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

Thomas M Jordan,Michael A Cooper,Dustin M Schroeder,Jonathan L Bamber,Martin J Siegert,Christopher N Williams,John D Paden

doi:10.5194/tc-11-1247-2017

Abstract

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 topographic-scale 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.

Highlights

In this study we used recent OIB radio-echo sounding (RES) data to demonstrate that subglacial roughness in northern Greenland exhibits self-affine scaling behaviour, with pronounced spatial variation in the Hurst exponent
We demonstrated an agreement between the predictions of the radar scattering model and the statistically distributed inverse relationship that is observed between the Hurst exponent and waveform abruptness
This enables us to conclude that self-affine statistics provide a valuable framework in understanding the topographic control which influences ice-penetrating radar scattering from glacier beds

Summary

Introduction

With the development of the newest generation of thermomechanical ice-sheet models, there has been a growing awareness that better constraining the physical properties of the glacier bed is essential for improving their predictive capability (e.g. Price et al, 2011; Seroussi et al, 2013; Nowicki et al, 2013; Shannon et al, 2013; Sergienko et al, 2014; Ritz et al, 2015; Cornford et al, 2015). With the development of the newest generation of thermomechanical ice-sheet models, there has been a growing awareness that better constraining the physical properties of the glacier bed is essential for improving their predictive capability Airborne radio-echo sounding (RES) is the only existing remote sensing technique that can acquire bed data with sufficient spatial coverage to enable subglacial information to be obtained across the ice sheets (refer to Pritchard (2014) and Bamber et al (2013a) for recent Antarctic and Greenland coverage maps). Data analysis methods which seek to improve the clarity and glaciological utility of RES-derived subglacial information are undergoing a period of rapid development

Methods

Results

Discussion

Conclusion