Application of a two-step approach for mapping ice thickness to various glacier types on Svalbard

• Published in: The Cryosphere
• Main Authors: J. J. Fürst, F. Gillet-Chaulet, T. J. Benham, J. A. Dowdeswell, M. Grabiec, F. Navarro, R. Pettersson, G. Moholdt, C. Nuth, B. Sass, K. Aas, X. Fettweis, C. Lang, T. Seehaus, M. Braun
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus Publications 2017
• Subjects: geo; envir; Svalbard; Vestfonna; glacier; Ice cap; The Cryosphere
• Online Access: https://doi.org/10.5194/tc-11-2003-2017; https://www.the-cryosphere.net/11/2003/2017/tc-11-2003-2017.pdf; https://doaj.org/article/6ee4e9d1c634478d9a9c326758ca71fd

The basal topography is largely unknown beneath most glaciers and ice caps, and many attempts have been made to estimate a thickness field from other more accessible information at the surface. Here, we present a two-step reconstruction approach for ice thickness that solves mass conservation over single or several connected drainage basins. The approach is applied to a variety of test geometries with abundant thickness measurements including marine- and land-terminating glaciers as well as a 2400 km2 ice cap on Svalbard. The input requirements are kept to a minimum for the first step. In this step, a geometrically controlled, non-local flux solution is converted into thickness values relying on the shallow ice approximation (SIA). In a second step, the thickness field is updated along fast-flowing glacier trunks on the basis of velocity observations. Both steps account for available thickness measurements. Each thickness field is presented together with an error-estimate map based on a formal propagation of input uncertainties. These error estimates point out that the thickness field is least constrained near ice divides or in other stagnant areas. Withholding a share of the thickness measurements, error estimates tend to overestimate mismatch values in a median sense. We also have to accept an aggregate uncertainty of at least 25 % in the reconstructed thickness field for glaciers with very sparse or no observations. For Vestfonna ice cap (VIC), a previous ice volume estimate based on the same measurement record as used here has to be corrected upward by 22 %. We also find that a 13 % area fraction of the ice cap is in fact grounded below sea level. The former 5 % estimate from a direct measurement interpolation exceeds an aggregate maximum range of 6–23 % as inferred from the error estimates here.