Toward Monitoring Surface and Subsurface Lakes on the Greenland Ice Sheet Using Sentinel-1 SAR and Landsat-8 OLI Imagery

Katie E Miles,Ian C Willis,Andrew G Williamson,Corinne L Benedek,Marco Tedesco

doi:10.3389/feart.2017.00058

Abstract

Supraglacial lakes are an important component of the Greenland Ice Sheet’s mass balance and hydrology, with their drainage affecting ice dynamics. This study uses imagery from the recently launched Sentinel-1A Synthetic Aperture Radar (SAR) satellite to investigate supraglacial lakes in West Greenland. A semi-automated algorithm is developed to detect surface lakes from Sentinel-1 images during the 2015 summer. A combined Landsat-8 and Sentinel-1 dataset, which has a comparable temporal resolution to MODIS (3 days versus daily) but a higher spatial resolution (25-40 m versus 250-500 m), is then used together with a fully-automated lake drainage detection algorithm. Rapid ( 4 days) drainages are investigated for both small (< 0.125 km2, the minimum size detectable by MODIS) and large (≥ 0.125 km2) lakes through the summer. Drainage events of small lakes occur at lower elevations (mean 159 m), and slightly earlier (mean 4.5 days) in the melt season than those of large lakes. The analysis is extended manually into the early winter to calculate the dates and elevations of lake freeze-through more precisely than is possible with optical imagery (mean 30 August; 1270 m mean elevation). Finally, the Sentinel-1 imagery is used to detect subsurface lakes and, for the first time, their dates of appearance and freeze-through (mean 9 August and 7 October, respectively). These subsurface lakes occur at higher elevations than the surface lakes detected in this study (mean 1593 m and 1185 m, respectively). Sentinel-1 imagery therefore provides great potential for tracking melting, water movement and freezing within both the firn zone and ablation area of the Greenland Ice Sheet.

Highlights

The rate of mass loss of the Greenland Ice Sheet (GrIS) has accelerated in recent decades and is increasingly dominated by surface meltwater runoff (Rignot et al, 2011; Vaughan et al, 2013; Csatho et al, 2014; Enderlin et al, 2014)
Lake drainage events are important in opening up moulins, which act as connections between the surface and bed, allowing continued delivery of water through the summer to the subglacial drainage system, facilitating its seasonal evolution and impacting basal water pressures and ice motion over the longer term (Bartholomew et al, 2010; Sole et al, 2013; Tedstone et al, 2013)
The total lake area classified by the Landsat Normalized Difference Water Index (NDWI) scheme was closer to the area produced using the Sentinel-1 HV polarization than the HH polarization, with the best match given when the lake areas produced from both the HV and HH polarizations were combined (Table 1)

Summary

Introduction

The rate of mass loss of the Greenland Ice Sheet (GrIS) has accelerated in recent decades and is increasingly dominated by surface meltwater runoff (Rignot et al, 2011; Vaughan et al, 2013; Csatho et al, 2014; Enderlin et al, 2014). Greenland Supraglacial Lakes from Sentinel-1 than that of the surrounding ice, so they enhance energy absorption and meltwater production (Lüthje et al, 2006; Tedesco and Steiner, 2011; Tedesco et al, 2012) They are significant as they affect the location, timing, volume, and rate of meltwater delivery to the bed, which impacts ice dynamics in different ways depending on whether the lakes drain rapidly, slowly, or do not drain at all and freeze at the end of the melt season (Joughin et al, 2013; Selmes et al, 2013; Tedesco et al, 2013). Increasing amounts of meltwater at higher elevations in the future may allow more lakes to form and drain in those locations, which may lead to an increased hydraulic efficiency of the subglacial drainage system through the formation of channels; this is unknown (Mayaud et al, 2014; Leeson et al, 2015)

Results

Discussion

Conclusion