Calving of glacial ice into the ocean from the Greenland Ice Sheet is an important component of global sea level rise. The calving process itself is relatively poorly observed, understood, and modeled; as such, it represents a bottleneck in improving future global sea level estimates in climate models. We organized a pilot project to observe the calving process at Helheim Glacier in East Greenland in an effort to better understand it. During an intensive one-week survey, we deployed a suite of instrumentation including a terrestrial radar interferometer, GPS receivers, seismometers, tsunameters, and an automated weather station. This effort captured a calving process and measured various glaciological, oceanographic, and atmospheric parameters before, during, and after the event. One outcome of our observations is evidence that the calving process actually consists of a number of discrete events, spread out over time, in this instance over at least two days. This time span has implications for models of the process. Realistic projections of future global sea level will depend on accurate parametrization of calving, which will require more sustained observations.
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Volume 29, No. 4
Pages 46 - 61
Holland, D.M., D. Voytenko, K. Christianson, T.H. Dixon, M.J. Mei, B.R. Parizek, I. Vaňková, R.T. Walker, J.I. Walter, K. Nicholls, and D. Holland. 2016. An intensive observation of calving at Helheim Glacier, East Greenland. Oceanography 29(4):46–61, https://doi.org/10.5670/oceanog.2016.98.
SUPPLEMENTAL FIGURE S1. Feature-tracking results showing glacier ice surface velocity vectors over a two-day period (August 13 00:10–August 15 00:10, 2014) overlain on a terrestrial radar interferometry (TRI)-derived digital elevation map (DEM) from August 11 01:00. Speeds are of order 20 m d–1 and the velocity vectors were determined by using two TRI intensity images and the OpenPIV software (Taylor et al., 2010). The tracking was performed on imagery with 15 m spacing, with a 16-pixel window size and an overlap of 8 pixels. Each velocity component was filtered with a 2-sigma filter and later a 3 × 3 median filter and (for plotting purposes) downsampled by a factor of 10. These results show two-dimensional velocity vectors obtained over a multihour-scale time period. However, TRI results typically rely on interferometry and provide single-component velocity measurements (toward the radar) but over minute-scale time steps. White indicates areas of no data. (324 KB jpg)
SUPPLEMENTAL FIGURE S2. Time series of displacement and velocity of a pixel observed by terrestrial radar interferometry (TRI) near the Helheim Glacier terminus in August 2014. (top) Line-of-sight displacement of a pixel as recorded by the TRI. Black solid vertical lines show the timing of the primary (left side) and secondary (right side) calving events. (middle) Detrended version of the top panel showing displacement “anomaly” over time and also some semidiurnal tidal variability. Despite the presence of the primary calving event, the displacement rate remained relatively constant. (bottom). The derived velocity time series illustrates calving-driven variations and tidal variations, along with instrument-related phase unwrapping errors (spikes). The red line shows the velocity as derived from a nearby GPS receiver (HEL0 in Figure 7 in the main article). Note the good agreement between the TRI and GPS velocities and that the TRI velocity is lower than the GPS velocity (within the noise level). This is because TRI measures a projection of the true velocity vector onto the TRI look vector (line-of-sight). (251 KB png)
SUPPLEMENTAL FIGURE S3. Animated sequence of glacier front position at Helheim Glacier from terrestrial radar interferometry (TRI) during deployment in August 2014, spanning the primary (August 12) and secondary (August 13) calving events. The animation is based on three TRI intensity images (August 12, 05:46 UTC; August 12 07:46 UTC; August 13, 11:40 UTC) overlain on top of a Landsat 8 image. First frame shows the pre-calving terminus during the primary event. Second frame shows the terminus after the primary calving event. Third frame shows the terminus after a smaller secondary calving event that affected only the southern trunk of the glacier, and was likely a continuation of the primary event. (6.3 MB gif)
SUPPLEMENTAL FIGURE S4. Animated sequence of glacier front position at Helheim Glacier from terrestrial radar interferometry (TRI) during deployment in August 2014, spanning the primary (August 12) and secondary (August 13) calving events. The glacier is flowing from top right toward lower left. Background color indicates coherence of radar return with yellow indicating high-coherence thus slow moving glacier; pink color indicates low-coherence thus fast moving glacier. A time index is shown in the lower right. (14.4 MB gif)
SUPPLEMENTAL FIGURE S5. Profiles of strain rate and surface elevation before (red), during (black), and after (blue) the primary calving event in the vicinity of the profile shown in Figure 6 in the main article. Strain rate (top curves) is given as a function of distance near the glacier front. Glacier front elevations (bottom curves) are obtained from hourly averages. The dashed line (top curves) shows transition from extension (positive strain rate, above) to compression (negative strain rate, below). Strain rates increase as the slope of the front becomes more pronounced. In all times, the mélange is under compression. (245 KB png)
SUPPLEMENTAL FIGURE S6. Animation of the elevation changes around the ~1 hour period associated with the primary calving event (~6:37 on August 12, 2014) on the northern trunk of Helheim Glacier. Each frame represents two minutes. Ice cliff elevations before calving are approximately 100 m. Note how the block of ice rotates during calving. (5.7 MB gif)
SUPPLEMENTAL FIGURE S7. A precursory seismic event detected at approximately 05:46 UTC on August 12, 2014, almost one hour prior to the primary calving event. The plots show the seismic energy as a function of time at (a) the on-land seismometer located on the south side of the fjord, (b) the on-land, north-side seismometer, and (c) the on-glacier, seismometer located near the calving front. Locations of the stations are seen in Figures 7 and 9 in the main article. (80 KB png)
SUPPLEMENTAL FIGURE S8. Animated one-year-long sequence of glacier front position at Helheim Glacier from an on-land automated weather station during the year preceding our pilot field campaign of August 2014. The winter calving advance and summer retreat as well as the aperiodic nature of calving is apparent. (7.9 MB gif)
SUPPLEMENTAL FIGURE S9. Simulation of elastic stresses in a two-dimensional (vertical slice) along-flow-line glacier model (glacier flow is from left to right). The stresses are generated by ocean tidal forcing at the glacier front, on the far right. (a) Tidal anomaly in front of Helheim Glacier from a seafloor Tsunameter pressure sensor. (b) Magnitude of deflection of the equilibrium glacier bed associated with a ~1.1 m low tide indicated by the red circle in (a), assuming a partially supported beam with Young’s Modulus of 4.8 GPa and Poisson ratio of 0.4. (c) Longitudinal deviatoric stress field within ~6.75 km of the glacier front due to the deflection in (b) (positive for tension and negative for compression). The glacier front is located at approximately position 41.5 km, at the far right. (459 KB jpg)
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