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Markus, T, Neumann T, Martino A, Abdalati W, Brunt K, Csatho B, Farrell S, Fricker H, Gardner A, Harding D, Jasinski M, Kwok R, Magruder L, Lubin D, Luthcke S, Morison J, Nelson R, Neuenschwander A, Palm S, Popescu S, Shum CK, Schutz BE, Smith B, Yang YK, Zwally J.  2017.  The Ice, Cloud, and land Elevation Satellite-2 (ICESat-2): Science requirements, concept, and implementation. Remote Sensing of Environment. 190:260-273.   10.1016/j.rse.2016.12.029   AbstractWebsite

The Ice, Cloud, and land Elevation Satellite (ICESat) mission used laser altimetry measurements to determine changes in elevations of glaciers and ice sheets, as well as sea ice thickness distribution. These measurements have provided important information on the response of the cryopshere (Earth's frozen surfaces) to changes in atmosphere and ocean condition. ICESat operated from 2003 to 2009 and provided repeat altimetry measurements not only to the cryosphere scientific community but also to the ocean, terrestrial and atmospheric scientific communities. The conclusive assessment of significant ongoing rapid changes in the Earth's ice cover, in part supported by ICESat observations, has strengthened the need for sustained, high accuracy, repeat observations similar to what was provided by the ICESat mission. Following recommendations from the National Research Council for an ICESat follow-on mission, the ICESat-2 mission is now under development for planned launch in 2018. The primary scientific aims of the ICESat-2 mission are to continue measurements of sea ice freeboard and ice sheet elevation to determine their changes at scales from outlet glaciers to the entire ice sheet, and from 105 of meters to the entire polar oceans for sea ice freeboard. ICESat carried a single beam profiling laser altimeter that produced similar to 70 m diameter footprints on the surface of the Earth at similar to 150 m along-track intervals. In contrast, ICESat-2 will operate with three pairs of beams, each pair separated by about 3 km cross-track with a pair spacing of 90 m. Each of the beams will have a nominal 17 m diameter footprint with an along -track sampling interval of 0.7 m. The differences in the ICESat-2 measurement concept are a result of overcoming some limitations associated with the approach used in the ICESat mission. The beam pair configuration of ICESat-2 allows for the determination of local cross -track slope, a significant factor in measuring elevation change for the outlet glaciers surrounding the Greenland and Antarctica coasts. The multiple beam pairs also provide improved spatial coverage. The dense spatial sampling eliminates along -track measurement gaps, and the small footprint diameter is especially useful for sea surface height measurements in the often narrow leads needed for sea ice freeboard and ice thickness retrievals. The ICESat-2 instrumentation concept uses a low energy 532 nm (green) laser in conjunction with single-photon sensitive detectors to measure range. Combining ICESat-2 data with altimetry data collected since the start of the ICESat mission in 2003, such as Operation IceBridge and ESA's CryoSat-2, will yield a 15+ year record of changes in ice sheet elevation and sea ice thickness. ICESat-2 will also provide information of mountain glacier and ice cap elevations changes, land and vegetation heights, inland water elevations, sea surface heights, and cloud layering and optical thickness. Published by Elsevier Inc. This is an open access article under the CC BY license

Carter, SP, Fricker HA, Siegfried MR.  2017.  Antarctic subglacial lakes drain through sediment-floored canals: theory and model testing on real and idealized domains. Cryosphere. 11:381-405.   10.5194/tc-11-381-2017   AbstractWebsite

Over the past decade, satellite observations of ice surface height have revealed that active subglacial lake systems are widespread under the Antarctic Ice Sheet, including the ice streams. For some of these systems, additional observations of ice-stream motion have shown that lake activity can affect ice-stream dynamics. Despite all this new information, we still have insufficient understanding of the lake-drainage process to incorporate it into ice-sheet models. Process models for drainage of ice-dammed lakes based on conventional "R-channels" incised into the base of the ice through melting are unable to reproduce the timing and magnitude of drainage from Antarctic subglacial lakes estimated from satellite altimetry given the low hydraulic gradients along which such lakes drain. We have developed an alternative process model, in which channels are mechanically eroded into the underlying deformable subglacial sediment. When applied to the known active lakes of the Whillans-Mercer ice-stream system, the model successfully reproduced both the inferred magnitudes and recurrence intervals of lake-volume changes, derived from Ice, Cloud and land Elevation Satellite (ICESat) laser altimeter data for the period 2003-2009. Water pressures in our model changed as the flood evolved: during drainage, water pressures initially increased as water flowed out of the lake primarily via a distributed system, then decreased as the channelized system grew, establishing a pressure gradient that drew water away from the distributed system. This evolution of the drainage system can result in the observed internal variability of ice flow over time. If we are correct that active subglacial lakes drain through canals in the sediment, this mechanism also implies that active lakes are typically located in regions underlain by thick subglacial sediment, which may explain why they are not readily observed using radio-echo-sounding techniques.

Massom, RA, Giles AB, Warner RC, Fricker HA, Legresy B, Hyland G, Lescarmontier L, Young N.  2015.  External influences on the Mertz Glacier Tongue (East Antarctica) in the decade leading up to its calving in 2010. Journal of Geophysical Research-Earth Surface. 120:490-506.   10.1002/2014jf003223   AbstractWebsite

The Mertz Glacier Tongue (MGT) in East Antarctica lost similar to 55% of its floating length in February 2010, when it calved large tabular iceberg C28 (78x35km). We analyze the behavior of the MGT over the preceding 12years using a variety of satellite data (synthetic aperture radar and Landsat imagery and Ice, Cloud, and land Elevation Satellite laser altimetry). Contact of its northwestern tip with the eastern flank of shoals from 2002/2003 caused eastward deflection of the ice flow by up to similar to 47 degrees. This change contributed to opening of a major rift system similar to 80km to the south, along which iceberg C28 eventually calved. Paradoxically, the seabed contact may have also held the glacier tongue in place to delay calving by similar to 8years. Our study also reveals the effects of other, more localized external influences on the MGT prior to calving. These include an abrupt sideways displacement of the glacier tongue front by at least similar to 145m following an apparent collision with iceberg C08 in early 2002 and calving of numerous small icebergs from the advancing northwestern front due to the chiseling action of small grounded icebergs and seabed contact, resulting in the loss of similar to 36km(2) of ice from 2001 to 2006. The example of the MGT confirms the need for accurate bathymetry in the vicinity of ice shelves and glacier tongues and suggests that the cumulative effect of external factors might be critical to understanding and modeling calving events and ice shelf stability, necessarily on a case-specific basis.

Beem, LH, Tulaczyk SM, King MA, Bougamont M, Fricker HA, Christoffersen P.  2014.  Variable deceleration of Whillans Ice Stream, West Antarctica. Journal of Geophysical Research-Earth Surface. 119:212-224.   10.1002/2013jf002958   AbstractWebsite

The Whillans Ice Stream Ice Plain (WIP) has been slowing since at least 1963. Prior constraints on this slowdown were consistent with a constant long-term deceleration rate. New observations of ice velocity from 11 continuous and 3 seasonal GPS sites indicate the deceleration rate varies through time including on interannual time scales. Between 2009 and 2012 WIP decelerated at a rate (6.1 to 10.9 2 m/yr(2)) that was double the multidecadal average (3.0 to 5.6 2 m/yr(2)). To identify the causes of slowdown, we used new and prior velocity estimates to constrain longitudinal and transverse force budget models as well as a higher-order inverse model. All model results support the conclusion that the observed deceleration of WIP is caused by an increase in basal resistance to motion at a rate of 10 to 40 Pa/yr. Subglacial processes that may be responsible for strengthening the ice stream bed include basal freeze on, changes in subglacial hydrology, or increases in the area of resistant basal substrate through differential erosion. The observed variability in WIP deceleration rate suggests that dynamics in subglacial hydrology, plausibly driven by basal freeze on and/or activity of subglacial lakes, plays a key role in modulating basal resistance to ice motion in the region.

Abdalati, W, Zwally HJ, Bindschadler R, Csatho B, Farrell SL, Fricker HA, Harding D, Kwok R, Lefsky M, Markus T, Marshak A, Neumann T, Palm S, Schutz B, Smith B, Spinhirne J, Webb C.  2010.  The ICESat-2 Laser Altimetry Mission. Proceedings of the IEEE . 98:735-751.   10.1109/jproc.2009.2034765   AbstractWebsite

Satellite and aircraft observations have revealed that remarkable changes in the Earth's polar ice cover have occurred in the last decade. The impacts of these changes, which include dramatic ice loss from ice sheets and rapid declines in Arctic sea ice, could be quite large in terms of sea level rise and global climate. NASA's Ice, Cloud and Land Elevation Satellite-2 (ICESat-2), currently planned for launch in 2015, is specifically intended to quantify the amount of change in ice sheets and sea ice and provide key insights into their behavior. It will achieve these objectives through the use of precise laser measurements of surface elevation, building on the groundbreaking capabilities of its predecessor, the Ice Cloud and Land Elevation Satellite (ICESat). In particular, ICESat-2 will measure the temporal and spatial character of ice sheet elevation change to enable assessment of ice sheet mass balance and examination of the underlying mechanisms that control it. The precision of ICESat-2's elevation measurement will also allow for accurate measurements of sea ice freeboard height, from which sea ice thickness and its temporal changes can be estimated. ICESat-2 will provide important information on other components of the Earth System as well, most notably large-scale vegetation biomass estimates through the measurement of vegetation canopy height. When combined with the original ICESat observations, ICESat-2 will provide ice change measurements across more than a 15-year time span. Its significantly improved laser system will also provide observations with much greater spatial resolution, temporal resolution, and accuracy than has ever been possible before.

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.