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SPRI Review 2005: Polar Physical Science

Polar Physical Science

Crevassed glacier surface, Lemaire Channel, Antarctica
Image as described adjacent

Topographic controls on the surface energy balance and mass balance of high Arctic valley glaciers

Glacier mass balance is generally measured using a series of stakes drilled into the glacier surface. Whilst such data form a very important part of world-wide research into changes in glacier mass balance, they have the disadvantage that it is often difficult to obtain adequate spatial coverage, even of small glaciers using such stake networks. Two-dimensional, distributed modelling of the surface energy balance, using Digital Elevation Models (DEMs) of glacier surfaces and measured meteorological data, can be used to give glacier-wide estimates of the energy and mass balance of glaciers which can fill the gaps between stake measurements. Such models also allow the climatic sensitivity of glaciers to be evaluated using estimates of possible climate changes from General Circulation Models of the atmosphere. We have developed a high resolution, spatially distributed model for the surface energy balance of high arctic valley glaciers. The model uses a very accurate DEM of a glacier, derived from airborne remotely-sensed data coupled with local meteorological measurements, and has successfully simulated the summer melt over the surface of a glacier, Midre Lovénbreen, in north west Svalbard, as measured at a network of stakes on the glacier. The model allows for shading of the glacier surface by the surrounding topography, the changing reflectance of the surface with the incident angle of the sun, the changing surface temperature of the glacier during the melt season, and the various energy gains and losses from the atmosphere. The model shows a very complex spatial pattern of melt, driven largely by small-scale topographic variations, and their impacts on solar energy receipt. These variations show the difficulties in obtaining a representative sample of the glacier mass balance from stake measurements. Future work in the Arctic will involve the use of the model on several different glaciers and ice caps in Svalbard as more high-resolution topographic data become available.

Neil Arnold and Gareth Rees

Evaluating the potential of high-resolution LiDAR data in glaciology

Following the successful acquisition in 2003 of a very high resolution LiDAR (Light Detection and Ranging) image of Midre Lovénbreen, a valley glacier in north-west Svalbard, we made another successful bid to the Natural Environment Research Council Airborne Remote Sensing Facility to acquire an additional image of the glacier in summer 2005. When post-collection processing of the data to correct for motion of the aircraft has been completed, these data will allow us to evaluate the use of such high-resolution imagery in studies of glacier motion, by tracking surface features such as crevasses and meltwater channels, which are clearly visible in the images. The two Digital Elevation Models for 2003 and 2005 will also allow us to begin to estimate glacier mass balance by examining differences in elevation between the two images. These high resolution remotely-sensed images will also provide a valuable complement to the very detailed (millimetre to metre) scale mapping of parts of the glacier surface which we have undertaken in work which seeks to understand the relationship between small scale topographic features and the aerodynamic roughness of ice surfaces. Surface roughness is an important control on glacier mass balance, as rough surfaces will typically experience higher melt rates than smooth ones due to increased turbulence in the atmospheric boundary layer above the glacier surface, and glacier-wide information on surface roughness should improve model-based estimates of mass balance.

Neil Arnold and Gareth Rees

Validation measurements for the Cryosat radar altimeter

In Spring 2005 we again participated in an international campaign to validate data collected by a new radar altimeter to be carried by the Cryosat satellite. A two-person team from SPRI joined Canadian and US glaciologists on the Devon Island Ice Cap in Arctic Canada. Measurements of snow density profiles were made at 1 m, 10 m, 100 m and 1 km intervals over a 1 km square site near the summit of the ice cap using an automatic profiling system based on neutron scattering. High band-width FMCW radar measurements were made over the same area and showed layering which matched the variations in density. These data will allow us to define the spatial and temporal variations in density for typical percolation-zone snow. Similar measurements were made in 2004 for a site in the dry-snow zone of Greenland and we were able to link these to radar returns from the airborne ASIRAS radar altimeter to determine annual accumulation along the flight line. Although the launch of the Cryosat satellite in September was unsuccessful, a second satellite will be launched in 2008. Work continued therefore on planning for the 2006 field season with minor adjustments, because the concurrent satellite data will come from ENVISAT rather than CryoSat. Theoretical work on neutron scattering in mixtures continued during the year and a series of experiments with rock-water mixtures was carried out with colleagues from CEH-Wallingford.

Liz Morris

Measuring surface change for polar glaciers from archival aerial photography using digital photogrammetry

Many polar glaciers are observed to be retreating due to regional climate warming. Efforts to measure the past extent of glaciers and to assess the importance of this trend have been hampered by difficulties in reconstructing past changes where field measurements are sparse. Photogrammetry using archival aerial photography is the best technique for measuring past changes in glacier surfaces and, hence, inferring glacier volume change over time. Many polar glaciers, including some on the Antarctic Peninsula, have a photographic record spanning 50 years or more. However, accurate photogrammetry for snow-covered areas is notoriously difficult to achieve, especially using historical aerial photographs. The project assesses, and presents strategies to maximise, the achievable accuracy for photogrammetric glacier-surface measurements from both historical and newly-acquired aerial photographs, with special reference to the Antarctic Peninsula. This includes, first, methods to overcome problems such as incomplete camera calibration metadata or missing negatives for historic aerial photography and, secondly, methods to optimise photogrammetry for surface measurement on polar glaciers. The project uses examples based on aerial photography and GPS survey data collected during fieldwork in Iceland, Svalbard and the Antarctic Peninsula.

Adrian Fox and Gareth Rees

Icebergs, glaciers and mountain peaks, Kangerdlussuaq Fjord, East Greenland
Image as described adjacent

Thermal regime, hydrology and dynamics of Arctic glaciers

Arctic glaciers are particularly sensitive because climate change in this region is especially marked. We have recently undertaken a study on the polythermal glacier Midre Lovénbreen, involving mapping its geometry and thermal structure (using Ground Penetrating Radar, GPR), investigating its hydrology (from calculations of surface melt and measurements of proglacial stream discharge) and monitoring its surface velocity and strain patterns. Water is stored beneath the warm-based core of the glacier in early summer and penetrates through the cold-based margin during a "breakthrough event" in mid-summer. The surface dynamics are affected, with high surface velocities during rapid summer inputs to storage which are directly forced by high basal motion beneath the warm-based core but indirectly forced in the cold-based margin due to longitudinal pushing from upglacier. The reverse is true during the breakthrough event, with direct basal forcing beneath the lower margin and indirect forcing in the upper tongue due to pulling from downglacier. We plan to extend this work to the adjacent Austre Lovénbreen and surge-type Pedersenbreen. As a precursor to a larger study, we will use GPR to map the geometry and thermal regime of these glaciers. All three glaciers are currently being monitored for surface mass balance and dynamics. This work is being done in collaboration with Jack Kohler (Norsk Polar Institute), Andy Hodson (Sheffield), Jiawen Ren (Cold & Arid Regions Environment & Engineering Institute, Chinese Academy of Sciences) and Ming Yan (Polar Research Institute of China).

Ian Willis

Glacial sedimentary processes in Iceland

We have just completed a study employing macro- and micro-sedimentological techniques on sediments exposed on recently deglaciated forefields at nine surging and non-surging glaciers in Iceland. Macro-scale evidence includes particle-size distribution and the shape, angularity and orientation of individual pebbles. Micro-scale evidence involves the identification of rotational structures, fold structures and unistrial plasmic fabric (evidence for ductile deformation) and alignment of grains, shear lines and crushed grains (evidence for brittle deformation) in thin sections viewed under a microscope. The techniques have been used to identify the types of sediments and therefore the processes of deposition. Many sediments show evidence of subglacial deformation and some also contain thin layers of waterlain sediment within the deformation till units suggesting ice-bed separation and basal sliding. All the evidence can be used to estimate the relative importance of sliding vs. sediment deformation beneath the glaciers, the styles (ductile vs. brittle) and depth of deformation, and whether these vary in space and time. Some important conclusions are i) the classic two tier deformation till (upper ductile; lower brittle) is relatively uncommon in Iceland, despite its documentation in the literature at a few glaciers; ii) the relative importance of sliding and deformation varies throughout the glacier surge cycle, presumably in response to variations in subglacial water pressure; iii) the relative importance of ductile and brittle deformation often reflects the particle-size characteristics of the sediments with coarser material showing more evidence of brittle deformation. This work was undertaken in collaboration with Anna Nelson (British Antarctic Survey) and Colm Ó Cofaigh (Durham).

Ian Willis

Flow-switching and large-scale deposition by ice streams draining former ice sheets

Today, fast-flowing ice streams and outlet glaciers drain over half the mass from the Antarctic and Greenland ice sheets. Some variability has been observed in the flow of modern West Antarctic ice streams, but major shifts in the location of ice streams flowing in deep channels has not been reported previously. We use extensive three-dimensional seismic data from the western Norwegian margin to explain how a 400 km-long ice stream has undergone major switching in flow direction from one glaciation to the next. The direction of ice flow is inferred from the pattern of build-up of large volumes of glacier-derived debris and observations of large-scale streamlined landforms on former subglacial beds. We demonstrate that ice streams can undergo major changes in flow direction through modification of their large-scale topographic setting. Whereas ice-stream switching in modern ice sheets has been regarded mainly as a reflection of internal changes in ice-sheet dynamics, switching over successive 100,000 year glacial cycles is in this case a response to the effects of continuing sediment deposition and the large-scale development of ice-influenced continental margins. This work is collaborative with Dag Ottesen and Leif Rise of the Norwegian Geological Survey.

Julian Dowdeswell

The meteorological instruments used and observations made during the 19th Century exploration of the Northwest Passage

Meteorological records from about 30 British Navy ships that overwintered in the Canadian Arctic islands between 1818 and 1859 are the earliest detailed baseline of direct historical data in this region against which modern and future climate trends can be assessed. We have investigated the types of meteorological instruments and the observational methods employed aboard these ships. For measuring air temperatures, both mercurial and spirit thermometers were used. Observations of atmospheric pressure were made using marine and aneroid barometers. Wind direction and speed were also logged. The Royal Navy's ordered daily regime was well-suited to regular scientific observations. Instruments on most Navy ships were calibrated against established Observatory standards before and after expeditions. Many recording officers commented on the relative unreliability of spirit thermometers below the freezing point of mercury. Little contemporary written evidence exists regarding absolute accuracy or precision of meteorological instruments taken to the Arctic, but some calibration data are available to assess typical instrument errors between 32º and -38ºF. Our comparison of minimum daily temperatures from four overwintering ships in 1853 and 1854 shows very high correlation coefficients. Although the absolute accuracy of temperatures recorded below the freezing point of mercury is in doubt, those above this point are relatively accurate. Guidance on the preferred methods of observing and recording were codified in, for example, the Admiralty's Manual of Scientific Enquiry (1851). The need to screen instruments from solar and terrestrial radiation was also recognised from the earliest expeditions. This unique set of ships' meteorological registers presents opportunities to investigate a variety of meteorological parameters for the Canadian high Arctic in the 19th Century, allowing quantitative assessment of change relative to contemporary climate.

Catharine Ward and Julian Dowdeswell

Vulnerability of European reindeer husbandry to climate change

This project forms a component of the EU-funded Balance programme, which has the aim of modelling the behaviour of the physical, biological and socio-economic response of the Barents Region (roughly, the European Arctic sector) to global climate change from today until 2080. Reindeer take on a surprisingly major role within this region. Approximately one million reindeer are managed on an area of around 750,000 square kilometres in the four mainland Arctic countries of Norway, Sweden, Finland and (the European part of) Russia. As well as their impact on and response to changing vegetation distribution, the reindeer form a significant aspect of the culture and local economy of the indigenous peoples of the North such as the Sámi and Nenets. Socio-economic aspects of our project are led by the Social Science and Humanities group, while we concentrate on the natural environment. We combine climate, vegetation and hydrological modelling data, generated by our colleagues within the Balance programme, with extensive field data collected from our detailed study area in the Nenets Okrug, Russia, and with data collated from the literature, to produce a quantitative model of the response of the spatial density of reindeer to changing environmental variables. Specifically, we consider the temperature regime, vegetation distribution, windiness, snow cover and the phenomenon of partial melting then refreezing of snow cover. This phenomenon, known as gololyod ('hungry ice') in Russian, can generate an impenetrable layer of ice which, in its most extreme form, can cause the deaths through starvation of many thousands of reindeer. Our preliminary results suggest that the combined effect of changing environmental factors will be negative in most of Sweden, the most northerly parts of Norway and Finland, and north-eastern part of the Nenets Okrug. There is no clear suggestion of a positive effect anywhere within the Balance region. If socio-economic factors and responses were to be ignored, we would predict decreases of around 50% in the densities of reindeer in Norway and Sweden, though much smaller decreases in Finland and possibly even small increases in Russia.

Gareth Rees and Fiona Danks