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Scambos, T, Fricker HA, Liu CC, Bohlander J, Fastook J, Sargent A, Massom R, Wu AM.  2009.  Ice shelf disintegration by plate bending and hydro-fracture: Satellite observations and model results of the 2008 Wilkins ice shelf break-ups. Earth and Planetary Science Letters. 280:51-60.   10.1016/j.epsl.2008.12.027   AbstractWebsite

Satellite remote sensing observations of three break-up events in 2008 for the Wilkins Ice Shelf (28 February to 6 March, 27 May to 31 May, and 28 June to mid-July) provide unprecedented detail of ice shelf calving during rapid break-up. The observations reveal that the Wilkins break-ups occur through a distinctive type of shelf calving, which we term 'disintegration', as well as more typical rifting and calving. Here we focus on the disintegration process, which is characterized by repeated rapid fracturing that creates narrow ice-edge-parallel blocks, with subsequent block toppling and fragmentation forming an expanding iceberg and ice rubble mass. We use these data to develop and test a model of floating ice plate disintegration in which ice plate bending stresses at the ice front arising from buoyancy forces can lead to runaway calving when free (mobile) water is available. High-resolution satellite images and laser altimetry of the first break-up event provide details of fracture spacings, ice thicknesses, and plate bending profiles that agree well with our model predictions. We suggest that surface or near-surface meltwater is the main pre-condition for disintegration, and that hydro-fracture is the main mechanism. Brine layers from near-waterline brine infiltration can support a similar process, but this is less effective unless regional ice stress patterns contribute to the net stress available at the crack tip for fracturing. A combination of brine-enhanced fracturing and changing internal net extensional stresses was the likely mechanism behind the latter two Wilkins events. (C) 2008 Elsevier B.V. All rights reserved.

Shepherd, A, Fricker HA, Farrell SL.  2018.  Trends and connections across the Antarctic cryosphere. Nature. 558:223-232.   10.1038/s41586-018-0171-6   AbstractWebsite

Satellite observations have transformed our understanding of the Antarctic cryosphere. The continent holds the vast majority of Earth's fresh water, and blankets swathes of the Southern Hemisphere in ice. Reductions in the thickness and extent of floating ice shelves have disturbed inland ice, triggering retreat, acceleration and draw-down of marine-terminating glaciers. The waxing and waning of Antarctic sea ice is one of Earth's greatest seasonal habitat changes, and although the maximum extent of the sea ice has increased modestly since the 1970s, inter-annual variability is high, and there is evidence of longer-term decline in its extent.

Shuman, CA, Zwally HJ, Schutz BE, Brenner AC, DiMarzio JP, Suchdeo VP, Fricker HA.  2006.  ICESat Antarctic elevation data: Preliminary precision and accuracy assessment. Geophysical Research Letters. 33   10.1029/2005gl025227   AbstractWebsite

Since 'first light' on February 20th, 2003, NASA's Ice, Cloud, and land Elevation Satellite (ICESat) has derived surface elevations from similar to 86 degrees N to 86 degrees S latitude. These unique altimetry data have been acquired in a series of observation periods in repeated track patterns using all three Geoscience Laser Altimeter System (GLAS) lasers. Here, we focus on Antarctic ice sheet elevation data that were obtained in 2003-2004. We present preliminary precision and accuracy assessments of selected elevation data, and discuss factors impacting elevation change detection. We show that for low slope and clear sky conditions, the precision of GLA12 Laser 2a, Release 21 data is similar to 2.1 cm and the relative accuracy of ICESat elevations is +/- 14 cm based on crossover differences.

Siegfried, MR, Fricker HA.  2018.  Thirteen years of subglacial lake activity in Antarctica from multi-mission satellite altimetry. Annals of Glaciology. 59:42-55.   10.1017/aog.2017.36   AbstractWebsite

The ability to detect the surface expression of moving water beneath the Antarctic ice sheet by satellite has revealed a dynamic basal environment, with implications for regional ice dynamics, grounding-line stability, and fluxes of freshwater and nutrients to the Southern Ocean. Knowledge of subglacial activity on timescales important for near-term prediction of ice-sheet fluctuations (decadal to century) is limited by the short observational record of NASA's Ice, Cloud, and land Elevation Satellite (ICESat) laser altimetry mission used to generate the last continent-wide survey (2003-08). Here, we use synthetic aperture radar-interferometric-mode data from ESA's CryoSat-2 radar altimetry mission (2010-present), which samples 45 of the ICESat-derived subglacial lakes, to extend their time series to the end of 2016. The extended time series show that there have been surface-height changes at 20 of the 45 lakes since 2008, indicating that some of these features are persistent and potentially cyclic, while other features show negligible changes, suggesting these may be transient or nonhydrological features. Continued monitoring of active lakes for both height and velocity changes, as well as developing methods for identifying additional lakes, is critical to quantifying the full distribution of active subglacial lakes in Antarctica.

Siegfried, MR, Fricker HA, Carter SP, Tulaczyk S.  2016.  Episodic ice velocity fluctuations triggered by a subglacial flood in West Antarctica. Geophysical Research Letters. 43:2640-2648.   10.1002/2016gl067758   AbstractWebsite

Height change anomalies in satellite altimeter data have been interpreted as the surface expressions of basal water moving into and out of subglacial lakes. These signals have been mapped throughout Antarctica on timescales of months to years, but only broad connections have been made between active lakes and ice dynamics. We present the first high-frequency observations of ice velocity evolution due to a cascading subglacial lake drainage event, collected over 5years (2010-2015) using Global Positioning System data on Whillans and Mercer ice streams, West Antarctica. We observed three episodic ice velocity changes over 2years, where flow speed increased by up to 4%, as well as an 11month disruption of the tidally modulated stick-slip cycle that dominates regional ice motion. Our observations reveal that basal conditions of an Antarctic ice stream can rapidly evolve and drive a dynamic ice response on subannual timescales, which can bias observations used to infer long-term ice sheet changes.

Siegfried, MR, Fricker HA, Roberts M, Scambos TA, Tulaczyk S.  2014.  A decade of West Antarctic subglacial lake interactions from combined ICESat and CryoSat-2 altimetry. Geophysical Research Letters. 41:891-898.   10.1002/2013GL058616   AbstractWebsite

We use CryoSat-2 interferometric satellite radar altimetry over the Mercer and Whillans ice streams, West Antarctica, to derive surface elevation changes due to subglacial lake activity at monthly resolution for the period 2010 to 2013. We validate CryoSat-2 elevation measurements, trends, and spatial patterns of change using satellite image differencing and in situ vertical movement from Global Positioning System (GPS) data. Two subglacial lake discharge events occur in the same subglacial-hydrological catchment within a 9 month period. Using GPS measurements that are spanning the gap between the Ice, Cloud, and land Elevation Satellite and Cryosat-2 missions, we cross-calibrate the two missions to establish the efficacy of CryoSat-2 altimetry to measure dynamic changes on the ice sheets.

Smith, BE, Fricker HA, Joughin IR, Tulaczyk S.  2009.  An inventory of active subglacial lakes in Antarctica detected by ICESat (2003-2008). Journal of Glaciology. 55:573-595. AbstractWebsite

Through the detection of surface deformation in response to water movement, recent satellite studies have demonstrated the existence of subglacial lakes in Antarctica that fill and drain on timescales of months to years. These studies, however, were confined to specific regions of the ice sheet. Here we present the first comprehensive study of these 'active' lakes for the Antarctic ice sheet north of 86 degrees S, based on 4.5 years (2003-08) of NASA's Ice, Cloud and land Elevation Satellite (ICESat) laser altimeter data. Our analysis has detected 124 lakes that were active during this period, and we estimate volume changes for each lake. The ICESat-detected lakes are prevalent in coastal Antarctica, and are present under most of the largest ice-stream catchments. Lakes sometimes appear to transfer water from one to another, but also often exchange water with distributed sources undetectable by ICESat, suggesting that the lakes may provide water to or withdraw water from the hydrologic systems that lubricate glacier flow. Thus, these reservoirs may contribute pulses of water to produce rapid temporal changes in glacier speeds, but also may withdraw water at other times to slow flow.

Sun, XL, Abshire JB, Borsa AA, Fricker HA, Yi DH, DiMarzio JP, Paolo FS, Brunt KM, Harding DJ, Neumann GA.  2017.  ICESAT/GLAS Altimetry Measurements: Received Signal Dynamic Range and Saturation Correction. Ieee Transactions on Geoscience and Remote Sensing. 55:5440-5454.   10.1109/tgrs.2017.2702126   AbstractWebsite

NASA's Ice, Cloud, and land Elevation Satellite (ICESat), which operated between 2003 and 2009, made the first satellite-based global lidar measurement of earth's ice sheet elevations, sea-ice thickness, and vegetation canopy structure. The primary instrument on ICESat was the Geoscience Laser Altimeter System (GLAS), which measured the distance from the spacecraft to the earth's surface via the roundtrip travel time of individual laser pulses. GLAS utilized pulsed lasers and a direct detection receiver consisting of a silicon avalanche photodiode and a waveform digitizer. Early in the mission, the peak power of the received signal from snow and ice surfaces was found to span a wider dynamic range than anticipated, often exceeding the linear dynamic range of the GLAS 1064-nm detector assembly. The resulting saturation of the receiver distorted the recorded signal and resulted in range biases as large as similar to 50 cm for ice-and snow-covered surfaces. We developed a correction for this "saturation range bias" based on laboratory tests using a spare flight detector, and refined the correction by comparing GLAS elevation estimates with those derived from Global Positioning System surveys over the calibration site at the salar de Uyuni, Bolivia. Applying the saturation correction largely eliminated the range bias due to receiver saturation for affected ICESat measurements over Uyuni and significantly reduced the discrepancies at orbit crossovers located on flat regions of the Antarctic ice sheet.