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Session 5A - Glaciers: multisensor observations and simulations
8:30am - 9:30am
Session Chair: David Burgess Session Chair: Rene Forsberg
8:30am - 8:50am
Challenges and Approaches to Altimeter-Data Analysis of Rapidly Changing and Heavily Crevassed Glacier Systems: The Bering Bagley Glacier System, Alaska, During Surge in the View of CryoSat-2, ICESat-2 and Airborne Altimetry
Ute Christina Herzfeld1, Thomas Trantow1, Veit Helm2, Kelly Brunt3,4, William Cook4
1University of Colorado Boulder, United States of America; 2Alfred Wegener Institut, Bremerhaven, Germany; 3University of Maryland, United States of America; 4NASA Goddard Space Flight Center, Greenbelt, United States of America
As a complex and large mountain glacier system with surging and non-surging glaciers, the Bering-Bagley Glacier System (BBGS) is both glaciologically very interesting and a great test bed for satellite-observation ground truthing and algorithm development aimed at advancing the limits of altimetry. Bering Glacier and the eastern Bagley Ice Field surged in 2011-2013, after the launch of CryoSat-2, while the western Bagley Ice Field was not affected by the surge. The signatures of the surge that result from sudden mass transfer, changes in the hydrological system and acceleration and are observable with altimetry include elevation changes and crevassing. Challenges result from the specific combination of large elevation changes occurring over very short time frames, the spatially complex and changing dynamics of the surge in the BBGS and the size range of spatial signatures as well as coincident changes in surface reflectance. In this paper we investigate the possibilities for utilizing altimeter data to study the surge. Three data types are available: CryoSat-2 data (in several forms of processing), airborne laser altimeter data collected by our group in 4 airborne campaigns in 2011-2013, and observations collected with the ICESat-2 simulator instrument MABEL (Multiple Altimeter Beam Experimental Lidar) in July 2014. Following spatial statistical analysis, comparison to flight data and detailed error analysis, we find that CryoSat-2 data provide time series of elevation and elevation change at 6-month intervals and hence can be used to monitor the complex mass transfer during the surge and is the only data available for this purpose. The relevance of CryoSat-2-based ice-surface observations as a constraint in modeling the surge dynamics will be presented in a companion paper. However, information on crevasse fields is not registered in CryoSat-2 data. High-resolution airborne altimetry reveals spatial characteristics of crevasse fields that result from different types of deformation, as will be illustrated for different times and ice provinces. Using an algorithm specifically designed for the analysis of micro-pulse photon-counting lidar altimeter data as will be collected by ICESat-2, we demonstrate that similar information on crevassed surfaces can be derived from MABEL data and hence from future ICESat-2 data. The presentation will conclude with glaciological implications of the altimeter-data analysis.
8:50am - 9:10am
Numerical Experiments of Surface Crevassing during the Surge of the Bering-Bagley Glacier System in 2011-2013 and Sensitivity to CryoSat-2 Processing
Thomas Trantow, Ute Christina Herzfeld
University of Colorado Boulder, United States of America
Glacier acceleration is one of the main sources of uncertainty in global sea-level rise estimates according to the IPCC 5th Assessment. Glacier surging is one type of acceleration found in ice systems around the world yet is still incompletely understood. After a long period of quiescent flow, a surge-type glacier will suddenly and rapidly advance resulting in large-scale elevation change and significant surface deformation. Crevasses are the most conspicuous manifestations of the surge dynamics and provide a source of geophysical information that allows reconstruction of deformational processes. The recent surge of the Bering-Bagley Glacier System (BBGS), Alaska, in 2011-2013 provides an excellent test case to study the surge through airborne and satellite observations together with numerical modeling.
A full-Stokes finite element model of the BBGS has been created using the Elmer/Ice software for structural and dynamical investigations of the surge. A von Mises condition is applied to modeled surface stresses so as to predict where crevassing might occur during the surge. The model is constrained by surface and bottom topography with associated boundary conditions all of which affect experiments investigating the glacier surface structure. CryoSat-2 measurements provide a time series of elevation at six-month intervals which are used as inputs for the numerical model. Satellite and airborne imagery of the BBGS during the surge provides us with observations for model-data comparisons with respect to the model's ability to predict surface crevassing. Sensitivity of results to different types of CryoSat-2 processing is investigated. As a result, the model-data comparison can be employed as a means for evaluation of CryoSat-2 processing.
9:10am - 9:30am
Calibration and Use of the Interferometric Mode of the CryoSat Radar Altimeter to Measure Height Change in the Periphery of the Greenland Ice Cap
Laurence Gray1, David Burgess2, Luke Copland1, Thorben Dunse3, Kirsty Langely4, Moholdt Geir5
1University of Ottawa, Canada; 2Geological Survey Canada; 3University of Oslo; 4Asiaq, Greenland; 5Norwegian Polar Institute
Geocoded heights derived from the interferometric mode (SARIn) of CryoSat are compared with surface heights from calibration-validation sites on Devon Ice Cap and West Greenland. Comparisons are included for both the heights derived from the first return (the ‘point-of-closest-approach’ or POCA) as well as heights derived from delayed waveform returns (‘swath’ processing). Using these results, we show that the pre-launch interferometric baseline coupled with an additional roll correction (~0.0075°), or equivalent phase correction (~0.0435 radians), provides an improved calibration of the interferometric SARIn mode.
The periphery of Greenland represents an important area for the use of the CryoSat SARIn mode: The ice loss in this area is significant and hard to quantify at high spatial resolution with existing satellite sensors. We show that the CryoSat SARIn mode can provide useful information on the summer melt through waveform signature and height change estimates, including height change of supraglacial lakes. An adaption of swath processing is used in this work and height accuracies of ~0.5 m are possible for the supraglacial lakes when they are positioned very close to the sub-satellite track.