| 1. |
- Hills, Benjamin H., et al.
(författare)
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Radar attenuation demonstrates advective cooling in the Siple Coast ice streams
- 2022
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Ingår i: Journal of Glaciology. - : Cambridge University Press. - 0022-1430 .- 1727-5652. ; 69:275, s. 566-576
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Tidskriftsartikel (refereegranskat)abstract
- Ice streams are warmed by shear strain, both vertical shear near the bed and lateral shear at the margins. Warm ice deforms more easily, establishing a positive feedback loop in an ice stream where fast flow leads to warm ice and then to even faster flow. Here, we use radar attenuation measurements to show that the Siple Coast ice streams are colder than previously thought, which we hypothesize is due to along-flow advection of cold ice from upstream. We interpret the attenuation results within the context of previous ice-temperature measurements from nearby sites where hot-water boreholes were drilled. These in-situ temperatures are notably colder than model predictions, both in the ice streams and in an ice-stream shear margin. We then model ice temperature using a 1.5-dimensional numerical model which includes a parameterization for along-flow advection. Compared to analytical solutions, we find depth-averaged temperatures that are colder by 0.7°C in the Bindschadler Ice Stream, 2.7°C in the Kamb Ice Stream and 6.2–8.2°C in the Dragon Shear Margin of Whillans Ice Stream, closer to the borehole measurements at all locations. Modelled cooling corresponds to shear-margin thermal strengthening by 3–3.5 times compared to the warm-ice case, which must be compensated by some other weakening mechanism such as material damage or ice-crystal fabric anisotropy.
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| 2. |
- Jacobel, Robert W., et al.
(författare)
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Spatial variation of radar-derived basal conditions on Kamb Ice Stream, West Antarctica
- 2009
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Ingår i: Annals of Glaciology. - : International Glaciological Society. - 0260-3055 .- 1727-5644. ; 50:51, s. 10-16
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Tidskriftsartikel (refereegranskat)abstract
- Radar profiles of bed echo intensity can survey conditions at the ice-bed interface and test for the presence or absence of water. However, extracting information about basal conditions from bed echo intensities requires an estimate of the attenuation loss through the ice. We used the relationship between bed echo intensities from constant-offset radar data and ice thickness to estimate depth-averaged attenuation rates at several locations on and near Kamb Ice Stream (KIS), West Antarctica. We found values varying from 29 dB km−1 at Siple Dome to 15 dB km−1 in the main trunk region of KIS, in agreement with a previous measurement and models. Using these attenuation-rate values, we calculated the relative bed reflectivity throughout our KIS surveys and found that most of the bed in the trunk has high basal reflectivities, similar to those obtained in the location of boreholes that found water at the bed. Areas of lower bed reflectivity are limited to the sticky spot, where a borehole found a dry bed, and along the margins of KIS. These results support previous hypotheses that the basal conditions at locations like the sticky spot on KIS control its stagnation and possible reactivation.
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| 3. |
- Richardson-Näslund, Cecilia, 1969-
(författare)
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Spatial distribution of snow in Antarctica and other glacier studies using ground-penetrating radar
- 2001
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The overall aim of this thesis is to develop and exemplify the use of ground-penetrating radar (GPR) within the field of glaciology. The main focus is on the spatial distribution of snow in Dronning Maud Land, East Antarctica. GPR soundings of shallow snow stratigraphy were made along two continuous profiles, 800 and 1000 km long, extending from the ice shelves to the high altitude plateau. Large local and regional spatial variations in net snow accumulation were observed. The complex accumulation pattern can be explained by the ice sheet surface topography and the distribution of air pressure systems and cyclone tracks. On ice shelves the spatial variability is less than 10%, in grounded coastal areas it ranges between 20-40% and on the high altitude plateau it ranges up to 20%. In areas subjected to intense wind erosion it often exceeds 50%. These values may serve as estimates of the spatial representativity of point measurements, such as firn and ice cores. The generally high variability in accumulation rates demonstrates that the spatial representativity of point measurements must be taken into consideration in studies of snow accumulation. It is shown that GPR can be used to quantify the degree to which the accumulation rate obtained at individual coring sites over- or underestimates the local average. Furthermore, a comparison between the figures for snow accumulation calculated by the parameterisations used in two numerical ice sheet models and the results of the GPR measurements shows that the models clearly overestimate the accumulation rates in the coastal region (elevations below 2000 m). The model parameterisations of snow accumulation were improved and adapted to regional conditions by fitting them to GPR data, and the new, fitted model parameterisations explain ~45% of the variations observed. Complemented by firn core studies, GPR constitutes a powerful tool to determine net accumulation rates along profiles made over vast distances.In addition, this thesis contains two GPR case studies, in which glaciers in northern Sweden were investigated from two view points, the potential of finding paleoclimatic signals preserved in the ice, and the development of glacial cirques. Both ice thickness and the thermal regimes of three glaciers were studied, and the results are discussed in the context of the Quaternary climates. It is suggested that the optimal place in Scandinavia to search for glacier ice that contains paleoclimatic signals is in the eastern part of the Scandinavian mountain range, where the climate is relatively dry and the glacier mass turnover is low. It is also concluded that glacial cirques are in general not good proxy data for climatic reconstructions, since the climatic conditions required for the excavation of glacial cirques are not well understood.
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| 4. |
- Sundström, Nils
(författare)
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Improving snow water equivalent estimates with ground penetrating radar by measuring on multiple channels
- 2012
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Snow water equivalent (SWE) of a snowpack is often measured along well-chosen transects representative of an area of interest, such as a drainage basin, to capture spatial distribution of SWE, which is of great interest for many applications. For example, it is a useful input to the new generation of hydrological models used for snow melt run-off predictions. A time-effective method to perform such measurements is to conduct them along one or several transects using a ground penetrating radar (GPR) operated from e.g. a snowmobile. Traditionally, a single-channel radar system has been used to estimate SWE from the radar wave two-way travel time via a linear formula, which can be calibrated for a particular snowpack with one or several manual measurements of snow density; this method typically relies on the assumption of a dry snowpack. However, if an unknown amount of liquid water is present in the snow, or if the snow density or the liquid water content varies substantially along the transect, SWE estimates are likely to be inaccurate.A different approach is to use a multi-channel GPR system with an array of antennas that makes it possible to simultaneously measure two-way travel time of several radar pulses that form a common mid-point (CMP) gather. Then the snow depth and the radar wave propagation velocity can be determined at each point with the CMP method under the assumption of a single-layer snowpack with parallel snow and ground surfaces. With liquid water content known or assumed to be zero, the snow density can be estimated from the propagation velocity via an empirical formula for mixtures, thus solving the problem of spatial variation in snow density. Finally, SWE is calculated from the snow depth and density. However, the CMP method is known to be sensitive to measurement errors in two-way travel time and to violations of its assumptions; and for a wet snowpack, the need to know the liquid water content at each measurement point to accurately estimate snow density presents a problem if the liquid water content varies along the transect.In this thesis, two methods that improve SWE estimates obtained with GPR are presented, both of which rely on measuring on multiple channels to obtain a CMP gather at each measurement point. The first method mitigates the impact of errors in CMP calculations on density estimates by establishing a depth-to-density function from the CMP data for all measurement points along a transect. This function, specific for each transect, is then used to determine snow density from snow depth. The second method (the PDA method) improves SWE estimates of wet snowpacks by determining liquid water content at each measurement point from path-dependent attenuation of two radar signals in the CMP gather. Both methods have been tested in field experiments and the sensitivity of the PDA method to built-in assumptions and measurement errors has been investigated in simulations.The field experiment conducted to test the first method has demonstrated that by applying a depth-to-density function, the accuracy of SWE estimates for a dry snowpack can be improved substantially. For the transect in the experiment, snow density and SWE estimated directly with the CMP method were overestimated by 34% and 36% on average; and when a depth-to-density function was used, snow density was underestimated by 2% and SWE was overestimated by less than 1%. The error was determined by comparison with manual measurements.In the field experiment conducted to test the PDA method, for a snowpack with the mean liquid water content of about 5 vol.%, the mean error in SWE was 16%, compared to 34% and 31% for two reference methods that both assumed liquid water content to be zero. Separately, the performed simulations suggest that the PDA method is very sensitive to measurement errors when liquid water content is close to zero; in such cases, one of the methods that assume dry snow should be used instead of the PDA method.
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