Waters at the Edge
Tracking Greenland’s Ice-Marginal Lakes with SWOT Observations
L.A.C. Reichel (TU Delft - Civil Engineering & Geosciences)
B. Wouters – Mentor (TU Delft - Physical and Space Geodesy)
D.C. Slobbe – Mentor (TU Delft - Physical and Space Geodesy)
S.L.M. Lhermitte – Graduation committee member (TU Delft - Mathematical Geodesy and Positioning)
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Abstract
The Surface Water and Ocean Topography (SWOT) mission has the goal to observe global lakes and reservoirs with a size as small as 1 ha and ocean circulations at sub-mesoscale with the help of the Ka-band Radar Interferometer (KaRIn) wide-swath altimeter. Launched in 2022, SWOT is observing an unprecedented amount of lakes globally at least once every 21 days. With a high spatial and temporal resolution, SWOT can be used to measure global water storage changes and improve climate modelling. This study contributes to assess the performance of SWOT in observing the Water Surface Elevation (WSE) of lakes, by analysing SWOT observations of Ice Marginal Lakes (IMLs) in southwest Greenland. These lakes are particularly difficult to observe with SWOT because the region is mountainous and the lake surfaces are covered in ice for most of the year. For this purpose, three lakes of different sizes were chosen and the WSEs obtained from the two main SWOT lake data products were compared to elevations from the Ice, Cloud, and land Elevation Satellite 2 (ICESat-2). While SWOTs Pixel Cloud Data Product (PIXC) product contains more observations during the ice-covered period and is better for finding error sources, the Lake Single Pass Vector Product (LakeSP) product is more convenient for analysing large numbers of lakes and both products have a similar accuracy after data editing. The SWOT-derived WSEs obtained in this study are not in compliance with the WSE mission requirements (1σ <10 cm for lakes >1 km² and 1σ <25 cm for lakes <1 km²), because the average WSE difference to ICESat-2 lies between 0.27-1.38 m for the three lakes that were analysed. This analysis indicates that the main error-sources are ice cover - leading to a low Normalised Radar Cross-Section (NRCS) and coherence, phase unwrapping errors and specular ringing - resulting in inconsistent lake outlines. It is recommended that more strict editing should be applied to the SWOT LakeSP product
when observing IMLs, in particular regarding ”dark water” pixels, and that the Prior Lake Database (PLD) should be updated to include more ice marginal lakes.