Elmer/Ice News

Seasonal evolution of basal environment conditions of Russell sector, West Greenland, inverted from satellite observation of surface flow

Seasonal evolution of basal environment conditions of Russell sector, West Greenland, inverted from satellite observation of surface flow

https://tc.copernicus.org/articles/15/5675/2021/

Due to increasing surface melting on the Greenland ice sheet, better constraints on seasonally evolving basal water pressure and sliding speed are required by models. Here we assess the potential of using inverse methods on a dense time series of surface speeds to recover the seasonal evolution of the basal conditions in a well-documented region in southwest Greenland. Using data compiled from multiple satellite missions, we document seasonally evolving surface velocities with a temporal resolution of 2 weeks between 2015 and 2019. We then apply the inverse control method using the ice flow model Elmer/Ice to infer the basal sliding and friction corresponding to each of the 24 surface velocity data sets. Near the margin where the uncertainty in the velocity and bed topography are small, we obtain clear seasonal variations that can be mostly interpreted in terms of an effective-pressure-based hard-bed friction law. We find for valley bottoms or “troughs” in the bed topography that the changes in modelled basal conditions directly respond to local modelled water pressure variations, while the link is more complex for subglacial “ridges” which are often non-locally forced. At the catchment scale, in-phase variations in the water pressure, surface velocities, and surface runoff variations are found. Our results show that time series inversions of observed surface velocities can be used to understand the evolution of basal conditions over different timescales and could therefore serve as an intermediate validation for subglacial hydrology models to achieve better coupling with ice flow models.

Read more: Derkacheva, A., Gillet-Chaulet, F., Mouginot, J., Jager, E., Maier, N., Cook, S., 2021. Seasonal evolution of basal environment conditions of Russell sector, West Greenland, inverted from satellite observation of surface flow. The Cryosphere 15, 5675–5704. https://doi.org/10.5194/tc-15-5675-2021

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Modeling Ice-Crystal Fabric as a Proxy for Ice-Stream Stability

lilien2021Glacier ice is composed of individual grains, or crystals. These grains can have different orientations, similar to how a pile of snow has many flakes all pointing in different directions. When ice accumulates as snowfall, the grains point in approximately random directions. However, as glaciers move, the orientation of the grains is changed, leading to characteristic "fabrics," where the grains tend to point in distinctive directions. Here, we try to understand how one could work backwards, from the orientation of the grains, to determine how the ice flowed in the past. We use an ice-flow model to understand whether instabilities in ice streams, "rivers" of fast flowing ice in Greenland and Antarctica, would be recorded in the fabric. We find that changes in ice-stream flow could indeed be seen in the fabric for thousands or tens of thousands of years, depending on the exact type of change. We then show that these changes to fabric are large enough that they could be measured in ice cores or with specialized ice-penetrating radars. This helps lay the groundwork for better understanding of long-term changes to ice flow, which is important but hard-to-measure context for modern ice-stream retreat.

Read more: Lilien, D. A., N. M. Rathmann, C. S. Hvidberg and D. Dahl-Jensen, 2021. Modeling ice-crystal fabric as a proxy for ice-stream stability. Journal of Geophysical Research: Earth Surface, 126, e2021JF006306, doi: 10.1029/2021JF006306


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Do Existing Theories Explain Seasonal to Multi-Decadal Changes in Glacier Basal Sliding Speed?

gimbert2021Basal sliding is an important component of glacier motion. However, our knowledge of the physics that controls basal sliding is incomplete. This causes large uncertainties in the contribution to sea-level rise predicted for ice sheets over the coming century. Here, we test our understanding of basal sliding against particularly unique observations, made via a rotating bicycle wheel that has been continuously measuring glacier basal motion over three decades within excavated tunnels under the Argentière Glacier in the French Alps. Due to stress changes from significant glacier thinning over the multi-decadal period we are able to establish an observationally derived sliding law and compare it with expectations from theory. We report many observational features that are in striking agreement with theoretical predictions from glacier sliding over bedrock beds. However, we also observe an undocumented behavior of stress stabilization during the melting period at a specific stress state known as the Iken's limit. This behavior causes long term sliding velocities to follow a simple power law scaling with bed shear stress. This finding has the potential of strongly simplifying and reducing uncertainty on predicting glaciers response to climate change.

Read more: Gimbert F., A. Gilbert, O.  Gagliardini, C.  Vincent and L. Moreau, 2021.  Do existing theories explain seasonal to multi-decadal changes in glacier basal sliding speed? Geophysical Research Letters,  48, e2021GL092858. https://doi.org/10.1029/2021GL092858

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