README: 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)