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Fricker, HA, Scambos T, Carter S, Davis C, Haran T, Joughin I.  2010.  Synthesizing multiple remote-sensing techniques for subglacial hydrologic mapping: application to a lake system beneath MacAyeal Ice Stream, West Antarctica. Journal of Glaciology. 56:187-199. AbstractWebsite

We present an analysis of the active hydrologic system of MacAyeal Ice Stream (MacIS), West Antarctica, from a synthesis of multiple remote-sensing techniques: satellite laser altimetry; satellite image differencing; and hydrologic potential mapping (using a satellite-derived DEM and a bedrock DEM from airborne radio-echo sounding). Combining these techniques augments the information provided by each one individually, and allows us to develop a protocol for studying subglacial hydrologic systems in a holistic manner. Our study reveals five large active subglacial lakes under MacIS, the largest of which undergoes volume changes of at least 1.0 km(3). We discuss the hydrologic properties of this system and present evidence for links between the lakes. At least three of the lakes are co-located with sticky spots, i.e. regions of high local basal shear stress. We also find evidence for surface elevation changes due to ice-dynamic effects (not just water movement) caused by changes in basal resistance. Lastly, we show that satellite radar altimetry is of limited use for monitoring lake activity on fast-flowing ice streams with surfaces that undulate on similar to 10 km length scales.

Fricker, HA, Hyland G, Coleman R, Young NW.  2000.  Digital elevation models for the Lambert Glacier-Amery Ice Shelf system, East Antarctica, from ERS-1 satellite radar altimetry. Journal of Glaciology. 46:553-560.   10.3189/172756500781832639   AbstractWebsite

The Lambert Glacier-Amery Ice Shelf system is a major component of the East Antarctic ice sheet. This paper presents two digital elevation models (DEMs) that have been generated for the Lambert-Amery system from validated European Remote-sensing Satellite (ERS-1) radar altimeter waveform data. The first DEM covers the Amery Ice Shelf only, and was produced using kriging on a 1 km grid. The second is a coarser (5 km) DEM of the entire Lambert-Amery system, generated via simple averaging procedures. The DEMs provide unprecedented surface elevation information for the Lambert-Amery system and allow new insight into the glaciology of the region.

Fricker, HA, Young NW, Coleman R, Bassis JN, Minster JB.  2005.  Multi-year monitoring of rift propagation on the Amery Ice Shelf, East Antarctica. Geophysical Research Letters. 32   10.1029/2004gl021036   AbstractWebsite

We use satellite imagery from four sensors (Multi-angle Imaging SpectroRadiometer (MISR), Enhanced Thematic Mapper (ETM), and RADARSAT and ERS Synthetic Aperture Radar (SAR) to monitor the lengths of two rifts on the Amery Ice Shelf, from 1996 to 2004. We find that the rifts have each been propagating at a steady annual rate for the past 5 years. Superimposed on this steady rate is a seasonal signal, where propagation rates are significantly higher in the summer period (i.e., September-April) than in the winter period (i.e., April-September). Possible causes of this summer-winter effect are changing properties of the ice melange, which fills the rifts, and seasonal changes in ocean circulation beneath the ice shelf.

Fricker, HA, Scambos T, Bindschadler R, Padman L.  2007.  An active subglacial water system in West Antarctica mapped from space. Science. 315:1544-1548.   10.1126/science.1136897   AbstractWebsite

Satellite laser altimeter elevation profiles from 2003 to 2006 collected over the lower parts of Whillans and Mercer ice streams, West Antarctica, reveal 14 regions of temporally varying elevation, which we interpret as the surface expression of subglacial water movement. Vertical motion and spatial extent of two of the largest regions are confirmed by satellite image differencing. A major, previously unknown subglacial lake near the grounding line of Whillans Ice Stream is observed to drain 2.0 cubic kilometers of water into the ocean over similar to 3 years, while elsewhere a similar volume of water is being stored subglacially. These observations reveal a widespread, dynamic subglacial water system that may exert an important control on ice flow and mass balance.

Fricker, HA, Siegfried MR, Carter SP, Scambos TA.  2016.  A decade of progress in observing and modelling Antarctic subglacial water systems. Philosophical Transactions of the Royal Society a-Mathematical Physical and Engineering Sciences. 374   10.1098/rsta.2014.0294   AbstractWebsite

In the decade since the discovery of active Antarctic subglacial water systems by detection of subtle surface displacements, much progress has been made in our understanding of these dynamic systems. Here, we present some of the key results of observations derived from ICESat laser altimetry, CryoSat-2 radar altimetry, Operation IceBridge airborne laser altimetry, satellite image differencing and ground-based continuous Global Positioning System (GPS) experiments deployed in hydrologically active regions. These observations provide us with an increased understanding of various lake systems in Antarctica: Whillans/Mercer Ice Streams, Crane Glacier, Recovery Ice Stream, Byrd Glacier and eastern Wilkes Land. In several cases, subglacial water systems are shown to control ice flux through the glacier system. For some lake systems, we have been able to construct more than a decade of continuous lake activity, revealing internal variability on time scales ranging from days to years. This variability indicates that continuous, accurate time series of altimetry data are critical to understanding these systems. On Whillans Ice Stream, our results from a 5-year continuous GPS record demonstrate that subglacial lake flood events significantly change the regional ice dynamics. We also show how models for subglacial water flow have evolved since the availability of observations of lake volume change, from regional-scale models of water routeing to process models of channels carved into the subglacial sediment instead of the overlying ice. We show that progress in understanding the processes governing lake drainage now allows us to create simulated lake volume time series that reproduce time series from satellite observations. This transformational decade in Antarctic subglacial water research has moved us significantly closer to understanding the processes of water transfer sufficiently for inclusion in continental-scale ice-sheet models.

Fricker, HA, Bassis JN, Minster B, MacAyeal DR.  2005.  ICESat's new perspective on ice shelf rifts: The vertical dimension. Geophysical Research Letters. 32   10.1029/2005gl025070   AbstractWebsite

The small footprint (similar to 70 m) and similar to 172 m along-track spacing of the Geoscience Laser Altimeter System (GLAS) on the Ice, Cloud and land Elevation Satellite (ICESat) provides unprecedented horizontal resolution for a satellite altimeter. This enables ICESat to map many previously unresolved features on ice shelves, such as crevasses, rifts, grounding zones and ice fronts. We present examples of ICESat-derived elevation data showing topography over rifts on the Amery and Ross ice shelves, widths of rifts and as estimates of the thickness of melange (a collection of ice and snow trapped inside the rifts). We show that melange thickness remains constant over the ICESat data period and tends to be thicker in older rifts. We validate the ICESat-derived melange depth estimate with an in situ measurement on the Ross Ice Shelf.

Fricker, HA, Padman L.  2012.  Thirty years of elevation change on Antarctic Peninsula ice shelves from multimission satellite radar altimetry. Journal of Geophysical Research-Oceans. 117   10.1029/2011jc007126   AbstractWebsite

We use data acquired between 1978 and 2008 by four satellite radar altimeter missions (Seasat, ERS-1, ERS-2 and Envisat) to determine multidecadal elevation change rates (dh(i)/dt) for six major Antarctic Peninsula (AP) ice shelves. In areas covered by the Seasat orbit (to 72.16 degrees S), regional-averaged 30-year trends were negative (surface lowering), with rates between -0.03 and -0.16 m a(-1). Surface lowering preceded the start of near-continuous radar altimeter operations that began with ERS-1 in 1992. The average rate of lowering for the first 14 years of the period was typically smaller than the 30-year average; the exception was the southern Wilkins Ice Shelf, which experienced negligible lowering between 2000 and 2008, when a series of large calving events began. Analyses of the continuous ERS/Envisat time series (to 81.5 degrees) for 1992-2008 reveal a period of strong negative dhi/dt on most ice shelves between 1992 and 1995. Based on prior studies of regional atmospheric and oceanic conditions, we hypothesize that the observed elevation changes on Larsen C Ice Shelf are driven primarily by firn compaction while the western AP ice shelves are responding to changes in both surface mass balance and basal melt rates. Our time series also show that large changes in dh(i)/dt can occur on interannual time scales, reinforcing the importance of long time series altimetry to separate long-term trends associated with climate change from interannual to interdecadal natural variability.

Fricker, HA, Popov S, Allison I, Young N.  2001.  Distribution of marine ice beneath the Amery Ice Shelf. Geophysical Research Letters. 28:2241-2244.   10.1029/2000gl012461   AbstractWebsite

We present a map of the marine ice accreted to the base of the Amery Ice Shelf (AIS), East Antarctica. This map is obtained by converting a Digital Elevation Model (DEM) of the AIS generated from satellite radar altimeter data to an ice thickness map, assuming hydrostatic equilibrium, and subtracting from that a second ice thickness map, derived from airborne radio-echo sounding (RES) measurements. The RES signal does not penetrate the marine ice, so the measurement is only to the meteoric-marine ice boundary, and therefore the difference between the two maps is the marine ice thickness. The marine ice is up to 190 m thick and accounts for about 9% of the shelf volume. It is concentrated in the northwest of the shelf, a result of the clockwise ocean circulation in the cavity below.

Fricker, HA, Allison I, Craven M, Hyland G, Ruddell A, Young N, Coleman R, King M, Krebs K, Popov S.  2002.  Redefinition of the Amery Ice Shelf, East Antarctica, grounding zone. Journal of Geophysical Research-Solid Earth. 107   10.1029/2001jb000383   AbstractWebsite

[1] New evidence is presented which shows that the Amery Ice Shelf, East Antarctica, extends similar to240 km upstream of the previously reported position. We combine a digital elevation model of the Amery Ice Shelf created from ERS-1 satellite radar altimetry with measured ice thicknesses and a simple density model in a hydrostatic (buoyancy) calculation to map the extent of the floating ice. This reveals that the ice is floating as far south as 73.2degreesS. The result is confirmed by static GPS measurements collected during three consecutive field campaigns on the Amery Ice Shelf where the vertical component of the GPS shows a clear tidal signal at 72.98degreesS. Other evidence for the grounding zone position comes from an analysis of satellite imagery, mass flux calculations, and ice radar data. The southward extension of the grounding line substantially alters the shape and dimensions of the ocean cavity beneath the ice shelf, which has implications for modeling studies of sub-ice shelf processes, such as basal melting and freezing, ocean circulation, and tides. The new grounding line position will also improve geophysical studies, where the computation of ocean tidal loading corrections is important for postglacial rebound estimates and correction of satellite altimetry measurements within the region.

Fricker, HA, Borsa A, Minster B, Carabajal C, Quinn K, Bills B.  2005.  Assessment of ICESat performance at the Salar de Uyuni, Bolivia. Geophysical Research Letters. 32   10.1029/2005gl023423   AbstractWebsite

The primary goal of the Ice, Cloud and land Elevation Satellite (ICESat) mission is ice sheet elevation change detection. Confirmation that ICESat is achieving its stated scientific requirement of detecting spatially-averaged changes as small as 1.5 cm/year requires continual assessment of ICESat-derived elevations throughout the mission. We use a GPS-derived digital elevation model (DEM) of the salar de Uyuni, Bolivia for this purpose. Using all twelve ICESat passes over the salar survey area acquired to date, we show that the accuracy of ICESat-derived elevations is impacted by environmental effects (e.g., forward scattering and surface reflectance) and instrument effects (e.g., pointing biases, detector saturation, and variations in transmitted laser energy). We estimate that under optimal conditions at the salar de Uyuni, ICESat-derived elevations have an absolute accuracy of <2 cm and precision of <3 cm.

Fricker, HA, Coleman R, Padman L, Scambos TA, Bohlander J, Brunt KM.  2009.  Mapping the grounding zone of the Amery Ice Shelf, East Antarctica using InSAR, MODIS and ICESat. Antarctic Science. 21:515-532.   10.1017/s095410200999023x   AbstractWebsite

We use a combination of satellite techniques (interferometric synthetic aperture radar (InSAR), visible-band imagery, and repeat-track laser altimetry) to develop a benchmark map for the Amery Ice Shelf (AIS) grounding zone (GZ), including its islands and ice rises. The break-in-slope, as an indirect estimate of grounding line location, was mapped for the entire AIS. We have also mapped similar to 55% of the landward edge and similar to 30% of the seaward edge of the ice shelf flexure boundary for the AIS perimeter. Vertical ice motion from Global Positioning System receivers confirms the location of the satellite-derived GZ in two regions. Our map redefines the extent of floating ice in the south-western AIS and identifies several previously unmapped grounded regions, improving our understanding of the stresses supporting the current dynamical state of the ice shelf. Finally, we identify three along-flow channels in the ice shelf basal topography, approximately 10 km apart, 1.5 km wide and 300-500 m deep, near the southern GZ. These channels, which form at the suture zones between ice streams, may represent zones of potential weakness in the ice shelf and may influence sub-ice-shelf ocean circulation.

Fricker, HA, Padman L.  2002.  Tides on Filchner-Ronne Ice Shelf from ERS radar altimetry. Geophysical Research Letters. 29   10.1029/2001gl014175   AbstractWebsite

[1] We use harmonic analysis of 8 years of ERS satellite radar altimeter (RA) data at orbital crossovers to retrieve complex amplitude (amplitude and phase) coefficients for several major tidal harmonics over the Filchner-Ronne Ice Shelf (FRIS), Antarctica. We describe a method for estimating the accuracy of this method, which ranges from similar to2 to 8 cm per harmonic. A comparison between M-2 complex amplitude from a recent ocean model and from our ERS RA analyses identifies two regions of the FRIS where the RA data are inconsistent with the model. In both regions the differences can be attributed to incorrect specification of the grounding line location in the model. Our study demonstrates the value of ERS RA data in Antarctic ice shelf tide modeling, and the potential for future altimeter satellites with high polar orbits to contribute to the definition of global tide height variations.

Fricker, HA, Padman L.  2006.  Ice shelf grounding zone structure from ICESat laser altimetry. Geophysical Research Letters. 33   10.1029/2006gl026907   AbstractWebsite

We present a technique for investigating the grounding zone (GZ) of Antarctic ice shelves using laser altimetry from the Ice, Cloud and land Elevation Satellite (ICESat). Most surface height variability in the GZ is easily resolved by the ICESat laser's similar to 65 m footprint and similar to 172 m along-track spacing. Comparisons of repeated tracks sampled at different phases of the ocean tide identify the landward and seaward limits of tide-forced ice flexure, providing GZ location and width information for each track. Using ICESat data in the Institute Ice Stream region of southern Ronne Ice Shelf, we demonstrate that the location of the GZ based on feature identification in satellite imagery or digital elevation models may be in error by several km. Our results show that ICESat will contribute significantly to improving knowledge of GZ structure and to studies requiring accurate GZ locations, e. g., ice mass balance calculations and ice-sheet/ocean modeling.

Fricker, HA, Powell R, Priscu J, Tulaczyk S, Anandakrishnan S, Christner B, Fisher AT, Holland D, Horgan H, Jacobel R, Mikucki J, Mitchell A, Scherer R, Severinghaus J.  2011.  Siple Coast subglacial aquatic environments; the Whillans ice stream subglacial access research drilling project. Geophysical Monograph. 192:199-219.   10.1029/2010gm000932   AbstractWebsite

The Whillians Ice Stream Subglacial Access Research Drilling (WISSARD) project is a 6-year (2009-2015) integrative study of ice sheet stability and subglacial geobiology in West Antarctica, funded by the Antarctic Integrated System Science Program of National Science Foundation's Office of Polar Programs, Antarctic Division. The overarching scientific objective of WISSARD is to assess the role of water beneath a West Antarctic Ice Stream in interlinked glaciological, geological, microbiological, geochemical, hydrological, and oceanographic systems. The WISSARD's important science questions relate to (1) the role that subglacial and ice shelf cavity waters and wet sediments play in ice stream dynamics and mass balance, with an eye on the possible future of the West Antarctic Ice Sheet and (2) the microbial metabolic and phylogenetic diversity in these subglacial environments. The study area is the downstream part of the Whillans Ice Stream on the Siple Coast, specifically Subglacial Lake Whillans and the part of the grounding zone across which it drains. In this chapter, we provide background on the motivation for the WISSARD project, detail the key scientific goals, and describe the new measurement tools and strategies under development that will provide the framework for conducting an unprecedented range of scientific observations.

Fricker, HA, Scambos T.  2009.  Connected subglacial lake activity on lower Mercer and Whillans Ice Streams, West Antarctica, 2003-2008. Journal of Glaciology. 55:303-315. AbstractWebsite

We examine patterns of localized surface elevation change in lower Mercer and Whillans Ice Streams, West Antarctica, which we interpret as subglacial water movement through a system of lakes and channels. We detect and measure the lake activity using repeat-track laser altimetry from ICESat and image differencing from MODIS image pairs. A hydrostatic-potential map for the region shows that the lakes are distributed across three distinct hydrologic regimes. Our analysis shows that, within these regimes, some of the subglacial lakes appear to be linked, with drainage events in one reservoir causing filling and follow-on drainage in adjacent lakes. We also observe changes near ice raft 'a' in lower Whillans Ice Stream, and interpret them as evidence of subglacial water and other changes at the bed. The study provides quantitative information about the properties of this complex subglacial hydrologic system, and a relatively unstudied component of ice-sheet mass balance: subglacial drainage across the grounding line.

Fricker, HA, Warner RC, Allison I.  2000.  Mass balance of the Lambert Glacier-Amery Ice Shelf system, East Antarctica: a comparison of computed balance fluxes and measured fluxes. Journal of Glaciology. 46:561-570.   10.3189/172756500781832765   AbstractWebsite

We combine European Remote-sensing Satellite (ERS-1) radar altimeter surface elevations (Fricker and others, 2000) with six different accumulation distributions to compute balance fluxes for the Lambert Glacier-Amery Ice Shelf drainage system. These interpolated balance fluxes are compared with fluxes derived from in situ measurements of ice thickness and velocity at 73 stations of the Lambert Glacier basin traverse and at 11 stations further downstream, to assess the system's state of balance. For the upstream line we obtain a range of imbalance estimates, from -23.8% to + 19.9% of the observed flux, reflecting the sensitivity to the accumulation distributions. For some of the accumulation distributions the imbalance estimates vary significantly between different parts of the line. Imbalance estimates for the downstream line range from - 17.7 % to + 70.2 %, with four of the estimates exceeding + 30%, again reflecting the sensitivity of the result to input accumulation, and strongly suggesting that the mass balance of the region between the two lines is positive. Our results confirm the importance of accurate estimates of accumulation in ice-sheet mass-balance studies. Furthermore, they suggest that it is not possible to accurately determine the state of balance of large Antarctic drainage basins on the basis of currently available accumulation distributions.

Fricker, HA, Carter SP, Bell RE, Scambos T.  2014.  Active lakes of Recovery Ice Stream, East Antarctica: a bedrock-controlled subglacial hydrological system. Journal of Glaciology. 60:1015-1030.   10.3189/2014JoG14J063   AbstractWebsite

A connected system of active subglacial lakes was revealed beneath Recovery Ice Stream, East Antarctica, by ICESat laser altimetry. Here we combine repeat-track analysis of ICESat (2003-09), Operation IceBridge laser altimetry and radio-echo sounding (2011 and 2012), and MODIS image differencing (2009-2011) to learn more about the lake activity history, the surface and bedrock topographic setting of the lakes and the constraints on water flow through the system. We extend the lake activity time series to 2012 for the three lower lakes and capture two major lake drainages. One lake underwent a large deflation between 2009 and 2011 while another lake, which had been continuously filling between 2003 and 2010, started to drain after 2011. Most of the active lakes are located in a similar to 1000 km long bedrock trough under the main trunk of Recovery Ice Stream, whose base is similar to 1500-2000 m below present-day sea level. The hydrologic system beneath Recovery Ice Stream is controlled by this unusually pronounced bedrock topography, in contrast to most Antarctic systems studied to date, which are controlled by the ice surface topography. Hydrologic connections among the lakes appear to be direct and responsive, and we reproduce the lake activity using a simple subglacial water model. We discuss potential causes of non-steady hydrologic behavior in major Antarctic catchments.

Fricker, HA, Young NW, Allison I, Coleman R.  2002.  Iceberg calving from the Amery Ice Shelf, East Antarctica. Annals of Glaciology, Vol 34, 2002. 34( Winther JG, Solberg R, Eds.).:241-246., Cambridge: Int Glaciological Soc   10.3189/172756402781817581   Abstract

We investigate the iceberg-calving cycle of the Amery Ice Shelf (AIS), East Antarctica, using evidence acquired between 1936 and 2000. The most recent major iceberg-calving event occurred between late 1963 and early 1964, when a large berg totalling about 10 000 km(2) in area broke From the ice front. The rate of forward advance of the ice front is presently 1300-1400 m a(-1). At this rate of advance, based on the present icefront position front recent RADARSAT imagery, it would take 20-25 years to attain the 1963 (pre-calve) position, suggesting that the AIS calving cycle has a period of approximately 60-70 years. Two longitudinal (parallel-to-flow) rifts, approximately 25 km apart at the AIS front, are observed in satellite imagery acquired over the last 14+ years. These rifts have formed at suture zones the shelf where neighbouring now-bands have separated in association with transverse spreading. The rifts were 15 km (rift A) and 26 km (rift B) in length in September 2000, and will probably become the sides of a large tabular iceberg (23 km x 25 km). A transverse (perpendicular-to-flow) fracture, visible at the upstream end of rift A in 1996, had propagated 6 km towards rift B by September 2000; when it meets rift B the iceberg will calve. A satellite image acquired in 1962 shows an embayment of this size in the AIS front, hence we deduce that this calving pattern also occurred during the last calving cycle, and therefore that the calving behaviour of the AIS apparently follows a regular pattern.