km3. For three of the events, optical imagery allows repeat photoclinometry (shape from shading) calculations to be made showing mean vertical collapse of the lake surfaces ranging between 1.21 ± 1.61 and 7.25 ± 1.61 m and drainage volumes of 0.002 ± 0.002968 to 0.044 ± 0.009858 km3. For one of these three, time-stamped ArcticDEM strips allow for DEM differencing, which demonstrates a mean collapse depth of 2.17 ± 0.28 m across the lake area. The findings show that lake drainage can occur in the winter in the absence of active surface melt and notable ice flow acceleration, which may have important implications for subglacial hydrology and biogeochemical processes.
Highlights
Lakes form each summer on the surface of the Greenland Ice Sheet (GrIS), in the upper ablation and lower accumulation areas (McMillan et al, 2007; Selmes et al, 2011; Liang et al, 2012; Pope et al, 2016; Williamson et al, 2017)
Rapid lake drainage provides a major shock to the ice sheet as millions of cubic metres of water are delivered to the bed in a few hours, and the resultant fracture may permit meltwater to reach the bed for the rest of the summer
We have developed an automated method for identifying large, anomalous, sudden and sustained backscatter changes in Sentinel-1 synthetic aperture radar (SAR) imagery, which we apply to images collected between October and May spanning three winter seasons
Summary
Lakes form each summer on the surface of the Greenland Ice Sheet (GrIS), in the upper ablation and lower accumulation areas (McMillan et al, 2007; Selmes et al, 2011; Liang et al, 2012; Pope et al, 2016; Williamson et al, 2017). Rapid lake drainage provides a major shock to the ice sheet as millions of cubic metres of water are delivered to the bed in a few hours, and the resultant fracture may permit meltwater to reach the bed for the rest of the summer. This lake drainage and subsequent water input generates a radiating subglacial water “blister” beneath the draining lake, which evolves into a conduit in the down-hydraulic-potential direction allowing the lake water and subsequent meltwater to be evacuated (Pimentel and Flowers, 2010; Tsai and Rice, 2010; Dow et al, 2015). High water pressures are generated transiently during lake drainage (Banwell et al, 2016), lifting the ice sheet off the bed and increasing temporarily its sliding velocity
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