Mirko Scheinert

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.

Sea surface topography retrieved from GNSS reflectometry phase data of the GEOHALO flight mission

Sea surface topography observations are deduced from an airborne reflectometry experiment. A GNSS (Global Navigation Satellite System) receiver dedicated for reflectometry was set up aboard the German HALO (High Altitude Long Range) research aircraft. Flights were conducted over the Mediterranean Sea about 3500 m above sea level. A signal path model divided into large- and small-scale contributions is used for phase altimetry. The results depict geoid undulations and resolve anomalies of the sea surface topography. For the whole experiment 65 tracks over the Mediterranean Sea are retrieved and compared with a topography model. Tracks differ between right-handed and left-handed circular polarization. The difference, however, is not significant for this study. Precision and spatial resolution decrease disproportionately at low elevations. Eight tracks with centimeter precision are obtained between 11° and 33° of elevation. At higher elevation angles the number of tracks is significantly reduced due to surface roughness. In future such retrievals could contribute to ocean eddy detection.

New Antarctic gravity anomaly grid for enhanced geodetic and geophysical studies in Antarctica

Gravity surveying is challenging in Antarctica because of its hostile environment and inaccessibility. Nevertheless, many ground-based, airborne and shipborne gravity campaigns have been completed by the geophysical and geodetic communities since the 1980s. We present the first modern Antarctic-wide gravity data compilation derived from 13 million data points covering an area of 10 million km2, which corresponds to 73% coverage of the continent. The remove-compute-restore technique was applied for gridding, which facilitated levelling of the different gravity datasets with respect to an Earth Gravity Model derived from satellite data alone. The resulting free-air and Bouguer gravity anomaly grids of 10 km resolution are publicly available. These grids will enable new high-resolution combined Earth Gravity Models to be derived and represent a major step forward towards solving the geodetic polar data gap problem. They provide a new tool to investigate continental-scale lithospheric structure and geological evolution of Antarctica.

Regional Geoid Improvement over the Antarctic Peninsula Utilizing Airborne Gravity Data

We present an improved quasigeoid of the Palmer Land Region, Antarctic Peninsula, derived from recent aerogravimetry profiles provided by the British Antarctic Survey (BAS). Special focus is given to the treatment of the ice layer covering the bedrock topography, the latter one being regarded as the boundary surface. The remove-compute-restore technique (RCR) with least-squares collocation (LSC) and a point mass modeling, respectively, are applied and compared. In addition to previous studies, an alternative strategy regarding downward continuation has been introduced. Furthermore, the Residual Terrain Model (RTM) has been enhanced to incorporate the individual densities of water, ice and bedrock.

Periodic surface features in coastal East Antarctica

We found evidence for highly periodic dunes in a near-coastal area of Dronning Maud Land, East Antarctica.Analysis of accumulation patterns, derived from ground-penetrating radar (GPR) internal layering, and GPS-based characteristics of surface topography, by means of auto- and cross-covariance, indicate quasi-harmonic oscillations of surface undulations, surface slope, and accumulation.The dunes occur at an elevation range of 1000-1350 m with the dominant wavelength of 5 km and undulations of about 10 m.Accumulation and slope are anticorrelated at zero lag, with a mean annual accumulation of some 140 kgm-2a-1 and variations on the order ofsome 100 kgm-2a-1.Our findings have implications for studies aiming at the mass balance of coastal areas in Antarctica.

Application of a spherical FFT approach in airborne gravimetry

The basic idea of this paper is to modifyPoissons integral for harmonic downward continuation into a convolution formula in the space domain. In this manner, the Fast Fourier Transform can be applied. The method is applied to airborne gravimetry, motivated especially by the Greenland survey. The accuracy of data continuation from the flight-level to the ground is analysed. In particular, the influence of latitudinal extension is investigated, since the introduced convolution formula is exact only for the mid-parallel of the analysed area. The results obtained justify the conclusion that the introduced method is applicable to processing of real data. Extended quadratic areas (up to 2500km × 2500km) in equatorial areas and up to 500km × 500km in regions with latitudes about 75°) can effectively be processed in one single procedure.

The Antarctic Geoid Project: Status Report and Next Activities

The Antarctic Geoid Project (AntGP) forms the Commission Project 2.4 within the IAG Subcommission 2.4 Regional Geoid Determination. The main scientific goal of AntGP is to compile gravity data for the entire Antarctic, thus to fill in the southern polax gap and to improve the terrestrial gravity data coverage. A completed Antarctic gravity dataset will substantially contribute to the determination of the global gravity field in combination with the new satellite missions and will serve as an excellent basis for regional and continental geoid improvement.

Regional geoid of the Weddell Sea, Antarctica, from heterogeneous ground-based gravity data

We present a geoid solution for the Weddell Sea and adjacent continental Antarctic regions. There, a refined geoid is of interest, especially for oceanographic and glaciological applications. For example, to investigate the Weddell Gyre as a part of the Antarctic Circumpolar Current and, thus, of the global ocean circulation, the mean dynamic topography (MDT) is needed. These days, the marine gravity field can be inferred with high and homogeneous resolution from altimetric height profiles of the mean sea surface. However, in areas permanently covered by sea ice as well as in coastal regions, satellite altimetry features deficiencies. Focussing on the Weddell Sea, these aspects are investigated in detail. In these areas, ground-based data that have not been used for geoid computation so far provide additional information in comparison with the existing high-resolution global gravity field models such as EGM2008. The geoid computation is based on the remove–compute–restore approach making use of least-squares collocation. The residual geoid with respect to a release 4 GOCE model adds up to two meters and more in the near-coastal and continental areas of the Weddell Sea region, also in comparison with EGM2008. Consequently, the thus refined geoid serves to compute new estimates of the regional MDT and geostrophic currents.

Subglacial Lake Vostok not expected to discharge water

The question whether Antarcticas largest lake, subglacial Lake Vostok, exchanges water is of interdisciplinary relevance but has been undecided so far. We present the potential pathway, outlet location, and threshold height of subglacial water discharge from this lake based on a quantitative evaluation of the fluid potential. If water left Lake Vostok, it would flow toward Ross Ice Shelf. Discharge would occur first to the east of the southern tip of the lake. At this location the bedrock threshold is 91 ± 23 m higher than the hydrostatic equipotential level of Lake Vostok. It is concluded that Lake Vostok is not likely to reach this level within climatic timescales and that no discharge of liquid water is to be expected. We show that in absence of the ice sheet the Lake Vostok depression would harbor a lake significantly deeper and larger than the present aquifer.

Progress and Prospects of the Antarctic Geoid Project (Commission Project 2.4)

The Antarctic Geoid Project (AntGP) aims at the improvement of terrestrial observations of the gravity field in Antarctica and, eventually, at the improvement of the Antarctic geoid. Until present, vast areas of Antarctica are still unexplored with regard to gravity measurements. The polar data gap due to the deflection from a polar inclination of the respective satellite and the limitation to a certain harmonic degree of resolution prevent a complete, high-resolution data coverage to be obtained from the dedicated gravity satellite missions only. In this context, the International Polar Year (March 2007 to February 2009) provided a framework for broad international and interdisciplinary collaboration, which opened also an excellent opportunity for the realization of new gravity surveys. Especially, there was a focus on airborne gravimetry which provides the most powerful technique to carry out observations in vast and remote areas. The paper will review the present situation and will give an outlook to further activities. The feasibility of the regional geoid improvement in Antarctica will be discussed, utilizing the heterogeneous gravity data available from different surveys and techniques.