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SPRI Review 2011: Polar Physical Science

Polar Physical Science

Microwave remote sensing of thaw lake dynamics

Anna Maria Trofaier continued her doctoral investigation of the seasonal fluctuations of permafrost thaw lakes using images from the European Space Agency’s Advanced Synthetic Aperture Radar (ASAR) on the Envisat satellite. The use of radar imagery for studying this highly dynamic phenomenon (thaw lakes can fill and drain in a few weeks) is particularly attractive because, unlike visible-wavelength or thermal infrared imagery, it is not affected by the presence of cloud cover or the absence of daylight. The current study area is located on the Yamal Peninsula, Russia. Preliminary analysis has confirmed the suitability of ASAR data for identifying thaw lakes and monitoring changes in their extent. These results were presented at the ESA DUE Permafrost Workshop, held at the University of Alaska, Fairbanks, in February and at the European Geosciences Union’s General Assembly meeting in Vienna in April. We are now broadening the geographical extent of the study to include comparable and contrasting sites in Alaska. This work is being undertaken in collaboration with Dr Annett Bartsch and her colleagues at the Institute of Remote Sensing and Photogrammetry, Vienna University of Technology.

Anna Maria Trofaier and Gareth Rees

Basler DC3, with under-wing radar antenna for sounding ice thickness, landing at Ilulissat in West Greenland

Basler DC3, with under-wing radar antenna for sounding ice thickness, landing at Ilulissat in West Greenland

Spectral and physical characterisation of glacier surfaces

The relationship between the optical properties of a glacier surface and its physical characteristics has been investigated by the collection of spectral reflectance data from the Langjökull ice cap in Iceland, providing a comparison with similar data collected from Midre Lovénbreen, Svalbard, in 2010. These data are of exceptionally high spectral resolution, covering the spectrum from a wavelength of around 400 nm, which is in the ultraviolet region, to 2400 nm, well into the infrared, with an effective resolution of around 10 nm. This gives much more spectral detail than is available from most existing or planned satellite imaging instruments, and is providing important insights into the scope that such instruments offer for drawing inferences about the surface characteristics, and ultimately mass balance, of glaciers. Initial results were presented at the American Geophysical Union’s Fall Meeting in San Francisco.

Allen Pope and Gareth Rees

The seismic architecture and geometry of grounding-zone wedges formed at the marine margins of past ice sheets

The grounding-zone of glaciers and ice sheets is where their marine margins cease to be in contact with the sea floor. It is important both glaciologically and geologically. Mass is lost at the grounding-zone through iceberg production and melting, with the latter taking place at both ice cliffs marking the marine ice-sheet margin and at the base if the ice sheet becomes ungrounded as an ice shelf and is exposed to ocean water. During the environmental oscillations of the Quaternary and earlier glaciations, the location of the grounding-zone has responded sensitively to climatically-induced changes in ice thickness and sea level. In addition, where fast-flowing ice streams are present, the grounding-zone is a major focus for sediment delivery at the marine margins of ice sheets.

Subglacial debris is transferred across the grounding-zone from deforming sedimentary beds. Subglacial meltwater channels may also provide point sources of sorted sediments to the grounding-zone. Grounding-zone wedges, formed by these processes of rapid sediment delivery to the ice-ocean interface, are up to 103 km2 in area and 103 km3 in volume. The wedges are important indicators of past ice-sheet dynamics in the geological record and allow former positions of the grounding-zone to be identified. This, in turn, provides independent geological evidence against which the predictions of time-dependent numerical ice-sheet models can be tested. This research was undertaken in collaboration with Dr Edith Fugelli of BP Norway.

Julian Dowdeswell

Hydrology and dynamics of the Greenland Ice Sheet

Predicting the response of the Greenland Ice Sheet to future warming requires improved understanding of the links between surface hydrology, basal hydrology and ice-sheet dynamics. We are developing a hydrological model that can be used to calculate patterns of melt across the ice sheet, the routing of surface water into depressions, where they may form lakes, the drainage of those lakes to the base of the ice sheet, and the routing of water across the bed to the ice-sheet edge. The model is applied to the Paakitsoq / Swiss Camp region of West Greenland. Recent model developments include a novel surface routing and lake filling / overflow algorithm that can be used to calculate supraglacial water discharge, the changing area and water volume of surface lakes that form in depressions, and the timing of lake overflow. We collected stream and lake-level data on the ice sheet over a three-week period in June 2011 to calibrate the model. Data show that lakes drain either by overflow and the delivery of water to nearby shafts (moulins) into the ice (“slow drainage”), or by hydro-fracture and the sudden injection of large volumes of water to the ice-sheet base (“fast drainage”). One lake reached a maximum volume of ~ 1.5 x 106 m3 (equivalent to 600 Olympic swimming pools) which then drained in 2½ hours. During drainage, the maximum water discharge entering the ice sheet was nearly 600 m3 s-1 (around 10 times the discharge of the River Thames, London). Analysis of dGPS data collected around the lakes shows that both “slow” and “fast” drainage affect the horizontal and vertical movement of the ice sheet, but in different ways. The work is being undertaken with PhD student Alison Banwell, and in collaboration with Dr Marco Tedesco (City College of New York) and Dr Andreas Ahlstrøm (Geological Survey of Denmark and Greenland).

Ian Willis and Neil Arnold

Cryosat

Liz Morris continued her participation in an international programme to validate data collected by a new radar altimeter (SIRAL) carried by the Cryosat-2 satellite, launched this year. A repeat traverse along the EGIG line on the Greenland Ice Sheet was made in summer 2011, during which measurements of snow-density profiles were made with an automated neutron-profiling system using access holes installed in 2010. This completed the series of measurements begun in 2004 and has provided a unique set of direct determinations of strain rate in polar snow.

Liz Morris

Modelling glacial meltwater-plume dynamics and sedimentation in high-latitude fjords

A numerical model, SedPlume, has been developed to simulate deposition of suspended sediment from meltwater plumes emerging from tidewater-glacier margins. Turbid meltwater entering a fjord from a subglacial channel rises as a buoyant plume due to salinity and temperature contrasts with the fjord water. A model is formulated for the conservation equations of volume, momentum, buoyancy and sediment flux along the path of a turbulent plume injected into stratified marine water. Sedimentation occurs from the plume when the sediment fall velocity is greater than the entrainment velocity. Flocculation, the aggregation of individual silt and clay particles, is modelled using empirical measurements of particle settling velocities in fjords to adjust the settling velocity of fine-grained sediments. The SedPlume model has been applied to McBride Inlet in Alaska, a temperate glaciated fjord where the majority of sedimentation originates from meltwater sources. The model predicts rates and patterns of sedimentation in good agreement with observations. This research has been published recently in the Journal of Geophysical Research.

Ruth Mugford and Julian Dowdeswell

New insight to the dynamics of Antarctic ice streams

Research on glacier dynamics at SPRI has produced a new representation of subglacial processes in a 3-dimensional ice flow model. The improved model physics allow thermo-mechanical interactions between ice-flow dynamics and subglacial processes. Application of the improved model reveals hitherto unsimulated and significant complexities in the flow of Antarctic ice streams. In a paper published in the Journal of Geophysical Research in 2011, the researchers show that slight changes in subglacial hydrology may have a profound effect on fast ice-stream flow, a key new finding in light of the recent discovery of interconnected subglacial lakes beneath many ice streams in Antarctica. The study also demonstrates how ice streams may posses a ‘memory’, insofar as their contemporary behaviour is governed by basal properties shaped by past aspects of flow and physical condition. The publication was selected as an Editors’ highlight and a contribution of special significance.

Marion Bougamont and Poul Christoffersen

Green Scholarships at the Scripps Institution of Oceanography

The Green Foundation, which supports research at the Institute of Geophysics and Planetary Physics across the University of California, awarded two Green Scholarships to researchers at SPRI in 2011; to Drs Poul Christoffersen and Marion Bougamont. The scholarships were funded with the aim of integrating ice-sheet modelling at SPRI with research on subglacial lakes by Prof. Helen Fricker and associated researchers at the Scripps Institution of Oceanography.

The collaborative research, which took place over a four-month period at the Scripps Institution of Oceanography, enabled the researchers to link modelling of subglacial hydrology with modelling of ice flow and basal processes. The coupled models are now being applied to Antarctic ice streams, with simulations running on the Darwin Supercomputer in Cambridge University.

Poul Christoffersen and Marion Bougamont

Ice cliffs of the Austfonna ice cap, eastern Svalbard

Ice cliffs of the Austfonna ice cap, eastern Svalbard