Surface velocity of the Northeast Greenland Ice Stream (NEGIS): Assessment of interior velocities derived from satellite data by GPS

• Main Authors: Hvidberg, Christine S., Grinsted, Aslak, Dahl-Jensen, Dorthe, Khan, Shfaqat Abbas, Kusk, Anders, Andersen, Jonas Kvist, Neckel, Niklas, Solgaard, Anne, Karlsson, Nanna B., Kjær, Helle Astrid, Vallelonga, Paul
• Format: Text
• Language: English
• Published: 2020
• Subjects: Arctic; Greenland; East Greenland; East Greenland Ice-core Project; Greenland ice core; Greenland Ice core Project; ice core
• Online Access: https://doi.org/10.5194/tc-2020-103; https://tc.copernicus.org/preprints/tc-2020-103/

The Northeast Greenland Ice Stream (NEGIS) extends around 600 km upstream from the coast to its onset near the ice divide in interior Greenland. Several maps of surface velocity and topography in the interior Greenland exist, but the accuracy is not well constrained by in situ observations and limiting detailed studies of flow structures and shear margins near the onset of NEGIS. Here we present the results from a GPS mapping of surface velocity in an area located approximately 150 km from the ice divide near the East Greenland Ice-core Project (EastGRIP) deep drilling site (75°38’ N, 35°60’ W). A GPS strain net consisting of 63 poles was established and observed over the years 2015–2019. The strain net covers 35 km along NEGIS and 40 km across NEGIS, including both shear margins. The ice flows with a uniform surface speed of approximately 55 m a −1 within a > 10 km wide central flow band with strain rates in the order of 10−4 a −1 . The strain rates increase in the shear margins by an order of magnitude, and 10–20 m deep shear margin troughs mark a zone with enhanced longitudinal stretching, transverse compression and shear. We compare the GPS results to the Arctic Digital Elevation Model (ArcticDEM) and a list of satellite-based surface velocity products in order to evaluate these products. For each velocity product, we determine the bias and precision of the velocity compared to the GPS observations, as well as the smoothing of the velocity products needed to obtain optimal precision. The best products have a bias and precision of ~0.5 m a −1 . We combine the GPS results with satellite-based products and show that organized patterns in flow and topography emerge in the NEGIS ice stream when the surface velocity exceeds approximately 55 m a −1 and are related to bedrock topography.