Polarimetric radar reveals the spatial distribution of ice fabric at domes in East Antarctica

Abstract

Abstract. Ice crystals are mechanically and dielectrically anisotropic. They progressively align under cumulative deformation, forming an ice crystal orientation fabric that, in turn, impacts ice deformation. However, almost all the observations of fabric are from ice core analysis and its interplay with the flow is unclear. Here, we present a non-linear inverse approach that combines radar polarimetry with vertical changes in anisotropic reflection to extract, for the first time, the full orientation tensor. The orientation tensor is routinely used to synthesize fabric information and it is used in anisotropic ice flow models. We validate our approach at two Antarctic ice-core sites (EDC and EDML) in contrasting flow regimes. Spatial variability of ice-fabric characteristics in the dome-to-flank transition near Dome C is quantified with 20 more sites located along a 36 km long cross-section. Local horizontal anisotropy increases under the dome summit and decreases away from the dome summit. We suggest that this is a consequence of the non-linear rheology of ice also known as Raymond effect. On larger spatial scales, horizontal anisotropy increases with increasing distance from the dome. At most of the sites, the main driver of ice-fabric evolution is vertical compression, yet our data show that ice fabric horizontal distribution is consistent with the present horizontal flow. Our method, which uses co- and cross polarimetric radar data suitable for profiling radar applications, can constrain ice-fabric distribution on a spatial scale comparable to ice flow observations and models.

Rapid and accurate polarimetric radar measurements of ice crystal fabric orientation at the Western Antarctic Ice Sheet (WAIS) Divide ice core site

“…The nomenclature attached to the h and v alignments is indicative of the electric field (Fig. 1) and is consistent with those used in previous polarimetric ApRES studies of ice fabric (Jordan et al, 2019(Jordan et al, , 2020cErshadi et al, 2021) with the exception of Brisbourne et al (2019), which reverses the two assignments (i.e. h in our study corresponds to v in their study, and v in our study corresponds to h in their study).…”

“…While older radar studies infer azimuthal fabric asymmetry at broad (hundreds to thousands of metres) depth resolution by investigating the depth between sequential "co-polarized nodes" (e.g. Fujita et al, 2006;Matsuoka et al, 2003Matsuoka et al, , 2012, more recent studies are able to quantitatively calculate the azimuthal fabric asymmetry via the polarimetric coherence method at comparatively higher resolutions (tens of metres) (Dall, 2010;Jordan et al, 2019;Ershadi et al, 2021). The ability to directly validate our results with measurements from the WAIS Divide ice core to a satisfactory degree gives confidence in our choice of processing parameters.…”

“…While the induced polarimetric dependence through these bubbles was calculated to be minimal, where microscopic vertically varying COF dominates the observed anisotropy, this effect was observed to be amplified at shallower depths. Although the observed anisotropic scattering can be exploited to infer the strength of the third eigenvalue (E 3 ) under assumptions of fabric isotropy at the ice surface (Ershadi et al, 2021), we do not attempt this method given our observations of significant fabric anisotropy in the firn layer (inset of Fig. 6).…”

“…Although polarimetric radar sounding data analysis has been implemented to detect horizontally asymmetric COF for almost half a century (e.g. Hargreaves, 1977), it has not yet been widely implemented, with the majority of radar studies that measure COF variations in ice being conducted within the last 15 years at coincident ice-coring sites for comparative analysis (Dall, 2010(Dall, , 2021Drews et al, 2012;Eisen et al, 2007;Ershadi et al, 2021;Fujita et al, 2006;Jordan et al, 2019Jordan et al, , 2020aLi et al, 2018;Matsuoka et al, 2003Matsuoka et al, , 2009Matsuoka et al, , 2012. Moreover, the majority of previous polarimetric radar studies infer COF at a coarse azimuthal resolution that is limited by the number of observations made along an acquisition plane that rotates around an azimuth centre (Brisbourne et al, 2019;Doake et al, 2002Doake et al, , 2003Jordan et al, 2020c;Matsuoka et al, 2003Matsuoka et al, , 2012.…”

Inferring Ice Fabric From Birefringence Loss in Airborne Radargrams: Application to the Eastern Shear Margin of Thwaites Glacier, West Antarctica

“…The resulting phase shift rotates the electric field and can cause polarization misalignment with linearly polarized antennas, resulting in power loss (Doake, 1981). Previous radar studies have utilized multi‐ and quadrature‐polarization setups to observe and quantify COF, and have shown good agreement with measurements from thin section analyses at coincident ice core sites (Fujita et al., 2006; Eisen et al., 2007; Ershadi et al., 2021; Dall, 2010, 2021; K. Matsuoka et al., 2003, 2009; Li et al., 2018; Jordan et al., 2019; Jordan, Besson, et al., 2020; Young et al., 2020) as well as provided evidence of more complex fabric at sites with more dynamic flow regimes (K. Matsuoka et al., 2012; Brisbourne et al., 2019; Jordan, Schroeder, et al., 2020; Jordan, Martín, et al., 2020). In‐situ observations of ice fabric have been crucial to understanding the stress patterns and behaviors of ice sheets over time and over large areas (e.g., Alley, 1988; Budd, 1972) and have provided constraints on the influence of crystal fabric on ice sheet flow (Azuma, 1994; Martín et al., 2009; Thorsteinsson et al., 2003).…”