ORCID Profile
0000-0001-7140-0931
Current Organisations
Københavns Universitet
,
Københavns Universitet Niels Bohr Instituttet
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Publisher: Cambridge University Press (CUP)
Date: 28-01-2021
DOI: 10.1017/JOG.2020.117
Abstract: Bulk directional enhancement factors are determined for axisymmetric (girdle and single-maximum) orientation fabrics using a transversely isotropic grain rheology with an orientation-dependent non-linear grain fluidity. Compared to grain fluidities that are simplified as orientation independent, we find that bulk strain-rate enhancements for intermediate-to-strong axisymmetric fabrics can be up to a factor of ten larger, assuming stress homogenization over the polycrystal scale. Our work thus extends previous results based on simple basal slip (Schmid) grain rheologies to the transversely isotropic rheology, which has implications for large-scale anisotropic ice-flow modelling that relies on a transversely isotropic grain rheology. In order to derive bulk enhancement factors for arbitrary evolving fabrics, we expand the c-axis distribution in terms of a spherical harmonic series, which allows the rheology-required structure tensors through order eight to easily be calculated and provides an alternative to current structure-tensor-based modelling.
Publisher: Springer Science and Business Media LLC
Date: 23-09-2022
DOI: 10.1038/S41467-022-32999-2
Abstract: Mass loss near the ice-sheet margin is evident from remote sensing as frontal retreat and increases in ice velocities. Velocities in the ice sheet interior are orders of magnitude smaller, making it challenging to detect velocity change. Here, we analyze a 35-year record of remotely sensed velocities, and a 6-year record of repeated GPS observations, at the East Greenland Ice-core Project (EastGRIP), located in the middle of the Northeast-Greenland Ice Stream (NEGIS). We find that the shear margins of NEGIS are accelerating, indicating a widening of the ice stream. We demonstrate that the widening of the ice stream is unlikely to be a response to recent changes at the outlets of NEGIS. Modelling indicates that the observed spatial fingerprint of acceleration is more consistent with a softening of the shear margin, e.g. due to evolving fabric or temperature, than a response to external forcing at the surface or bed.
Publisher: American Geophysical Union (AGU)
Date: 09-01-2022
DOI: 10.1029/2021GL096244
Abstract: We introduce a transfer matrix model for radio‐wave propagation through layered anisotropic ice that permits an arbitrary dielectric permittivity tensor in each layer. The model is used to investigate how crystal orientation fabrics without a vertical principal direction affect polarimetric radar returns over glaciers and ice sheets. By expanding the c ‐axis orientation distribution in terms of a spherical harmonic series, we find that radar returns from synthetic fabric profiles are relatively insensitive to the harmonic mode responsible for a nonvertical principal direction however, only for normally incident waves. Consequently, the strength of this mode might be relatively difficult to infer in glaciers and ice sheets, which in turn has implications for the ability to determine the full second‐order structure tensor, needed to infer the local flow regime, flow history, or to represent the directional viscosity structure of glacier ice for ice‐flow modeling.
Publisher: American Geophysical Union (AGU)
Date: 09-2021
DOI: 10.1029/2021JF006306
Abstract: The crystal structure within an ice sheet evolves in response to deformation hence ice‐crystal fabric records ice‐flow history. However, the complexity of crystal‐fabric evolution, and the lack of model results with which to compare data, limit the usefulness of fabric measurements, particularly in areas with complex ice dynamics. Here, we use an ice‐flow model to identify characteristic fabrics associated with ice‐stream onset, with the goal of aiding interpretation of fabric measurements. Using time‐dependent model simulations, we identify how crystal fabric may be used to diagnose changes in an ice stream's speed or lateral position. Consistent with previous work, we find that fabric within an ice stream is generally a vertical girdle, though horizontal shear can lead to a horizontal single maximum. Transient simulations demonstrate that effects from changes in flow may be recorded in the crystal fabric for thousands of years after an ice stream activates and more than ten thousand years after an ice stream stagnates. Both transient and steady effects on fabric are sufficiently large as to be measurable in ice cores or with polarized radar, suggesting that in certain scenarios fabric could be used to identify past flow changes. These results could be used to design radar surveys in areas where ice streams are known to deactivate, such as the Siple Coast in Antarctica, or where they may migrate laterally or widen, such as in Northeast Greenland.
No related grants have been discovered for Nicholas Rathmann.