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The Basal Roughness of Ice Sheets from Radio-Echo Sounding

Periodic Reporting for period 2 - BRISRES (The Basal Roughness of Ice Sheets from Radio-Echo Sounding)

Berichtszeitraum: 2019-08-21 bis 2020-08-20

Radar-sounding enables constraints to be placed on the basal topography of ice sheets, the physical properties of the basal interface, and the internal structure of ice sheets. In turn, this radar-derived information enables more accurate model predictions of ice-sheet evolution and the contribution of ice sheets to global sea-level rise in a warming world. Radar-sounding also enables exploration and physical characterization of inaccessible regions of the earth's surface, with the `discovery’ aspect of the technique of wide-interest in society. This research fellowship focused on two distinct research themes within field of radar-sounding, and these are described separately below.

First, this fellowship focused on using radar-sounding data to constrain basal properties of the Greenland Ice Sheet - primarily bed roughness, but also basal water and temperature. As a primary objective, this involved producing a new map of `fine-scale’ roughness (hypothesized to relate to friction as a glacier slides over its bed) and comparing this with ice motion. A basal water map was also produced and compared with basal temperature and geothermal heat. Within this research theme, additional collaborations were carried out in Antarctic subglacial hydrology, Greenland geothermal heat, and adaptation of similar techniques to planetary radar.

Secondly, this fellowship has focused on the measurement of ice microstructure (fabric) from polarimetric radar sounding, and incorporating these measurements into ice-flow models. The fellowship objectives included the development of a new polarimetric measurement technique, development of an ice-flow model parameterization scheme, and application of the new methods to fast-flowing ice streams (the main drainage pathways of ice sheets). Within this general research theme, additional work was performed in modelling the impact of fabric on in-ice neutrino experiments and developing a processing scheme for englacial velocity estimation from radar-sounding interferometry.

The scientific conclusions from the first research theme include:
• Widespread basal water storage is present in the northern and eastern interior of the Greenland Ice Sheet, where there is generally higher geothermal heat flux. Notably, a quasilinear “corridor” of basal water, which extends from the central interior to the northern margin, is present (Jordan et al. 2019).
• Basal roughness exhibits an exponential scaling relationship with ice surface velocity parallel, but not perpendicular, to flow direction in fast-flowing regions of the Greenland Ice Sheet (Cooper et al. 2019).
• Many slow-flowing regions of the Greenland Ice Sheet have smoother beds, which, through combination with analyses of the underlying geology, is likely due to the presence of an undeformable hard bed (Cooper et al. 2019).

The scientific conclusions from the second research theme include:
• The NEEM ice divide has near unchanging azimuthal fabric orientation with ice depth, which is consistent with a stable ice-flow history (Jordan et al. 2019).
• Ice fabric within Whillans Ice stream develops in the firn layer, and exhibits a rotation within the ice column (Jordan et al. 2020a)
•`Radio polarization time delays’, due ice fabric, were demonstrated to facilitate range reconstruction in neutrino experiments (Jordan et al. 2020b).
• Ice fabric with Rutford Ice Stream is consistent with flow-induced development (Jordan et al. 2020c).
• Ice fabric within Rutford Ice stream acts to enhance lateral-shear in the ice-stream margins and compression in the ice-stream center (Jordan et al. 2020c).
• Englacial layers behave as `persistent scatterers’, enabling measurement of englacial velocity from radar-sounding interferometry (Castelletti et al. 2020).
Work performed within first research theme included extraction of Greenland bed-echo data/waveforms from NASA IceBridge campaigns, developing a data combination scheme for different radar systems, and exploration of the relationship between roughness, ice velocity and subglacial geology for Greenland. Work performed within the second research theme included the development of a polarimetric radar data analysis method to measure ice fabric microstructure, adaptation of polarimetric radar backscatter model to interpret data, validation using the method using ice core fabric data, and development of a parametrization scheme for ice-flow models.

Within the first research theme, Jordan et al. 2018 and Cooper et al. 2019 both produced new constraints for the basal properties of the Greenland Ice Sheet (water/thermal regime and bed roughness respectively). A notable finding in Jordan et al. 2018 was the basal water distribution in the interior of the ice sheet supports recent predictions for the path of Iceland hotspot. A notable finding in Cooper at al. 2019 was that many of the contiguous smoother regions in the interior likely correspond to hard bedrock (rather than the previous interpretation of large pools of water).

Within the second research theme, Jordan et al. 2019 developed a new technique to measure ice crystal orientation fabric from polarimetric radar sounding. The paper performed a detailed comparison/validation with ice core data from the NEEM ice divide, Greenland. Jordan et al. 2020a extended this method to the ground-based radar system (commonly used glacier geophysics field investigations) and performed an exploratory investigation of the technique within Whillans Ice Stream. A notable result was the observation of fabric rotation within the ice column. Jordan et al. 2020b adapted radio propagation models from polarimetric radar-sounding, to be used in the calibration of in-ice neutrino experiments. Jordan et al. 2020c developed a parametrization scheme for anisotropic ice-flow models using the radar fabric measurements. This paper then trialed the new framework in Rutford Ice Stream, and explored the impact on ice viscosity and deformation within the Ice Stream. Castelletti et al. 2020 developed a new method to measure englacial velocities from airborne radar-sounding interferometry.
State of the art: Within the first research theme, Jordan et al. 2018 and Cooper et al. 2019 both developed radar data analysis methods that addressed new approaches to combine radar systems and compiled with more radar data-sets than previous studies. Jordan et al. 2018 performed a new comparison between radar-derived basal water, other predictions for the basal thermal regime, geothermal heat flux. Cooper et al. 2019 contained the first assessment of bed roughness-anisotropy across the Greenland Ice Sheet and the first assessment of scattering-derived roughness at the ice-sheet scale. Within the second research theme, Jordan et al. 2019, demonstrated that the new (phase-based) technique to measure fabric improved significantly upon past (power-based) methods. Jordan et al. 2020c, developed a novel scheme to input radar ice fabric measurements into ice-flow models, via parametrization of ice viscosity.

Wider society: The fellowship research provided new radar-derived constraints and parameterization schemes for numerical ice-sheet models are used to predict sea-level rise
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