Abstract
ABSTRACTThe basal interface of glaciers is generally not directly observable. Geophysical inverse methods are therefore used to infer basal parameters from surface observations. Such methods can also provide information about potential inadequacies of the forward model. Ideally an inverse problem can be regularized so that the differences between modeled and observed surface velocities reflect observational errors. However, deficiencies in the forward model usually result in additional errors. Here we use the spatial pattern of velocity residuals to discuss the main error sources for basal stress inversions for Jakobshavn Isbræ, Greenland. Synthetic tests with prescribed patterns of basal yield stress with varying length scales are then used to investigate different weighting functions for the data-model misfit and for the ability of the inversion to resolve details in basal yield stress. We also test real-data inversions for their sensitivities to prior estimate, forward model parameters, data gaps, and temperature fields. We find that velocity errors are not sufficient to explain the residual patterns of real-data inversions. Conversely, ice-geometry errors and especially simulated errors in model simplifications are capable of reproducing similar error patterns and magnitudes. We suggest that residual patterns can provide useful guidance for forward model improvements.
Highlights
Due to the changing climate and its implication on sea level, our ability to predict centennial-scale future ice-sheet behavior through modeling has received considerable attention (IPCC, 2013)
High residuals upstream Improved residual upstream, but wave-like features remain Wave-like residual structure disappears, but Shallow Shelf Approximation (SSA) not realistic upstream Wave-like residual structure disappears Small-scale structure is not corrected of experiments, we investigate which of the possible sources of error is most likely causing these residual patterns
An investigation of the residual patterns shows that the bed topography experiment (Exp. 6) has high residual values in the fast-flowing terminus region, they do not reach the widespread values of 50–100 m a−1 seen in the real-data inversion
Summary
Due to the changing climate and its implication on sea level, our ability to predict centennial-scale future ice-sheet behavior through modeling has received considerable attention (IPCC, 2013). In a series of synthetic experiments, the velocity residual pattern and magnitude for surface velocity, ice geometry, and modeling errors are examined All these experiments concentrate on the terminus region (first ∼100 km), where the residuals are high in real-data inversions. Away from the terminus and the deep trough, residual values are small and the residual patterns suggest that the errors in velocity observations are the main cause for the mismatch. It is unclear, how far upstream basal yield stress can be resolved with present-day ice velocities. We discuss self-consistency of ice geometry and model, and how this affects inversion results
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