Radar attenuation and temperature within the Greenland Ice Sheet

MacGregor, Joseph A.; Li, Jilu; Paden, John; Catania, G. A.; Clow, Gary D.; Fahnestock, M. A.; Gogineni, S.; Grimm, R. E.; Morlighem, Mathieu; Nandi, Soumyaroop; Seroussi, Hélène; Stillman, D. E.

doi:10.1002/2014jf003418

Cited by 92 publications

(154 citation statements)

References 96 publications

(210 reference statements)

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“…These relatively small uncertainty values (compared with the pattern in Fig. 8a) are conservative estimates since potential errors from assumed ice chemistry or surface temperature values are negligible compared with the difference between models ( MacGregor and others, 2015). Therefore, any comparison of the observed (Fig.…”

Section: Resultsmentioning

confidence: 83%

“…Together, these approaches stand to enable observationbased estimates of the entire vertical attenuation and temperature profile across entire ice sheets and glacier catchments. (Wolff and others, 1997;MacGregor and others, 2015). Background contours are ice thickness (Fretwell and others, 2013).…”

Section: Discussionmentioning

confidence: 99%

“…Recent work has demonstrated the use of radar reflections from englacial layers to constrain attenuation rates and temperatures as a function of depth within the upper portion of the Greenland ice sheet ( MacGregor and others, 2015). This technique exploits the dependence of radar attenuation on ice temperature (Gudmandsen, 1971;Bogorodsky and others, 1985;Corr and others, 1993;Wolff and others, 1997;MacGregor and others, 2007) by assuming constant reflectivity values for internal layers (Drews and others, 2009;MacGregor and others, 2015).…”

Section: Introductionmentioning

confidence: 99%

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Adaptively constraining radar attenuation and temperature across the Thwaites Glacier catchment using bed echoes

et al. 2016

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ABSTRACT. Englacial temperature is a major control on ice rheology and flow. However, it is difficult to measure at the glacier to ice-sheet scale. As a result, ice-sheet models must make assumptions about englacial temperature and rheology, which affect sea level projections. This is problematic if fundamental processes are not captured by models due to a lack of observationally constrained ice temperature values. Although radar sounding data have been exploited to constrain the temperature structure of the Greenland ice sheet using englacial layers, this approach is limited to areas and depths where these layers exist intact. In order to extend empirical radar-based temperature estimation beyond this limitation, we present a new technique for estimating englacial attenuation rates for the entire ice column using adaptive fitting of unfocused radar bed echoes based on the correlation of ice thickness and corrected bed echo power. We apply this technique to an airborne survey of Thwaites Glacier in West Antarctica and compare the results with temperatures and attenuation rates from a numerical ice-sheet model. We find that the estimated attenuation rates reproduce modelled patterns and values across the catchment with the greatest differences near steeply sloping bed topography.

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Section: Resultsmentioning

confidence: 83%

Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Adaptively constraining radar attenuation and temperature across the Thwaites Glacier catchment using bed echoes

et al. 2016

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“…Bulk material properties of the glacier bed can, in principle, be determined using the basal reflectivity (Bogorodsky et al, 1983;Peters et al, 2005;Jacobel et al, 2009;Schroeder et al, 2016). Performing basal reflectivity analysis on ice-sheet-wide scale is, however, greatly limited by uncertainty and spatial variation in englacial radar attenuation (Matsuoka, 2011;Matsuoka et al, 2012;MacGregor et al, 2012MacGregor et al, , 2015Jordan et al, 2016). In contrast to bulk properties, subglacial roughness analysis methods are (nearly) independent of radar attenuation.…”

Section: Introductionmentioning

confidence: 99%

Self-affine subglacial roughness: consequences for radar scattering and basal water discrimination in northern Greenland

et al. 2017

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Abstract. Subglacial roughness can be determined at a variety of length scales from radio-echo sounding (RES) data either via statistical analysis of topography or inferred from basal radar scattering. Past studies have demonstrated that subglacial terrain exhibits self-affine (power law) roughness scaling behaviour, but existing radar scattering models do not take this into account. Here, using RES data from northern Greenland, we introduce a self-affine statistical framework that enables a consistent integration of topographicscale roughness with the electromagnetic theory of radar scattering. We demonstrate that the degree of radar scattering, quantified using the waveform abruptness (pulse peakiness), is topographically controlled by the Hurst (roughness power law) exponent. Notably, specular bed reflections are associated with a lower Hurst exponent, with diffuse scattering associated with a higher Hurst exponent. Abrupt waveforms (specular reflections) have previously been used as a RES diagnostic for basal water, and to test this assumption we compare our radar scattering map with a recent prediction for the basal thermal state. We demonstrate that the majority of thawed regions (above pressure melting point) exhibit a diffuse scattering signature, which is in contradiction to the prior approach. Self-affine statistics provide a generalised model for subglacial terrain and can improve our understanding of the relationship between basal properties and ice-sheet dynamics.

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“…Ice thickness profiles, internal layers, bed topography, and basal conditions revealed by radar echo sounding are widely used for analysis and modeling of ice mass balance, temperature, flow, and dynamics (Shepherd et al, 2012;MacGregor, 2015;Bamber et al, 2013;Hughes et al, 2016). Many previous studies show that ice properties, such as the birefringence and crystal orientation fabric (COF), could be inferred from multi-polarization radar measurements.…”

Section: Introductionmentioning

confidence: 99%

Multi-channel and multi-polarization radar measurements around the NEEM site

González

Leuschen

et al. 2018

The Cryosphere

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Abstract. Ice properties inferred from multi-polarization measurements, such as birefringence and crystal orientation fabric (COF), can provide insight into ice strain, viscosity, and ice flow. In 2008, the Center for Remote Sensing of Ice Sheets (CReSIS) used a ground-based VHF (very high frequency) radar to take multi-channel and multi-polarization measurements around the NEEM (North Greenland Eemian Ice Drilling) site. The system operated with 30 MHz bandwidth at a center frequency of 150 MHz. This paper describes the radar system, antenna configurations, data collection, and processing and analysis of this data set. Within the framework derived from uniaxial ice crystal model, we found that ice birefringence dominates the power variation patterns of co-polarization and cross-polarization measurements in the area of 100 km 2 around the ice core site. The phase shift between ordinary and extraordinary waves increases nonlinearly with depth. The ice optic axis lies in planes that are close to the vertical plane and perpendicular or parallel to the ice divide depending on depth. The ice optic axis has an average tilt angle of about 11.6 • vertically, and its plane may rotate either clockwise or counterclockwise by about 10 • across the 100 km 2 area, and at a specific location the plane may rotate slightly counterclockwise as depth increases. Comparisons between the radar observations, simulations, and ice core fabric data are in very good agreement. We calculated the effective colatitude at different depths by using azimuth and colatitude measurements of the c axis of ice crystals. We obtained an average effective c axis tilt angle of 9.6 • from the vertical axis, very comparable to the average optic axis tilt angle estimated from radar polarization measurements. The comparisons give us confidence in applying this polarimetric radio echo sounding technique to infer profiles of ice fabric in locations where there are no ice core measurements.

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Radar attenuation and temperature within the Greenland Ice Sheet

Cited by 92 publications

References 96 publications

Adaptively constraining radar attenuation and temperature across the Thwaites Glacier catchment using bed echoes

Adaptively constraining radar attenuation and temperature across the Thwaites Glacier catchment using bed echoes

Self-affine subglacial roughness: consequences for radar scattering and basal water discrimination in northern Greenland

Multi-channel and multi-polarization radar measurements around the NEEM site

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