DOI: 10.5061/dryad.4j0zpc8n6
title: Data For: Grounding zone of Helheim Glacier, Greenland, from terrestrial radar interferometry

description: Helheim Glacier, in East Greenland, is a major outlet glacier that
 discharges about 33 Gigatons of ice per year into the ocean. Helheim has
 been undergoing a retreat since 2003. Using a portable terrestrial radar
 interferometer that scans the glacier every 2 minutes, we characterize its
 regime of tidal flexure at the ice front margin in relation to oceanic
 tide, bed slope, and iceberg calving events. The data reveals the presence
 of a central bed ridge that acts as a fulcrum, with one side of the
 glacier moving in phase with the tide, while the other is moving out of
 phase with a reduced amplitude. We detect seawater intrusions of one full
 thickness beneath the glacier front at high tide. Following tabular
 calving events, most ice flexing ceases, indicating that the ice front
 becomes grounded and that calving events involve ice blocks that are
 already afloat. 
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
 6 days. 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 ## Description of the
 data and file structure All the data are geocoded in Polar Stereographic
 format, with coordinate reference system (CRS) EPSG:3413 The dataset is
 compatible with GIS software (e.g. QGIS). The grounding line and grounding
 zone data are provided in shapefile format (.shp). Note that the companion
 files (.shx, .dbf, etc.) are essential for the proper functioning of the
 shapefile but are not intended to be viewed individually. 1. DEM folder ;
 "ArcticDEM_20140731_msl.tif" is a DEM tile of ArcticDEM from
 2014-07-31 in geotiff format, referenced to mean sea level. 2.
 Differential_interferograms folder ; Differential_interferograms folder:
 There are 547 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 6 shape files of grounding lines for Helheim. The naming
 convention is "GL_YYYYMMDD" 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
 Helheim. 5. Speed_maps folder ; There are 2 speed maps in geotiff format.
 All files use a unit of "km/yr". ref_speed_x.tif : speed map in
 east direction (geocoded) ref_speed_y.tif : speed map in north direction
 (geocoded)