R. Rosenau

Mass, volume and velocity of the Antarctic Ice Sheet: present-day changes and error effects

This study examines present-day changes of the Antarctic ice sheet (AIS) by means of different data sets. We make use of monthly gravity field solutions acquired by the Gravity Recovery and Climate Experiment (GRACE) to study mass changes of the AIS for a 10-year period. In addition to ‘standard’ solutions of release 05, solutions based on radial base functions were used. Both solutions reveal an increased mass loss in recent years. For a 6-year period surface-height changes were inferred from laser altimetry data provided by the Ice, Cloud, and land Elevation Satellite (ICESat). The basin-scale volume trends were converted into mass changes and were compared with the GRACE estimates for the same period. Focussing on the Thwaites Glacier, Landsat optical imagery was utilised to determine ice-flow velocities for a period of more than two decades. This data set was extended by means of high-resolution synthetic aperture radar (SAR) data from the TerraSAR-X mission, revealing an accelerated ice flow of all parts of the glacier. ICESat data over the Thwaites Glacier were complemented by digital elevation models inferred from TanDEM-X data. This extended data set exhibits an increased surface lowering in recent times. Passive microwave remote sensing data prove the long-term stability of the accumulation rates in a low accumulation zone in East Antarctica over several decades. Finally, we discuss the main error sources of present-day mass-balance estimates: the glacial isostatic adjustment effect for GRACE as well as the biases between laser operational periods and the volume–mass conversion for ICESat.

Assessing the Current Evolution of the Greenland Ice Sheet by Means of Satellite and Ground-Based Observations

The present study utilises different satellite and ground-based geodetic observations in order to assess the current evolution of the Greenland Ice Sheet (GIS). Satellite gravimetry data acquired by the Gravity Recovery and Climate Experiment are used to derive ice-mass changes for the period from 2003 to 2012. The inferred time series are investigated regarding long-term, seasonal and interannual variations. Laser altimetry data acquired by the Ice, Cloud, and land Elevation Satellite (ICESat) are utilised to solve for linear and seasonal changes in the ice-surface height and to infer independent mass-change estimates for the entire GIS and its major drainage basins. We demonstrate that common signals can be identified in the results of both sensors. Moreover, the analysis of a Global Positioning System (GPS) campaign network in West Greenland for the period 1995–2007 allows us to derive crustal deformation caused by glacial isostatic adjustment (GIA) and by present-day ice-mass changes. ICESat-derived elastic crustal deformations are evaluated comparing them with GPS-observed uplift rates which were corrected for the GIA effect inferred by model predictions. Existing differences can be related to the limited resolution of ICESat. Such differences are mostly evident in dynamical regions such as the Disko Bay region including the rapidly changing Jakobshavn Isbræ, which is investigated in more detail. Glacier flow velocities are inferred from satellite imagery yielding an accelerated flow from 1999 to 2012. Since our GPS observations cover a period of more than a decade, changes in the vertical uplift rates can also be investigated. It turns out that the increased mass loss of the glacier is also reflected by an accelerated vertical uplift.

Temporal flow variations of major outlet glaciers in Greenland using Landsat data

We derived flow velocity fields over the last decade for all outlet glaciers with a frontal width larger than 1 km along the Greenland coast using a feature tracking approach in Landsat imagery. The velocity fields were used to determine both the linear trend and the seasonal variation of the flow regime. Additionally, we map the advance or retreat of the frontal position for individual glaciers. In this paper we focused on two regions in West Greenland (Jakobshavn Isbræ) and Southeast Greenland (Køge Bugt). For the Jakobshavn Isbræ the frontal glacier area accelerated from 15 m/day in 1999 to over 30 m/day in 2011 and thereby retreated over 12 km. In the Køge Bugt region, some glaciers show a similar acceleration in flow velocity while some do not vary in flow velocity significantly.

Utilizing InSAR for the measurement of ice flow velocities and ocean tide induced height changes of ice shelves at their grounding zones and surroundings

The knowledge of accurate surface topography heights and ice dynamics is a key issue for mass flux and mass balance calculations. In this study we use interferometry to derive surface heights and horizontal ice flow velocities for grounded parts in Western Dronning Maud Land, Antarctica. Additionally, a feature and speckle tracking is performed to also estimate horizontal ice flow velocities for the Riiser-Larsen ice shelf and its grounding zone. This flow velocity field is afterwards used to remove the horizontal flow velocity component from the interferograms in order to obtain the vertical changes in the ice shelf area which are caused by ocean tides. The measured height changes are than compared to the predicted height changes of current ocean tide models which allow a validation of the models quality.