G32A-01 INVITED
Glacial Isostatic Adjustment Signals in Geodetic Data Sets
Present-Day Glacial Isostatic Adjustment (GIA) following melt of the Late-Pleistocene ice sheets is a world-wide phenomenon. Its signals intermingle with those from other geophysical sources in geodetic data sets representing gravity, sea level and rotational variations or crustal deformation. Isolating and consecutively removing the GIA contributions in these data sets in order to obtain estimates on ongoing mass transports, like current melt of continental ice, hydrological variations and sea level change, is far from trivial. Though reasonably accurate GIA estimates exist for global scale purposes, based on simplified earth and Pleistocene ice models and on suitable (mainly Relative Sea Level and GPS) data sets, such estimates might not always have the required accuracy for regional applications. Lateral variations in earth structure, existence of shallow low-viscosity zones in lower crust or asthenosphere in many tectonic provinces, and limited knowledge of the Pleistocene ice sheets are a few examples to illustrate that regional GIA-solutions might have strong, mostly still unknown, deviations from solutions based on global models. This will be illustrated by two regional studies, one on sea level change for Northern Europe and the other for ice mass changes in Antarctica. For example, for Northern Europe the effects of regional shallow low-viscosity crustal and asthenospheric zones will be shown to be of importance for refining European sea level change estimations based on tide gauges, satellite altimetry, gravity and other data sets. In Antarctica, the contribution of GIA to the total mass change has a magnitude comparable to what has been measured by the latest gravity and altimetry satellite missions. As a consequence, ice mass balance estimates are strongly dependent on the assumed Pleistocene ice models and on the local Earth structure.
G32A-02
Glacial isostatic adjustment on the Northern Hemisphere - new results from GRACE
The Earth's gravity field mapped by the Gravity Recovery and Climate Experiment (GRACE) satellite mission shows variations due to the integral effect of mass variations in the atmosphere, hydrosphere and geosphere. The Earth's gravity field is provided in form of monthly solutions by several institutions, e.~g. GFZ Potsdam, CSR and JPL. During the GRACE standard processing of these analysis centers, oceanic and atmospheric contributions as well as tidal effects are reduced. The solutions of the analysis centers differ slightly, which is due the application of different reduction models and center-specific processing schemes. We present our investigation of mass variations in the areas of glacial isostatic adjustment (GIA) in North America and Northern Europe from GRACE data. One key issue is the separation of GIA parts and the reduction of the observed quantities by applying dedicated filters (e.~g. isotropic, non-isotropic, and destriping filters) and global models of hydrological variations (e.~g. WGHM, LaDWorld, GLDAS). In a further step, we analyze the results of both regions regarding their reliability, and finally present a comparison to results of a geodynamical modeling and absolute gravity measurements. Our results clearly show that the quality of the GRACE-derived gravity- change signal benefits from improved reduction models and chosen analysis techniques. Nevertheless, the comparison to results of geodynamic models still reveals differences, and thus further studies are in progress.
G32A-03 INVITED
Contributions of Numerical Simulations of Atmosphere-Hydrosphere Dynamics to the Interpretation of Geodetic Observations
Mass redistributions within and mass exchanges among the Earth's subsystems are reflected in the fundamental observables of geodesy, i.e., the Earth's shape, its rotation and gravity field. Since all these parameters of the Earth are precisely observed with various space- and ground-based geodetic techniques, in principle, underlying dynamical processes in the Earth system are monitored. However, due to the integral character of geodetic observations and restrictions concerning resolution in time and space, the processing as well as the interpretation and utilization of the data require independent, interdisciplinary, and consistent methods, e.g., from theory and modelling. Here, a numerical model approach covering transient dynamics and corresponding mass redistributions in the atmosphere-hydrosphere system is presented. This modular system model allows for the connection of fundamentally different measurements as, e.g., atmospheric and hydrospheric mass anomalies, load induced vertical deformations, sea-surface height variations as well as changes in Earth rotation, and therefore provides the opportunity to jointly interpret different types of geodetic and geophysical observations. Exemplarily, ocean mass anomalies from GRACE satellite gravimetry and sea surface height anomalies from Jason 1 satellite altimetry will be connected by means of output of the global ocean model OMCT consistently forced by operational analyses from ECMWF and freshwater fluxes simulated with a hydrological discharge model. Beside the discussion of individual short-comings of both observation techniques as well as of the applied ocean model, it will be demonstrated how reliable information about climate relevant oceanic heat transports can be derived from the global monitoring data.
G32A-04
Non-periodic near-surface mass variations and their impact on Earth's rotation
The variability in the Earth's rotation rate and the orientation of its axes can be observed by modern geodetic space techniques. % To identify the underlying geophysical excitation potential it is common practise to describe the dynamical mechanisms in the Earth's subsystems with independent methods (in-situ observations or numerical models). % In this investigation we analyse the contribution of near-surface mass redistributions on continents using three hydrological models in terms of variations of the effective angular momentum functions χ. % A focus is laid in this study on episodic and quasi-periodic signals in the hydrological χ-functions. % Wavelet analyses reveal similar and significant non-periodic signals in all investigated hydrological models. % These results are verified by comparisons with geodetic observations after accounting for tidal, atmospheric and oceanic effects. % Typical climate relevant variability patterns, such as the Indian monsoon, the El Niño Southern Oscillation (ENSO), the Quasibiennial Oscillation (QBO) and the Tropospheric Biennial Oscillation (TBO), are identified as possible causes of episodic and quasi-periodic variations in continental water storage and, consequently, in those of the χ-functions. % The application of wavelet analyses to individual continents points out that such non-periodic variations in the total hydrological χ-functions are mainly caused by water storage changes in South America, Australia and Asia.
G32A-05
The Effect of Mantle Heterogeneity on Tidal Gravity and Surface Deformations
Precise knowledge of Earth tides is crucial for correcting the signal from satellite gravity surveys or superconducting gravimeters with nanoGal precision. Some local observations show unexplained perturbations to the tidal gravity signal. This noise is generally considered to result from unmodeled loading induced by oceanic tides, but it is possible that an unmodeled component of Earth's body tides contributes to this noise. In fact, the strain and gravity responses of the Earth to body tides are generally computed assuming a radially (or elliptically) stratified Earth. However, seismic tomography surveys and fluid dynamic studies show that thermal convection in Earth's mantle produces significant heterogeneity. Superplumes beneath the Pacific and South Africa superswells, and descending slabs that ring the Pacific are examples of this internal heterogeneity. We have determined body tides for a non-radially symmetric earth model using a spectral element method. The structure of the laterally-heterogeneous mantle interior has been inferred from a tomographic model. We couple this heterogeneity to a viscous convection model to compute dynamic topography on surface and CMB, and verify the complete Earth model using geoid observations. We investigated semi-diurnal, diurnal and a few longer periods wave tides. We show that laterally-varying mantle structure induces perturbations in surface tidal response of the solid Earth of about 1 per mil. Using a tidal catalogue, we show that the maximum perturbation of radial tidal and geoid displacements are about 0.25 mm and about 0.1 mm, respectively, and the maximum perturbation of tidal gravity variation is about 120 nanoGal. This gravity perturbation is 100 times larger than the present precision of superconducting gravimeters and is at the level of the observed tidal noise. However, the amplitudes of tidal perturbations depend strongly on location, and are particularly high above large dense slabs (such as those near South America, Indonesia, and the Marianas) or over hotspots of Hawaii or Iceland, and over the East African Rift. Some of these regions exhibit unusually large tidal noise, suggesting that perturbations to the body tides might be an important source of this noise.
G32A-06
Towards a Future Predictive Non-Linear Terrestrial Reference Frame for Improved Geodetic Monitoring of the Global Hydrological Cycle
Geodetic techniques have a great potential to monitor mass transport in the Earth system. In particular, the combination of space-geodetic techniques present in the Global Geodetic Observing System (GGOS) constitutes a unique system for monitoring the mass transport in the hydrological cycle on regional to global scale. However, in order to separate the signals induced by water transport from other geophysical signals and instrumental effects, consistency of the models used in the space-geodetic analyses is crucial. Moreover, the conceptual approach to the reference frame has to ensure preservation of the water-related signals (i.e., no aliasing into other quantities, no biasing, and no reduction due to implicit filtering). The current concept for the International Terrestrial Reference Frame (ITRF) is that of a secular (linear) polyhedron augmented by a small set of conventional models for high-frequency surface deformation largely decoupled from variations in gravity and rotation. This concept is not appropriate at the accuracy level of 10-9 or better that is required to fully develop GGOS into a water-cycle monitoring system. Moreover, the traditional interpretation of geodetic observations aims at the separation of effects. However, in the hydrological cycle all mass movements are interrelated. Consequently, the geodetic observations capture signals from the atmosphere, terrestrial hydrosphere, cryosphere and ocean, which interact with each other through gravity forces and surface displacements at a level detectable by present-day geodetic techniques. Therefore, an integrated approach to reference frame modeling is required in geodesy, similar to, for example, the modeling of ocean circulation, climate, and weather. In these cases, assimilation of data into integrated predictive models has proved to be a successful strategy. In order to resolve the current inconsistencies of the models and the conceptual inadequacy of the ITRF for high- accuracy monitoring of the global and regional hydrological cycle, a Dynamic Earth Reference Model (DREM) is proposed which consistently models the changes in Earth's shape, gravity field and rotation induced, in particular, through mass transport in the hydrological cycle. This DREM will have to account for geophysical processes with a predefined target accuracy derived from the monitoring goals, and the conservation of water mass in the Earth system. It will require ongoing and consistent assimilation of geodetic observations of surface displacements, gravity field changes and Earth's rotation perturbations. The DREM will provide a basis for an extended reference frame concept with high temporal and spatial resolution, which accounts for intraseasonal to interannual variations in the geodetic parameters caused by mass transport. We will present the principle components of the DREM, discuss the main challenges in implementation, indicate the necessary extensions of the theory describing deformations of the solid Earth due to surface and body forces, and summarize the current status.
G32A-07
Designing a Global Geodetic Network to Support GGOS
Space geodesy is entrusted with the establishment and maintenance of reference frames that are widely used by the scientific and other user communities. Over the past decade, the burden of this task was primarily carried by the services of the International Association of Geodesy (IAG), led by IERS--the International Earth Rotation and Reference Systems Service. The new IAG initiative, the Global Geodetic Observing System--GGOS, places the utmost importance on the development, maintenance and wide distribution of an International Terrestrial Reference Frame (ITRF) of high accuracy and stability. At present, the goal is the definition of the origin accurate to 1 mm or better (at epoch) and a temporal stability on the order of 0.1 mm/y, with similar numbers for the scale and orientation components. The stability, integrity and applicability of the ITRF are directly related to how accurately we can account for mass redistribution during the analysis and reduction process of the data used for its development. Long wavelength variations of the gravity field driven by these mass redistributions produce geometric effects that are manifested as changes in the origin and orientation between the instantaneous and the mean reference frame. An uneven distribution of the stations that realize the ITRF on the globe generates biases and distortions in the combined product due to the dissimilarity of the combined networks and the de facto lopsided overlap of the combined networks. The poor geometry of the constituent networks results in increased correlations between the similarity transformation parameters, and they thus lead to biased and unstable results. The currently existing networks do not support high accuracy products and it is widely accepted that they are urgently in need of serious modernization and resource redistribution. Using simulations of geodetic data that we expect to collect with the future geodetic networks (SLR and VLBI), we provide preliminary options for the design of the complementary networks that will ensure the desired accuracy in the origin, scale and orientation definition of the ITRF.
G32A-08
GOCE: ESA's gravity mission getting ready for launch
The Gravity field and steady-state Ocean Circulation Explorer (GOCE) Mission is the first Earth Explorer Core mission of the Living Planet programme of the European Space Agency (ESA). The satellite is in the final stages of testing and is getting ready for launch in spring of 2008. The primary objective of the GOCE mission is to provide global and regional models of the Earth gravity field and the geoid, its reference equipotential surface, with unprecendented spatial resolution and accuracy. The high resolution static gravity field and gravimetric geoid measured by GOCE will stimulate research in a wide range of disciplines spanning studies of ocean circulation, cryosphere, solid-earth physics, natural hazards, geodesy and surveying. In this paper we will present the status of the development and testing activities of the GOCE satellite proto-flight model, the instruments and the ground segment elements, including the expected performance of the GOCE data products. http://www.esa.int/livingplanet/goce