The recent acceleration of mass loss of the Greenland ice sheet as determined from large-scale, satellite-derived geodetic and gravimetric observations is well documented. However, longer-term in situ elevation change measurements are scarce. Here, we present a 23-year time series (1991 to 2014) of bi-annual in situ geodetic observations at two sites in West Greenland. Repeated GPS measurements at Swiss Camp (1170 m a.s.l.) and 15 km downglacier at site ST2 (1100 m a.s.l.) were used to determine elevation changes, ice flow velocities, and strain rates. Meteorological observations were used to interpret the results. These were compared to satellite-derived evaluations. Surface elevation at Swiss Camp dropped by 14.4 m between 1991 and 2014 (0.62 m/year on average) with accelerated elevation drops in recent years. The same tendency was also apparent at ST2. Here the surface elevation dropped by 12.2 m between 2004 and 2014 (1.2 m/year on average). The elevation changes were not constant over the survey areas, there were, however, pronounced systematic local differences. The causes of these (e.g. albedo, humidity, etc.) have not yet been established. The velocity of flow does not behave uniformly. At SWC the ice flows faster (0.32 m/d) than at ST2 (0.19 m/d), and although at SWC acceleration arises, ST2 demonstrates a reduction in speed, probably because of the ascending border mountains. Strain rates vary strongly and show that the local underground of the moved measuring field affects the results strongly. The strain rates in longitudinal direction between both sites are negative, indicating compression of the ice, which results in dynamic thickening. The influence of individual meteorological parameters on the elevation changes was examined using correlation analysis. However, significant correlations were rarely found; in both survey areas these were most clear for the summer albedo. Air temperature and net radiation proved to be less relevant. Comparisons of the geodetic mass balance with the specific SMB from stake measurements show different results. At SWC the cumulative geodetic mass balance from 1991 to 2014 results in -13.0 m w.e, and SMB from stakes -10.1 m w.e (2014). The dynamic portion results from the difference in relation to the geodetically-determined mass balance. The results at SWC and at ST2 are contradictory. The dynamic part is derived from the strain rates using the continuity (incompressibility) condition. However, consistent results appear only at the beginning of our measurements. Remote sensing results were compared to ground truths with GPS. ICESat altimetry from 2005 demonstrates surface height deviations of -0.13 ± 0.06 m on average. Elevation changes from CryoSat-2 between 2011 and 2014 differ by 0.24 m/a (SWC) and 0.48 m/a (ST2). Heights from a TanDEM-X elevation model in August 2014 show an offset from the GPS elevation model of 3.14 ±0.025 m, the standard deviation of one point being 1.07 m. The ice flow velocity from Sentinel-1 IW, TerraSAR-X, and ALOS PALSAR was compared to ground measurements with GPS. Data from the years 2010-2017 mostly agree well (with the exception of PALSAR) within the measuring accuracy of the satellite procedures.
Supplement to: Stober, Manfred; Hepperle, Jörg (2019): Glacial-geodetic long-term study on mass balance and ice dynamics near the equilibrium line of the Greenland ice sheet. Polarforschung, 88(2)
