Abstract
Given high-resolution satellite-derived surface elevation and velocity data, ice-sheet models generally estimate mechanical basal boundary conditions using surface-to-bed inversion methods. In this work, we address the sensitivity of results from inversion methods to the accuracy of the bed elevation data on Pine Island Glacier. We show that misfit between observations and model output is reduced when high-resolution bed topography is used in the inverse model. By looking at results with a range of detail included in the bed elevation, we consider the separation of basal drag due to the bed topography (form drag) and that due to inherent bed properties (skin drag). The mean value of basal shear stress is reduced when more detailed topography is included in the model. This suggests that without a fully resolved bed a significant amount of the basal shear stress recovered from inversion methods may be due to the unresolved bed topography. However, the spatial structure of the retrieved fields is robust as the bed accuracy is varied; the fields are instead sensitive to the degree of regularisation applied to the inversion. While the implications for the future temporal evolution of PIG are not quantified here directly, our work raises the possibility that skin drag may be overestimated in the current generation of numerical ice-sheet models of this area. These shortcomings could be overcome by inverting simultaneously for both bed topography and basal slipperiness.
Highlights
Pine Island Glacier (PIG) has been one of the fastest flowing and most rapidly retreating glaciers in Antarctica over the past few decades (e.g., Mouginot et al, 2014; Rignot et al, 2014; Smith et al, 2017)
The maximum in basal shear stress overlies the sharp transition in bed elevation, whether or not the domain is defined with this resolved in the topography
This is true across all six of the sites where we have the high-resolution topography from DELORES radar measurements. We propose that this result is due to the form drag that the topography introduces into the problem
Summary
Pine Island Glacier (PIG) has been one of the fastest flowing and most rapidly retreating glaciers in Antarctica over the past few decades (e.g., Mouginot et al, 2014; Rignot et al, 2014; Smith et al, 2017). Spatial variations illustrate changes in slipperiness at the ice-bed boundary; these may be due to changes in basal conditions (e.g. more water at the boundary results in slippy conditions) or due to inaccuracies in the data, with the model producing slipperiness perturbations in an attempt to better match the data With regard to the latter, given the accuracy of modern satellite data, the main source of error is usually in the ice depth field, i.e., how well the bed elevation is known. We make comparisons with results that use both the smoother BEDMAP2 dataset and a completely flat bed, while keeping the surface elevation resolved to a high resolution in all cases This allows us to consider the robustness of estimates of basal slipperiness and resulting basal stress to the resolution of the bed topography. As well as absolute values of the derived fields, and consider if there is a way to take how well the bed elevation is known into account when interpreting basal stress fields
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
