100 research outputs found
Five decades of radioglaciology
Radar sounding is a powerful geophysical approach for characterizing the subsurface conditions of terrestrial and planetary ice masses at local to global scales. As a result, a wide array of orbital, airborne, ground-based, and in situ instruments, platforms and data analysis approaches for radioglaciology have been developed, applied or proposed. Terrestrially, airborne radar sounding has been used in glaciology to observe ice thickness, basal topography and englacial layers for five decades. More recently, radar sounding data have also been exploited to estimate the extent and configuration of subglacial water, the geometry of subglacial bedforms and the subglacial and englacial thermal states of ice sheets. Planetary radar sounders have observed, or are planned to observe, the subsurfaces and near-surfaces of Mars, Earth's Moon, comets and the icy moons of Jupiter. In this review paper, and the thematic issue of the Annals of Glaciology on ‘Five decades of radioglaciology’ to which it belongs, we present recent advances in the fields of radar systems, missions, signal processing, data analysis, modeling and scientific interpretation. Our review presents progress in these fields since the last radio-glaciological Annals of Glaciology issue of 2014, the context of their history and future prospects
Impact of environmental, instrumental and data processing parameters on the performance of the Radar for Icy Moon Exploration
Il radar sounding è una tecnica molto promettente per la ricerca di ambienti abitabili sulle lune ghiacciate di Giove, poiché permetterà di osservare direttamente sotto la superficie fino a profondità di diversi chilometri. In questo lavoro si è seguita una metodologia basata sull'utilizzo di dati raccolti su terreni analoghi di altri corpi del sistema solare, per valutare l'impatto di alcuni parametri fondamentali sulle prestazioni di RIME (Radar for Icy Moon Exploration)
How dynamic are ice-stream beds?
Projections of sea-level rise contributions from West Antarctica's dynamically thinning ice streams contain high uncertainty because some of the key processes involved are extremely challenging to observe. An especially poorly observed parameter is sub-decadal stability of ice-stream beds, which may be important for subglacial traction, till continuity and landform development. Only two previous studies have made repeated geophysical measurements of ice-stream beds at the same locations in different years, but both studies were limited in spatial extent. Here, we present the results from repeat radar measurements of the bed of Pine Island Glacier, West Antarctica, conducted 3–6 years apart, along a cumulative ∼ 60 km of profiles. Analysis of the correlation of bed picks between repeat surveys shows that 90 % of the bed displays no significant change despite the glacier increasing in speed by up to 40 % over the last decade. We attribute the negligible detection of morphological change at the bed of Pine Island Glacier to the ubiquitous presence of a deforming till layer, wherein sediment transport is in steady state, such that sediment is transported along the basal interface without inducing morphological change to the radar-sounded basal interface. Given the precision of our measurements, the upper limit of subglacial erosion observed here is 500 mm a‾¹, far exceeding erosion rates reported for glacial settings from proglacial sediment yields, but substantially below subglacial erosion rates of 1.0 m a‾¹ previously reported from repeat geophysical surveys in West Antarctica
A new bed elevation model for the Weddell Sea sector of the West Antarctic Ice Sheet
We present a new bed elevation digital elevation model (DEM), with a 1 km spatial resolution, for the Weddell Sea sector of the West Antarctic Ice Sheet. The DEM has a total area of ~125,000 km2 covering the Institute, Möller and Foundation ice streams and the Bungenstock ice rise. In comparison with the Bedmap2 product, our DEM includes several new aerogeophysical datasets acquired by the Center for Remote Sensing of Ice Sheets (CReSIS) through the NASA Operation IceBridge (OIB) program in 2012, 2014 and 2016. We also update bed elevation information from the single largest existing dataset in the region, collected by the British Antarctic Survey (BAS) Polarimetric Airborne Survey Instrument (PASIN) in 2010-11, as BEDMAP2 included only relatively crude ice thickness measurements determined in the field for quality control purposes. This have resulted in the deep parts of the topography not being visible in the fieldwork non-SAR processed radargrams. While the gross form of the new DEM is similar to Bedmap2, there are some notable differences. For example, the position and size of a deep trough (~ 2 km below sea level) between the ice sheet interior and the grounding line of Foundation ice stream has been redefined. From the revised DEM, we are able to better derive the expected routing of basal water at the ice-bed interface, and by comparison with that calculated using Bedmap2 we are able to assess regions where hydraulic flow is sensitive to change. Given the sensitivity of this sector of the ice sheet to ocean-induced melting at the grounding line, especially in light of improved definition of the Foundation ice stream trough, our revised DEM will be of value to ice-sheet modelling in efforts to quantify future glaciological changes in the region, and therefore the potential impact on global sea level. The new 1 km bed elevation product of the Weddell Sea sector, West Antarctica can be found in the http://doi.org/10.5281/zenodo.1035488
Subglacial controls on dynamic thinning at Trinity-Wykeham Glacier, Prince of Wales Ice Field, Canadian Arctic
Mass loss from glaciers and ice caps represents the largest terrestrial component of current sea level rise. However, our understanding of how the processes governing mass loss will respond to climate warming remains incomplete. This study explores the relationship between surface elevation changes (dh/dt), glacier velocity changes (du/dt), and bedrock topography at the Trinity-Wykeham Glacier system (TWG), Canadian High Arctic, using a range of satellite and airborne datasets. We use measurements of dh/dt from ICESat (2003-2009) and CryoSat-2 (2010-2016) repeat observations to show that rates of surface lowering increased from 4 m yr-1 to 6 m yr-1 across the lowermost 10 km of the TWG. We show that surface flow rates at both Trinity Glacier and Wykeham Glacier doubled over 16 years, during which time the ice front retreated 4.45 km. The combination of thinning, acceleration and retreat of the TWG suggests that a dynamic thinning mechanism is responsible for the observed changes, and we suggest that both glaciers have transitioned from fully grounded to partially floating. Furthermore, by comparing the separate glacier troughs we suggest that the dynamic changes are modulated by both lateral friction from the valley sides and the complex geometry of the bed. Further, the presence of bedrock ridges induces crevassing on the surface and provides a direct link for surface meltwater to reach the bed. We observe supraglacial lakes that drain at the end of summer and are concurrent with a reduction in glacier velocity, suggesting hydrological connections between the surface and the bed significantly impact ice flow. The bedrock topography thus has a primary influence on the nature of the changes in ice dynamics observed over the last decade.</p
A Semiautomated Multilayer Picking Algorithm for Ice-sheet Radar Echograms Applied to Ground-Based Near-Surface Data
Snow accumulation over an ice sheet is the sole mass input, making it a primary measurement for understanding the past, present, and future mass balance. Near-surface frequency-modulated continuous-wave (FMCW) radars image isochronous firn layers recording accumulation histories. The Semiautomated Multilayer Picking Algorithm (SAMPA) was designed and developed to trace annual accumulation layers in polar firn from both airborne and ground-based radars. The SAMPA algorithm is based on the Radon transform (RT) computed by blocks and angular orientations over a radar echogram. For each echogram's block, the RT maps firn segmented-layer features into peaks, which are picked using amplitude and width threshold parameters of peaks. A backward RT is then computed for each corresponding block, mapping the peaks back into picked segmented-layers. The segmented layers are then connected and smoothed to achieve a final layer pick across the echogram. Once input parameters are trained, SAMPA operates autonomously and can process hundreds of kilometers of radar data picking more than 40 layers. SAMPA final pick results and layer numbering still require a cursory manual adjustment to correct noncontinuous picks, which are likely not annual, and to correct for inconsistency in layer numbering. Despite the manual effort to train and check SAMPA results, it is an efficient tool for picking multiple accumulation layers in polar firn, reducing time over manual digitizing efforts. The trackability of good detected layers is greater than 90%
Basal Conditions of Petermann Glacier and Jakobshavn Isbrae derived from Airborne Ice Penetrating Radar Measurements
Understanding ice dynamics and ice basal conditions is important because of their impacts on sea level rise. Radio echo sounding has been extensively used for characterizing the ice sheets. The radar reflectivity of the ice bed is of special importance because it can discriminate frozen and thawed ice beds. The knowledge of the spatial distribution of basal water is crucial in explaining the flow velocity and stability of glaciers and ice sheets. Basal echo reflectivity used to identify the areas of basal melting can be calculated by compensating ice bed power for geometric losses, rough interface losses, system losses and englacial attenuation. Two important outlet glaciers of Greenland, Petermann glacier and Jakobshavn isbrae have been losing a lot of ice mass in recent years, and are therefore studied to derive its basal conditions from airborne radar surveys in this thesis. The ice surface and bed roughness of these glaciers are estimated using Radar Statistical Reconnaissance (RSR) method and validated using roughness derived from NASA’s Airborne Topographic Mapper (ATM) and Ku band altimeter. Englacial attenuation is modeled using Schroeder’s variable attenuation method. After compensating for these losses, the basal reflectivity for the two glaciers is estimated and validated using cross over analysis, geophysics, hydraulic potential, abruptive index and coherence index. The areas of basal melting i.e. areas with higher reflectivity are identified. Petermann glacier is found to have alternate frozen and thawed regions explaining the process of ice movement by friction and freezing. Due to the lack of topographic pinning the glacier is subject to higher ice flow speed. Jakobshavn glacier has several areas of basal melting scattered in the catchment area with most concentration near the glacier front which is likely due to surface water infiltration into ice beds via moulins and sinks. The ice bed channels and retrograde slope of this glacier are also important in routing subglacial water and ice mass. The basal conditions of these two glaciers presented in this study can help in modeling the behavior of these glaciers in the future
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Variability of Subglacial Drainage Across the Greenland Ice Sheet: A Joint Model/Radar Study
Over the last several decades, the majority of the Greenland outlet glaciers have accelerated due to the increased warming in both the atmosphere and the oceans around the polar latitudes. While there is a clear overall acceleration trend over this period, there is significant variability in the glacier responses to climate on seasonal and year-to-year timescales. This variability observed around Greenland is very likely tied to the differences in internal dynamics of individual glaciers and the complex interaction with its local environment. Here I investigate the interaction between ice and water along the ice base as an important mechanism contributing to the observed variability among glaciers in Greenland. I use a range of modeling and radar sounding approaches to study the subglacial hydrology for three types of outlet glaciers, including slow moving, marine terminating glaciers in the west, a land-terminating system in the southwest, and a fast moving, marine-terminating glacier in northern Greenland. These case studies allow me to characterize the basal water distribution, its variability throughout the year and how this drainage behavior varies across different regions of Greenland.
To start, I use a hydrological routing model to characterize the subglacial hydrology for three neighboring slow moving (< 100myr−1), marine terminating glaciers in western Greenland. The hydrologic model allows me to examine the sensitivity of basal water routing to subtle changes in basal water pressures. My results reveal that Greenland subglacial drainage can be rerouted across 100’s of km in response to changes in basal water pressures as small as 10%. I conclude that water piracy and subsequent dramatic changes in ice velocity, similar to that observed around the Siple Coast in West Antarctica, can occur in Greenland. Next, I move to a more data-orientated approach and use airborne radar sounding to examine the seasonal variability of basal water distribution. To robustly characterize basal water from radar bed power, I use a novel radar analysis approach that integrates a thermomechanical ice-sheet model to predict the spatial variations of radar attenuation. I improve this approach by including a least-squares minimization to correct for power offsets due to the different radar systems deployed in multiple field seasons.
This improved method is first applied to two land-terminating glaciers in the southwest, Russell Glacier, and Isunnguata Sermia. Using two seasons of radar sounding data, I find that the basal water distribution can change between the wintertime and the summertime. My results reveal that during the winter, water resides primarily in small pockets on top of bedrock ridges. In the summer, these pockets of water on the ridges connect and drain into the nearby basal troughs. This seasonal shift in the basal water distribution is actively controlled by the material properties of the bed. Therefore, in addition to the bed topography, the permeability of the bed and the presence of basal sediments could also exert a critical influence on the seasonal development of subglacial drainage.
Finally, I apply the radar analysis approach to a fast-flowing marine terminating glacier for Petermann Glacier in Northern Greenland. Here I incorporate an additional step to address the spatial variation in ice chemistry and its effect on radar attenuation. I use this approach to examine the relationship between basal water, ice deformation and the onset of glacier flow. In addition to finding basal water in the fastest flowing region near the ice margin, I identify substantial basal water in the ice sheet interior where meltwater must either be related to the advection of water from upstream or be generated by internal heating due to ice deformation. My results show there are three basal water networks beneath Petermann that connect the ice sheet interior to the margin. Together, the interaction between these basal water networks and the ice deformation enhances and sustains fast flow in the interior of the Petermann catchment. Overall, the research presented in this dissertation suggests that subglacial hydrology is high variable in both space and time. This variation in the hydrologic system can influence the fundamental structure of the ice sheet through changing the transport and storage of basal water and through interacting with ice deformation and the thermal properties of the bed
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