Altimetry, gravimetry, GPS and viscoelastic modeling data for the joint inversion for glacial isostatic adjustment in Antarctica (ESA STSE Project REGINA)

The poorly known correction for the ongoing deformation of the solid Earth caused by glacial isostatic adjustment (GIA) is a major uncertainty in determining the mass balance of the Antarctic ice sheet from measurements of satellite gravimetry and to a lesser extent satellite altimetry. In the past decade, much progress has been made in consistently modeling ice sheet and solid Earth interactions; however, forward-modeling solutions of GIA in Antarctica remain uncertain due to the sparsity of constraints on the ice sheet evolution, as well as the Earth's rheological properties. An alternative approach towards estimating GIA is the joint inversion of multiple satellite data – namely, satellite gravimetry, satellite altimetry and GPS, which reflect, with different sensitivities, trends in recent glacial changes and GIA. Crucial to the success of this approach is the accuracy of the space-geodetic data sets. Here, we present reprocessed rates of surface-ice elevation change (Envisat/Ice, Cloud,and land Elevation Satellite, ICESat; 2003–2009), gravity field change (Gravity Recovery and Climate Experiment, GRACE; 2003–2009) and bedrock uplift (GPS; 1995–2013). The data analysis is complemented by the forward modeling of viscoelastic response functions to disc load forcing, allowing us to relate GIA-induced surface displacements with gravity changes for different rheological parameters of the solid Earth. The data and modeling results presented here are available in the PANGAEA database (https://doi.org/10.1594/PANGAEA.875745). The data sets are the input streams for the joint inversion estimate of present-day ice-mass change and GIA, focusing on Antarctica. However, the methods, code and data provided in this paper can be used to solve other problems, such as volume balances of the Antarctic ice sheet, or can be applied to other geographical regions in the case of the viscoelastic response functions. This paper presents the first of two contributions summarizing the work carried out within a European Space Agency funded study: Regional glacial isostatic adjustment and CryoSat elevation rate corrections in Antarctica (REGINA)

Remote Sensing by Satellite Gravimetry

Over the last two decades, satellite gravimetry has become a new remote sensing technique that provides a detailed global picture of the physical structure of the Earth. With the CHAMP, GRACE, GOCE and GRACE Follow-On missions, mass distribution and mass transport in the Earth system can be systematically observed and monitored from space. A wide range of Earth science disciplines benefit from these data, enabling improvements in applied models, providing new insights into Earth system processes (e.g., monitoring the global water cycle, ice sheet and glacier melting or sea-level rise) or establishing new operational services. Long time series of mass transport data are needed to disentangle anthropogenic and natural sources of climate change impacts on the Earth system. In order to secure sustained observations on a long-term basis, space agencies and the Earth science community are currently planning future satellite gravimetry mission concepts to enable higher accuracy and better spatial and temporal resolution. This Special Issue provides examples of recent improvements in gravity observation techniques and data processing and analysis, applications in the fields of hydrology, glaciology and solid Earth based on satellite gravimetry data, as well as concepts of future satellite constellations for monitoring mass transport in the Earth system

Joint inversion estimate of regional glacial isostatic adjustment in Antarctica considering a lateral varying Earth structure (ESA STSE Project REGINA)

A major uncertainty in determining the mass balance of the Antarctic ice sheet from measurements of satellite gravimetry, and to a lesser extent satellite altimetry, is the poorly known correction for the ongoing deformation of the solid Earth caused by glacial isostatic adjustment (GIA). Although much progress has been made in consistently modelling the ice-sheet evolution throughout the last glacial cycle, as well as the induced bedrock deformation caused by these load changes, forward models of GIA remain ambiguous due to the lack of observational constraints on the ice sheet's past extent and thickness and mantle rheology beneath the continent. As an alternative to forward modelling GIA, we estimate GIA from multiple space-geodetic observations: GRACE, Envisat/ICESat and GPS. Making use of the different sensitivities of the respective satellite observations to current and past surface mass (ice mass) change and solid Earth processes, we estimate GIA based on viscoelastic response functions to disc load forcing. We calculate and distribute the viscoelastic response functions according to estimates of the variability of lithosphere thickness and mantle viscosity in Antarctica. We compare our GIA estimate with published GIA corrections and evaluate its impact in determining the ice mass balance in Antarctica from GRACE and satellite altimetry. Particular focus is applied to the Amundsen Sea Sector in West Antarctica, where uplift rates of several cm/yr have been measured by GPS. We show that most of this uplift is caused by the rapid viscoelastic response to recent ice-load changes, enabled by the presence of a low-viscosity upper mantle in West Antarctica. This paper presents the second and final contribution summarizing the work carried out within a European Space Agency funded study, REGINA, (www.regina-science.eu)

GRACE Hydrological Monitoring of Australia: Current Limitations and Future Prospects

The Gravity Recovery and Climate Experiment (GRACE) twin-satellite gravimetry mission has been monitoring time-varying changes of the Earth's gravitational field on a near-global scale since 2002. One of the environmentally important signals to be detected is temporal variations induced by changes in the distribution of terrestrial water storage (i.e., hydrology).Since water is one of Australia's precious resources, it is logical to monitor its distribution, and GRACE offers one such opportunity. The second and fourth releases (referred to as RL02and RL04) of the 'standard' monthly GRACE solutions with respect to their annual mean are analysed. When compared to rainfall data over the same time period, GRACE is shown to detect hydrological signals over Australia, with the RL04 data showing better results. However, the relatively small hydrological signal typical for much of Australia is obscured by deficiencies in the standard GRACE data processing and filtering methods. Spectral leakage of oceanic mass changes also still contaminates the small hydrological signals typical over land. It is therefore recommended that Australia-focussed reprocessing of GRACE data is needed for useful hydrological signals to be extracted. Naturally,this will have to be verified by independent 'insitu' external sources such as rainfall, soil moisture and groundwater bore hole piezometer data over Australia

Upper mantle rheology from GRACE and GPS postseismic deformation after the 2004 Sumatra‐Andaman earthquake

International audience[1] Mantle rheology is one of the essential, yet least understood, material properties of our planet, controlling the dynamic processes inside the Earth's mantle and the Earth's response to various forces. With the advent of GRACE satellite gravity, measurements of mass displacements associated with many processes are now available. In the case of mass displacements related to postseismic deformation, these data may provide new constraints on the mantle rheology. We consider the postseismic deformation due to the M w = 9.2 Sumatra 26 December 2004 and M w = 8.7 Nias 28 March 2005 earthquakes. Applying wavelet analyses to enhance those local signals in the GRACE time varying geoids up to September 2007, we detect a clear postseismic gravity signal. We supplement these gravity variations with GPS measurements of postseismic crustal displacements to constrain postseismic relaxation processes throughout the upper mantle. The observed GPS displacements and gravity variations are well explained by a model of visco-elastic relaxation plus a small amount of afterslip at the downdip extension of the coseismically ruptured fault planes. Our model uses a 60 km thick elastic layer above a viscoelastic asthenosphere with Burgers body rheology. The mantle below depth 220 km has a Maxwell rheology. Assuming a low transient viscosity in the 60–220 km depth range, the GRACE data are best explained by a constant steady state viscosity throughout the ductile portion of the upper mantle (e.g., 60–660 km). This suggests that the localization of relatively low viscosity in the asthenosphere is chiefly in the transient viscosity rather than the steady state viscosity. We find a 8.10 18 Pa s mantle viscosity in the 220–660 km depth range. This may indicate a transient response of the upper mantle to the high amount of stress released by the earthquakes. To fit the remaining misfit to the GRACE data, larger at the smaller spatial scales, cumulative afterslip of about 75 cm at depth should be added over the period spanned by the GRACE models. It produces only small crustal displacements. Our results confirm that satellite gravity data are an essential complement to ground geodetic and geophysical networks in order to understand the seismic cycle and the Earth's inner structure

Atmospheric effects and spurious signals in GPS analyses

Improvements in the analyses of Global Positioning System (GPS) observations yield resolvable millimeter to submillimeter differences in coordinate estimates, thus providing sufficient resolution to distinguish subtle differences in analysis methodologies. Here we investigate the effects on site coordinates of using different approaches to modeling atmospheric loading deformation (ATML) and handling of tropospheric delays. The rigorous approach of using the time-varying Vienna Mapping Function 1 yields solutions with lower noise at a range of frequencies compared with solutions generated using empirical mapping functions. This is particularly evident when ATML is accounted for. Some improvement also arises from using improved a priori zenith hydrostatic delays (ZHD), with the combined effect being site-specific. Importantly, inadequacies in both mapping functions and a priori ZHDs not only introduce time-correlated noise but significant periodic terms at solar annual and semiannual periods. We find no significant difference between solutions where nontidal ATML is applied at the observation level rather than as a daily averaged value, but failing to model diurnal and semidiurnal tidal ATML at the observation level can introduce anomalous propagated signals with periods that closely match the GPS draconitic annual (∼351.4 days) and semiannual period (∼175.7 days). Exacerbated by not fixing ambiguities, these signals are evident in both stacked and single-site power spectra, with each tide contributing roughly equally to the dominant semiannual peak. The amplitude of the propagated signal reaches a maximum of 0.8 mm with a clear latitudinal dependence that is not correlated directly with locations of maximum tidal amplitude.Australian Research Council’s Discovery Project

On the reliability of the so far performed tests for measuring the Lense-Thirring effect with the LAGEOS satellites

In this paper we will show in detail that the performed attempts aimed at the detection of the general relativistic Lense-Thirring effect in the gravitational field of the Earth with the existing LAGEOS satellites are often presented in an optimistic and misleading way which is inadequate for such an important test of fundamental physics. E.g., in the latest reported measurement of the gravitomagnetic shift with the nodes of the LAGEOS satellites and the 2nd generation GRACE-only EIGEN-GRACE02S Earth gravity model over an observational time span of 11 years a 5-10% total accuracy is claimed at 1-3sigma, respectively. We will show that, instead, it might be 15-45% (1-3sigma) if the impact of the secular variations of the even zonal harmonics is considered as well.Comment: LaTex2e, 22 pages, 1 figure, 1 table, 60 references. Conclusions and Table of Contents added. Estimates of the impact of J6dot on the node-node-perigee combination presented. Typos corrected and minor stylistic changes. Small changes due to G. Melki useful remarks. Lense-Thirring 'memory' effect in EIGEN-GRACE02S discusse

Loading Deformation On Various Timescales Using Gps And Grace Measurements

Thesis (Ph.D.) University of Alaska Fairbanks, 2012Tidal, seasonal and long-term surface mass movements cause the earth to deform and the gravity field to change. Current geodetic satellites, GPS and GRACE, accurately measure these geophysical signals. I examine the effect on GPS solutions of using inconsistent reference frames to model ocean tidal loading (OTL). For seasonal loading, I choose two study areas, Nepal Himalaya and southern Alaska, and compare GPS-measured and GRACE-modeled seasonal hydrological ground loading deformation. Globally distributed stations are employed to compare GPS coordinate solutions with OTL corrections computed in different reference frames: center of mass of the solid Earth (CE), and center of mass of the Earth system (CM). A strong spectral peak at a period of ~14 days appears when inconsistent OTL models are applied along with smaller peaks at ~annual and ~semi-annual periods. Users of orbit/clock products must ensure to use OTL coefficients computed in the same frame as the OTL coefficients used by the analysis centers; otherwise, systematic errors will be introduced into position solutions. Continuous GPS measurements of seasonal deformation in Nepal Himalaya are compared with load model predictions derived from GRACE observations. The GPS seasonal height variation and GRACE-modeled seasonal vertical displacement due to the changing hydrologic load exhibit consistent results, for both amplitude and phase. GRACE indicates a long-term mass loss in the Himalaya region, which leads to crustal uplift since the earth behaves as an elastic body. We model this effect and remove it from GPS observed vertical rates. Then most GPS vertical rates can be explained by interseismic strain from the Main Himalayan Thrust. In southern Alaska, vertical seasonal loading deformation observed by GPS stations and modeled displacements due to seasonal hydrological loading inferred from GRACE are highly correlated. The effects of atmosphere and non-tidal ocean loading are important. Adding the AOD1B de-aliasing model to the GRACE solutions improves the correlation between these two geodetic measurements, because the displacements due to these loads are present in the GPS data. Weak correlations are found for some stations located in areas where the magnitude of the load changes over a short distance, due to GRACE's limited spatial resolution