Thorben Dunse

Recent fluctuations in the extent of the firn area of Austfonna, Svalbard, inferred from GPR

Abstract In spring during 2004–07 we conducted ground-penetrating radar (GPR) measurements on the Austfonna ice cap, Svalbard, with the original aim of mapping the thickness and distribution of winter snow. Here, we further exploit the information content of the data and derive a multi-year sequence of glacier-facies distribution that provides valuable spatial information about the total surface mass balance (SMB) of the ice cap, beyond the usually evaluated winter balance. We find that following an initial decrease in the extent of the firn area (2003–04), the firn line lowered within two subsequent years by ∼40–100m elevation in the north and west and 150–230m in the south and east of the ice cap, corresponding to a lateral expansion of the firn area along the profiles by up to 7.3 and 13.3 km, respectively. The growth of the firn area is in line with stake measurements from Etonbreen that indicate a trend towards less negative SMB over the corresponding period.

Permanent fast flow versus cyclic surge behaviour: numerical simulations of the Austfonna ice cap, Svalbard

A large part of the ice flux within ice caps occurs through spatially limited fast-flowing units. Some of them permanently maintain fast flow, whereas others operate in an oscillatory mode, characterized by short-lived active phases followed by long quiescent phases. This surge-type behaviour results from intrinsic rather than external factors, thus complicating estimates of glacier response to climate change. Here we present numerical model results from Austfonna, an ice cap on Svalbard that comprises several surge-type basins. Previous studies have suggested a thermally controlled soft-bed surge mechanism for Svalbard. We systematically change the parameters that govern the nature of basal motion and thereby control the transition between permanent and oscillatory fast flow. Surge-type behaviour is realized by a relatively abrupt onset of basal sliding when basal temperatures approach the pressure-melting point and enhanced sliding of marine grounded ice. Irrespective of the dynamic regime, the absence of considerable volumes of temperate ice, both in the observed and simulated ice cap, indicates that fast flow is accomplished by basal motion over a temperate bed. Given an idealized present-day climate, the equilibrium ice-cap size varies significantly, depending on the chosen parameters.

Characteristics and small-scale variability of GPR signals and their relation to snow accumulation in Greenland's percolation zone

We investigate snowpack properties at a site in west-central Greenland with ground- penetrating radar (GPR), supplemented by stratigraphic records from snow pits and shallow firn cores. GPR data were collected at a validation test site for CryoSat (T05 on the ExpGlaciologiques Internationales au Groenland (EGIG) line) over a 100 m × 100 m grid and along 1 km sections at fre- quencies of 500 and 800 MHz. Several internal reflection horizons (IRHs) down to a depth of 10 m were tracked. IRHs are usually related to ice-layer clusters in vertically bounded sequences that obtain their initial characteristics near the surface during the melt season. Warm conditions in the following melt season can change these characteristics by percolating meltwater. In cold conditions, smaller melt volumes at the surface can lead to faint IRHs. The absence of simple mechanisms for internal layer origin emphasizes the need for independent dating to reliably interpret remotely sensed radar data. Our GPR-derived depth of the 2003 summer surface of 1.48 m (measured in 2004) is confirmed by snow-pit observations. The distribution of IRH depths on a 1 km scale reveals a gradient of increasing accumu- lation to the northeast of about 5 cm w.e. km −1 . We find that point measurements of accumulation in this area are representative only over several hundred metres, with uncertainties of about 15% of the spatial mean.

Comparison of Airborne Radar Altimeter and Ground-based Ku-band Radar Measurements on the Ice Cap Austfonna, Svalbard

We compare data from the European Space Agencys (ESA) Airborne SAR/Interferometric Radar Altimeter System (ASIRAS) with ground-based Very High Bandwidth (VHB) stepped frequency radar measurements in the Ku-band. Using the VHB radar we have been able to pinpoint the major backscatter sources within the accumulation area. The ground-based and airborne waveforms show good agreement and we therefore find the ground-based measurements valuable for validation and interpretation of the airborne altimeter waveforms. The comparison shows that the surface and the Last year Summer Surface (LSS) can be tracked in the airborne data, but fails at lower elevations with snow depths less than ~1 m. For ground truth, i.e. snow depth, we use ground based radar profiles (800 MHz), snow pits, snow probing, and density retrieved from 7-12 m deep boreholes using a neutron probe.