DOI: 10.5061/dryad.1c59zw423
title: Data For: Seawater intrusion at the grounding line of Jakobshavn Isbrae, Greenland, from Terrestrial Radar Interferometry (TRI)

description: Jakobshavn Isbrae is a major outlet glacier in West Greenland that lost
 its protective ice shelf in 2002 and has been speeding up and retreating
 since. We image its grounding line for the first time with a ground
 portable radar interferometer deployed in 2016 and detect its migration at
 tidal frequencies. The southern half of the glacier develops a floating
 section (3 km x 3 km) that migrates in phase with the tide up to a
 distance of 1.5 km, which is far more than expected from flotation. We
 attribute the migration to kilometer-scale seawater intrusions, 10-20 cm
 in height, occurring at high tide. The intrusions reveal that the glacier
 bed must be 100-600 m deeper than expected on the south side, which
 illustrates that our knowledge of bed topography remains limited in this
 sector. We expect seawater intrusions to cause rapid melt of basal ice and
 play a major role in the glacier evolution. 
description: The GPRI complex images in slant-range geometry were combined to
 generate interferograms and geocoded into an Earth-fixed grid. The
 pointing angle of the GPRI was refined by adjusting image features in the
 geocoded datga with a reference Landsat image. Phase unwrapping was
 conducted from a stationary point on the glacier side margin. Here, we
 form interferograms using data acquired 16-minutes apart and we difference
 tthe interferograms every 6 hours. We extend the analysis over a period of
 2 weeks. From the differential interferograms, we locate the transition
 boundary where the ice surface is first displaced vertically, or first
 fringe of displacement, which is our proxy for the grounding line. By
 repeating the exercise over time, we measure how the grounding line
 migrates back and forth with tide.
description: # Data for: Seawater intrusion at the grounding line of Jakobshavn Isbrae,
 Greenland, from Terrestrial Radar Interferometry Jakobshavn Isbrae is a
 major outlet glacier in West Greenland that lost its protective ice shelf
 in 2002 and has been speeding up and retreating since. We image its
 grounding line for the first time with a ground portable radar
 interferometer deployed in 2016 and detect its migration at tidal
 frequencies. The southern half of the glacier develops a floating section
 (3 km x 3 km) that migrates in phase with the tide up to a distance of 1.5
 km, which far more than expected from flotation. We attribute the
 migration to kilometer-scale seawater intrusions, 10-20 cm in height,
 occurring at high tide. The intrusions reveal than the glacier bed must be
 100-600 m deeper than expected on the south side, which illustrates that
 our knowledge of bed topography remains limited in this sector. We expect
 seawater intrusions to cause rapid melt of basal ice and play a major role
 in the glacier evolution. GPRI complex images in slant-range geometry are
 combined in pairs to generate interferograms and geocoded into an
 Earth-fixed grid. The pointing angle of the GPRI was refined by adjusting
 image features in the geocoded datga with a reference Landsat image. Here,
 we form interferograms using data acquired 16-minutes apart and we
 difference the interferograms every 6 hours. We extend the analysis over a
 period of 2 weeks. Phase unwrapping is conducted from a stationary point
 on the glacier side margin using the "grasses" algorithm. From
 the differential, unwrapped interferograms, we locate the transition
 boundary where the ice surface is first displaced vertically, or first
 fringe of displacement, which is our proxy for the grounding line. By
 repeating the exercise over time, we measure how the grounding line
 migrates back and forth with changes in oceanic tide. ## Description of
 the data and file structure All the data are geocoded in Polar
 Stereographic format, with coordinate reference system (CRS) EPSG:3413 1.
 DEM folder: "ArcticDEM\_20160616\_msl.tif" is a DEM tile of
 ArcticDEM from 2016-06-16 in geotiff format, referenced to mean sea level.
 2. Differential\_interferograms folder: There are 1,488 differential
 interferograms in geotiff format. The naming convention is
 "YYYYMMDD\_HHMMSS\_HHMMSS-YYYYMMDD\_HHMMSS\_HHMMSS.unw.geo.tif".
 The first "YYYYMMDD\_HHMMSS\_HHMMSS" before hyphen "-"
 stands for the date and time of the primary interferogram. The second
 "YYYYMMDD\_HHMMSS\_HHMMSS" after hyphen "-" stands for
 the date and time of the secondary interferogram. unw means unwrapped, geo
 means geocoded, and tif means geotiff format. 3. Grounding\_lines folder:
 There are 12 shape files of grounding lines for Jakobshavn. The naming
 convention is "GL\_geo\_YYYYMMDD\_corr" by the date of YYYYMMDD.
 All grounding lines were hand-drawn by the author using differential
 interferograms in the "Differential\_interferograms" folder. 4.
 Grounding\_zone folder: There is 1 shape file of grounding zone in
 Jakobshavn. 5. Speed\_maps folder: There are 4 speed maps in geotiff
 format. All files use a unit of "km/yr". ref\_speed\_r.tif :
 speed map in slant-range direction (geocoded) ref\_speed\_z.tif : speed
 map in azimuth direction (geocoded) ref\_speed\_x.tif : speed map in east
 direction (geocoded) ref\_speed\_y.tif : speed map in north direction
 (geocoded)