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Session Chair: Anna Hogg Session Chair: Laurence Gray
2:00pm - 2:20pm
Measuring Surface Processes for the Interpretation of CryoSat-2 and IceBridge Altimetry in the Accumulation Zone of Greenland.
Santiago de la Peña1, Ian Howat1, Alberto Behar2, James Crowell2
1The Ohio State University, United States of America; 2Arizona State University, United States of America
Inter-annual fluctuations in the firn volume of the Greenland Ice Sheet have been observed in recent years by repeated altimetry from CryoSat-2. Although an order of magnitude smaller than ice thinning rates measured in some areas at the margins of the ice sheet, these short-term departures in surface elevation trends occur over most of the accumulation zone of Greenland. Changes in the thickness of the firn column are influenced by variability in surface mass balance, firn compaction, and abrupt seasonal densification near the surface caused by refreezing at depth of variable amounts of surface meltwater produced during the summer. These processes and dynamic thinning cannot be differentiated from each other by altimetry alone.
Here we use direct, continuous measurements of firn density and surface mass balance along with annual estimates of firn ice content used to assess observed elevation change in the percolation zone of western Greenland in relation to firn processes. Since 2012, autonomous in-situ firn compaction sensors have monitored several sites in the catchment area of Jakobshavn Isbrae, and since 2015 surface mass balance and surface displacement has been measured continuously using a combination of sensors. In addition to identify the different components in the altimetry signal, the temporal resolution of the data acquired provide a means to monitor short-term changes in the near-surface firn, and identifying individual events causing surface elevation displacement independently from ice dynamics in western Greenland.
2:20pm - 2:40pm
Swath Processing of CryoSat-2 for Improved Coverage of the Grounding Zone
Geoffrey Joseph Dawson, Jonathan Bamber
University of Bristol, United Kingdom
The synthetic aperture radar interferometric (SARin) mode of CryoSat-2, which uses a cross-track interferometer to determine the origin of off-nadir reflections, has performed better over steeper margins of the ice sheets compared to previous conventional radar altimetry. In the grounding zone of ice sheets where there is generally a break in slope, measurements based on the point of closest approach (POCA) are limited as the POCA points tends to be clustered upslope of the grounding line, reducing coverage in this critical zone. Swath processing maps the time-delayed footprint beyond the POCA allowing us to obtain downslope elevation estimates close to nadir. In this study, we focus on the Antarctic Peninsula and the Ross Ice Shelf to investigate the accuracy and potential for improved coverage using swath processing compared to the POCA data around the grounding zone. We compare the results with ICESat to assess the accuracy, and with the POCA data to assess the improvement in coverage.
2:40pm - 3:00pm
Fourteen Years of Subglacial Lake Activity in Antarctica from Multi-Mission Altimetry
Matthew R. Siegfried, Helen A. Fricker
Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, United States of America
The ability to infer the movement of water beneath the Antarctic ice sheet using remote sensing techniques has fundamentally altered our perception of the subglacial environment. What used to be considered system in steady-state is now known to be a dynamic environment, with inferred variability on timescales ranging from sub-annual to multi-century or longer. The changing basal environment has direct implications on regional glaciological parameters (e.g., ice flow and grounding line dynamics), as well as on boundary conditions for other disciplines (e.g., freshwater flux to the ocean and global carbon cycling). Our understanding of subglacial hydrological variability on timescales likely important for the near-term prediction of ice sheet fluctuations driven by global climate change (multi-year to decadal) is limited by the short temporal window of our current record of active subglacial hydrology (2003-2008). Here we extend the record of subglacial lake activity for all lakes covered by CryoSat-2 SARIn mode-data, to generate a fourteen year time series. We reassess the ice surface height anomalies and explore what we can infer about Antarctic subglacial hydrologic processes and variability from this extended time-series.
3:00pm - 3:20pm
Comparison of Regional Scheme Techniques for Estimating the Canadian Arctic Archipelago Land Ice Mass Changes from ICESat
Iliana Tsalis, Michael Sideris
University of Calgary, Canada
Mountain glaciers and ice caps of the Canadian Arctic Archipelago (CAA) - sensitive indicators of climate change - have recently been characterized as significant contributors to the sea level rise. Therefore, our goal is to estimate the elevation changes of the land ice covered area of the North and South Canadian Arctic Archipelago (NCAA and SCAA), using high spatial resolution satellite laser altimetry data from the ICESat satellite. In order to estimate the mass variations, we seek a spatial correlation scheme for predicting the elevation change values in the unmeasured off-track glaciated areas, convert them to volume changes, and then into mass changes.
Our focus lies on applying and comparing the deterministic method of inverse distance weighting and the geostatistical method of ordinary kriging for investigating the variability of the regional volume changes. The data used are the GLAS/ICESat L2 Global Land Surface Altimetry data (product GLA14) of release 34, referring to the period from 2003.02.20 until 2009.10.10. Repeat-track method is followed for estimating the elevation rates, after gross error detection. The glaciated areas of the CAA are defined as refined polygons of glaciers and ice caps obtained from the GLIMS Glacier Viewer.
We evaluate our results by performing cross-validation comparison on the implemented techniques on a dense and a sparse measurement sampling of the NCAA and the SCAA respectively. Our results show that although both methods are giving similar ice mass change estimates, approximately ~40 Gt/year for the NCAA and ~25Gt/year for the SCAA, in agreement with previous studies, ordinary kriging is the preferred method for estimating the mass balance of ice caps and glaciers, since it can work with noisy data and provide uncertainties in the predicted results.