References of "van Dam, Tonie 50003245"
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See detailThe use of GPS horizontals for loading studies, with applications to northern California and southeast Greenland
Wahr, John; Khan, Shfaqat; van Dam, Tonie UL et al

in Journal of Geophysical Research. Solid Earth (2013), 118

We describe how GPS measurements of horizontal crustal motion can be used to augment vertical crustal motion measurements, to improve and extend GPS studies of surface loading. We show that the ratio of ... [more ▼]

We describe how GPS measurements of horizontal crustal motion can be used to augment vertical crustal motion measurements, to improve and extend GPS studies of surface loading. We show that the ratio of the vertical displacement to the horizontal displacement, combined with the direction of the horizontal motion, can help determine whether nearby loading is concentrated in a small region (for example, in a single lake or glacier), and where that region is. We illustrate this method by applying it to two specific cases: an analysis of GPS data from northern California to monitor the level of Lake Shasta, and the analysis of data from a single GPS site in southeast Greenland to determine mass variability of two large, nearby outlet glaciers: Helheim Glacier and Midgaard Glacier. The California example serves largely as a proof-of-concept, where the results can be assessed by comparing with independent observations (Lake Shasta tide gauge data, in this case). Our Greenland results show that both Helheim and Midgaard have experienced notable interannual variations in mass loss rate over the last decade. Helheim’s mass loss accelerated rapidly in mid-2003, decelerated in late 2005, and increased again in 2008–2009 before returning to about its pre-2003 rate in late 2010. Midgaard’s mass loss accelerated in mid-2004, and remained more-or-less constant before returning to its pre-2003 rate in late 2008. [less ▲]

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See detailAn estimate of the influence of loading effects on tectonic velocities in the Pyrenees
Ferenc, Marcell; Nicolas, Joelle; van Dam, Tonie UL et al

in Studia Geophysica & Geodaetica (2013)

Surface displacements due to temporal changes in environmental mass redistributions are observable in the coordinate time series of many Global Navigation Satellite System (GNSS) sites. In this study, we ... [more ▼]

Surface displacements due to temporal changes in environmental mass redistributions are observable in the coordinate time series of many Global Navigation Satellite System (GNSS) sites. In this study, we investigated the effect of loading on estimates of tectonic velocity computed from campaign-style GNSS observations. The study region is in the Pyrenees mountain range between France and Spain (ResPyr campaigns). In this area, seismic activity is continuous and moderate and the expected amplitude of the horizontal tectonic velocity is less than 0.5 mm/yr. In order to determine the velocity, 4 sparse GNSS campaigns were carried out from 1995 to 2010. Considering this small rate of deformation, loading phenomena can contribute a non-negligible artifact to the velocity computation that could affect our geodynamical interpretation. In this investigation, we specifically considered the atmospheric, hydrological, and non-tidal ocean loading phenomena. The computed loading deformations for this region show the horizontal displacements are dominated by the non-tidal ocean loading (maximum 4 mm for the North and 3.1 mm for the East components); the main vertical contributions are due to the atmospheric and continental water storage loading (maximum 14.3 for the atmosphere and 8.1 mm for the hydrology, respectively). We have found that the dominant loading effect on the horizontal velocity is the non-tidal ocean loading (mean of 0.11 mm/yr), whereas the vertical component is dominated by the hydrological loading (mean of 0.21 mm/yr). Since the study area is in a mountainous region, we also analyzed the difference between the atmospheric and the topography dependent atmospheric loading models at our GNSS campaign sites. We did not find any significant difference between the two atmospheric loading models in terms of horizontal velocity. Finally, we performed simulations to identify the optimum timing and frequency of future GNSS campaigns in this area that would minimize the loading effects on tectonic velocity estimates. [less ▲]

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See detailVertical and horizontal surface displacements near Jakobshavn Isbræ driven by melt-induced and dynamic ice loss
Nielsen, Karina; Khan, Shfaqat A.; Spada, Giorgio et al

in Journal of Geophysical Research. Solid Earth (2013), 118(4), 1837--1844

We analyze Global Positioning System (GPS) time series of relative vertical and horizontal surface displacements from 2006 to 2012 at four GPS sites located between ∼5 and ∼150 km from the front of ... [more ▼]

We analyze Global Positioning System (GPS) time series of relative vertical and horizontal surface displacements from 2006 to 2012 at four GPS sites located between ∼5 and ∼150 km from the front of Jakobshavn Isbræ (JI) in west Greenland. Horizontal displacements during 2006–2010 at KAGA, ILUL, and QEQE, relative to the site AASI, are directed toward north-west, suggesting that the main mass loss signal is located near the frontal portion of JI. The directions of the observed displacements are supported by modeled displacements, derived from NASA's Airborne Topographic Mapper (ATM) surveys of surface elevations from 2006, 2009, and 2010. However, horizontal displacements during 2010–2012 at KAGA and ILUL are directed more towards the west suggesting a change in the spatial distribution of the ice mass loss. In addition, we observe an increase in the uplift rate during 2010–2012 as compared to 2006–2010. The sudden change in vertical and horizontal displacements is due to enhanced melt-induced ice loss in 2010 and 2012. [less ▲]

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See detailEarth System Mass Transport Mission (e.motion): A Concept for Future Earth Gravity Field Measurements from Space
Panet, I.; Flury, J.; Biancale, R. et al

in Surveys in Geophysics (2013), 34(2), 141-163

In the last decade, satellite gravimetry has been revealed as a pioneering technique for mapping mass redistributions within the Earth system. This fact has allowed us to have an improved understanding of ... [more ▼]

In the last decade, satellite gravimetry has been revealed as a pioneering technique for mapping mass redistributions within the Earth system. This fact has allowed us to have an improved understanding of the dynamic processes that take place within andbetween the Earth’s various constituents. Results from the Gravity Recovery And Climate Experiment (GRACE) mission have revolutionized Earth system research and have established the necessity for future satellite gravity missions. In 2010, a comprehensive team of European and Canadian scientists and industrial partners proposed the e.motion (Earth system mass transport mission) concept to the European Space Agency. The proposal is based on two tandem satellites in a pendulum orbit configuration at an altitude of about 370 km, carrying a laser interferometer inter-satellite ranging instrument and improved accelerometers. In this paper, we review and discuss a wide range of mass signals related to the global water cycle and to solid Earth deformations that were outlined in the e.motion proposal. The technological and mission challenges that need to be addressed in order to detect these signals are emphasized within the context of the scientific return. This analysis presents a broad perspective on the value and need for future satellite gravimetry missions. [less ▲]

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See detailObserving and understanding the Earth system variations from space geodesy
Jin, Shuanggen; van Dam, Tonie UL; Wdowinski, Shimon

in Journal of Geodynamics (2013), 72

The interaction and coupling of the Earth system components that include the atmosphere, hydrosphere, cryosphere, lithosphere, and other fluids in Earth's interior, influence the Earth's shape, gravity ... [more ▼]

The interaction and coupling of the Earth system components that include the atmosphere, hydrosphere, cryosphere, lithosphere, and other fluids in Earth's interior, influence the Earth's shape, gravity field and its rotation (the three pillars of geodesy). The effects of global climate change, such as sea level rise, glacier melting, and geoharzards, also affect these observables. However, observations and models of Earth's system change have large uncertainties due to the lack of direct high temporal–spatial measurements. Nowadays, space geodetic techniques, particularly GNSS, VLBI, SLR, DORIS, InSAR, satellite gravimetry and altimetry provide a unique opportunity to monitor and, therefore, understand the processes and feedback mechanisms of the Earth system with high resolution and precision. In this paper, the status of current space geodetic techniques, some recent observations, and interpretations of those observations in terms of the Earth system are presented. These results include the role of space geodetic techniques, atmospheric–ionospheric sounding, ocean monitoring, hydrologic sensing, cryosphere mapping, crustal deformation and loading displacements, gravity field, geocenter motion, Earth's oblateness variations, Earth rotation and atmospheric-solid earth coupling, etc. The remaining questions and challenges regarding observing and understanding the Earth system are discussed. [less ▲]

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See detailComparative analysis of different environmental loading methods and their impacts on the GPS height time series
Jiang, Weiping; Li, Zhao UL; van Dam, Tonie UL et al

in Journal of Geodesy (2013), 87(7), 687-703

Three different environmental loading methods are used to estimate surface displacements and correct nonlinear variations in a set of GPS weekly height time series. Loading data are provided by (1) Global ... [more ▼]

Three different environmental loading methods are used to estimate surface displacements and correct nonlinear variations in a set of GPS weekly height time series. Loading data are provided by (1) Global Geophysical Fluid Center (GGFC), (2) Loading Model of Quasi-Observation CombinationAnalysis software (QLM) and (3) our own daily loading time series (we call itOMDfor optimum model data). We find that OMD has the smallest scatter in height across the selected 233 globally distributed GPS reference stations, GGFC has the next smallest variability, and QLM has the largest scatter. By removing the load-induced height changes from the GPS height time series, we are able to reduce the scatter on 74, 64 and 41 % of the stations using the OMD models, the GGFC model and QLM model respectively. We demonstrate that the discrepancy between the center of earth (CE) and the center of figure (CF) reference frames can be ignored. The most important differences between the predicted models are caused by (1) differences in the hydrol- ogy data from the National Center for Atmospheric Research (NCEP) vs. those from the Global Land Data Assimilation System (GLDAS), (2) grid interpolation, and (3) whether the topographic effect is removed or not. Both QLM and GGFC are extremely convenient tools for non-specialists to use to calculate loading effects. Due to the limitation ofNCEP reanalysis hydrology data compared with theGLDAS model, theGGFCdataset is much more suitable thanQLMfor applying environmental loading corrections to GPS height time series. However, loading results for Greenland from GGFC should be discarded since hydrology data from GLDAS in this region are not accurate. The QLM model is equivalent to OMD in Greenland and, hence, could be used as a complement to the GGFC product to model the load in this region. We find that the predicted loading from all three models cannot reduce the scatter of the height coordinate for some stations in Europe. [less ▲]

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See detailMonthly crustal loading corrections for satellite altimetery
Ray, R.; Luthcke, S.B.; van Dam, Tonie UL

in J. Atmos. Oceanic Technol. (2013)

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See detailLarge scale time variability from high-low SST - filling the gap between GRACE and GFO
Weigelt, Matthias UL; van Dam, Tonie UL; Jäggi, A. et al

Scientific Conference (2012, September)

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See detailNontidal ocean loading: amplitudes and potential effects in GPS height time series
van Dam, Tonie UL; Collilieux, X.; Wuite, J. et al

in Journal of Geodesy (2012), 86(11), 1043-1057

Ocean bottom pressure (OBP) changes are caused by a redistribution of the ocean’s internal mass that are driven by atmospheric circulation, a change in the mass entering or leaving the ocean, and/or a ... [more ▼]

Ocean bottom pressure (OBP) changes are caused by a redistribution of the ocean’s internal mass that are driven by atmospheric circulation, a change in the mass entering or leaving the ocean, and/or a change in the integrated atmospheric mass over the ocean areas. The only previous global analysis investigating the magnitude of OBP surface displacements used older OBP data sets (van Dam et al. in J Geophys Res 129:507–517, 1997). Since then significant improvements in meteorological forcing models used to predict OBP have been made, augmented by observations from satellite altimetry and expendable bathythermograph profiles. Using more recent OBP estimates from the Estimating the Circulation and Climate of the Ocean (ECCO) project, we reassess the amplitude of the predicted effect of OBP on the height coordinate time series from a global distribution of GPS stations. OBP-predicted loading effects display an RMS scatter in the height of between 0.2 and 3.7 mm, larger than previously reported but still much smaller (by a factor of 2) than the scatter observed due to atmospheric pressure loading. Given the improvement in GPS hardware and data analysis techniques, the OBP signal is similar to the precision of weekly GPS height coordinates. We estimate the effect of OBP on GPS height coordinate time series using the MIT reprocessed solution, mi1. When we compare the predicted OBP height time series with mi1, we find that the scatter is reduced over all stations by 0.1 mm on average with reductions as high as 0.7 mm at some stations. More importantly we are able to reduce the scatter on 65 % of the stations investigated. The annual component of the OBP signal is responsible for 80 % of the reduction in scatter on average.We find that stations located close to semi-enclosed bays or seas are affected by OBP loading to a greater extent than other stations. [less ▲]

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See detailAssimilation of GRACE terrestrial water storage into a land surface model: Evaluation and potential value for drought monitoring in western and central Europe
Li, B.; Rodell, M.; Zaitchik, B. F. et al

in Journal of Hydrology (2012), 446-447

A land surface model's ability to simulate states (e.g., soil moisture) and fluxes (e.g., runoff) is limited by uncertainties in meteorological forcing and parameter inputs as well as inadequacies in ... [more ▼]

A land surface model's ability to simulate states (e.g., soil moisture) and fluxes (e.g., runoff) is limited by uncertainties in meteorological forcing and parameter inputs as well as inadequacies in model physics. In this study, anomalies of terrestrial water storage (TWS) observed by the Gravity Recovery and Climate Experiment (GRACE) satellite mission were assimilated into the NASA Catchment land surface model in western and central Europe for a 7-year period, using a previously developed ensemble Kalman smoother. GRACE data assimilation led to improved runoff estimates (in temporal correlation and root mean square error) in 17 out of 18 hydrological basins, even in basins smaller than the effective resolution of GRACE. Improvements in root zone soil moisture were less conclusive, partly due to the shortness of the in situ data record. GRACE data assimilation also had significant impacts in groundwater estimates including trend and seasonality. In addition to improving temporal correlations, GRACE data assimilation also reduced increasing trends in simulated monthly TWS and runoff associated with increasing rates of precipitation. The assimilation downscaled (in space and time) and disaggregated GRACE data into finer scale components of TWS which exhibited significant changes in their dryness rankings relative to those without data assimilation, suggesting that GRACE data assimilation could have a substantial impact on drought monitoring. Signals of drought in GRACE TWS correlated well with MODIS Normalized Difference Vegetation Index (NDVI) data in most areas. Although they detected the same droughts during warm seasons, drought signatures in GRACE derived TWS exhibited greater persistence than those in NDVI throughout all seasons, in part due to limitations associated with the seasonality of vegetation. Mass imbalances associated with GRACE data assimilation and challenges of using GRACE data for drought monitoring are discussed. © 2012 Elsevier B.V.. [less ▲]

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See detailGeocenter motion and its geodetic and geophysical implications
Wu, X.; Ray, J.; van Dam, Tonie UL

in Journal of Geodynamics (2012), 58

The horizontal transport of water in Earth’s surface layer, including sea level change, deglaciation, and surface runoff, is a manifestation of many geophysical processes. These processes entail ocean and ... [more ▼]

The horizontal transport of water in Earth’s surface layer, including sea level change, deglaciation, and surface runoff, is a manifestation of many geophysical processes. These processes entail ocean and atmosphere circulation and tidal attraction, global climate change, and the hydrological cycle, all having a broad range of spatiotemporal scales. The largest atmospheric mass variations occur mostly at synoptic wavelengths and at seasonal time scales. The longest wavelength component of surface mass transport, the spherical harmonic degree-1, involves the exchange of mass between the northern and southern hemispheres. These degree-1 mass loads deform the solid Earth, including its surface, and induce geocenter motion between the center-of-mass of the total Earth system (CM) and the center-of-figure (CF) of the solid Earth surface. Because geocenter motion also depends on the mechanical properties of the solid Earth, monitoring geocenter motion thus provides an additional opportunity to probe deep into Earth’s interior. Most modern geodetic measurement systems rely on tracking data between ground stations and satellites that orbit around CM. Consequently, geocenter motion is intimately related to the realization of the International Terrestrial Reference Frame (ITRF) origin, and, in various ways, affects many of our measurement objectives for global change monitoring. In the last 15 years, there have been vast improvements in geophysical fluid modeling and in the global coverage, densification, and accuracy of geodetic observations. As a result of these developments, tremendous progress has been made in the study of geocenter motion over the same period. This paper reviews both the theoretical and measurement aspects of geocenter motion and its implications. [less ▲]

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See detailStrategies to mitigate aliasing of loading signals while estimating GPS frame parameters
Collilieux, Xavier; van Dam, Tonie UL; Ray, Jim et al

in Journal of Geodesy (2012), 86(1), 1-14

Although GNSS techniques are theoretically sensitive to the Earth center of mass, it is often preferable to remove intrinsic origin and scale information from the estimated station positions since they ... [more ▼]

Although GNSS techniques are theoretically sensitive to the Earth center of mass, it is often preferable to remove intrinsic origin and scale information from the estimated station positions since they are known to be affected by systematic errors. This is usually done by estimating the parameters of a linearized similarity transformation which relates the quasi-instantaneous frames to a long-term frame such as the International Terrestrial Reference Frame (ITRF). It is well known that non-linear station motions can partially alias into these parameters. We discuss in this paper some procedures that may allow reducing these aliasing effects in the case of the GPS techniques. The options include the use of well-distributed sub-networks for the frame transformation estimation, the use of site loading corrections, a modification of the stochastic model by downweighting heights, or the joint estimation of the low degrees of the deformation field. We confirm that the standard approach consisting of estimating the transformation over the whole network is particularly harmful for the loading signals if the network is not well distributed. Downweighting the height component, using a uniform sub-network, or estimating the deformation field perform similarly in drastically reducing the amplitude of the aliasing effect. The application of these methods to reprocessed GPS terrestrial frames permits an assessment of the level of agreement between GPS and our loading model, which is found to be about 1.5 mm WRMS in height and 0.8 mm WRMS in the horizontal at the annual frequency. Aliased loading signals are not the main source of discrepancies between loading displacement models and GPS position time series. [less ▲]

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See detailBedrock displacements in Greenland manifest ice mass variations, climate cycles and climate change
Bevis, Michael; Wahr, John; Khan, Shfaqat A. et al

in Proceedings of the National Academy of Sciences of the United States of America (2012), 109(30), 11944-11948

The Greenland GPS Network (GNET) uses the Global Positioning System (GPS) to measure the displacement of bedrock exposed near the margins of the Greenland ice sheet. The entire network is uplifting in ... [more ▼]

The Greenland GPS Network (GNET) uses the Global Positioning System (GPS) to measure the displacement of bedrock exposed near the margins of the Greenland ice sheet. The entire network is uplifting in response to past and present-day changes in ice mass. Crustal displacement is largely accounted for by an annual oscillation superimposed on a sustained trend. The oscillation is driven by earth’s elastic response to seasonal variations in ice mass and air mass (i.e., atmospheric pressure). Observed vertical velocities are higher and often much higher than predicted rates of postglacial rebound (PGR), implying that uplift is usually dominated by the solid earth’s instantaneous elastic response to contemporary losses in ice mass rather than PGR. Superimposed on longer-term trends, an anomalous ‘pulse’ of uplift accumulated at many GNET stations during an approximate six-month period in 2010. This anomalous uplift is spatially correlated with the 2010 melting day anomaly. [less ▲]

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See detailThe effect of using inconsistent ocean tidal loading models on GPS coordinate solutions
Fu, Y.; Freymueller, J.; van Dam, Tonie UL

in Journal of Geodesy (2011)

We use up to a 6-year span of GPS data from 85 globally distributed stations to compare solutions using ocean tidal loading (OTL) corrections computed in differ- ent reference frames: center of mass of ... [more ▼]

We use up to a 6-year span of GPS data from 85 globally distributed stations to compare solutions using ocean tidal loading (OTL) corrections computed in differ- ent reference frames: center of mass of the solid Earth (CE), and center of mass of the Earth system (CM). We compare solution sets that differ only in the frame used for the OTL model computations, for three types of GPS solutions. In global solutions with all parameters including orbits estimated simultaneously, we find coordinate differences of ∼0.3mm between solutions using OTL computed in CM and OTL computed in CE. When orbits or orbits and clocks are fixed, larger biases appear if the user applies an OTL model inconsistent with that used to derive the orbit and clock products. Network solutions (orbits fixed, satellite clocks estimated) show differences smaller than 0.5 mm due to model inconsistency, but PPP solutions show distortions at the ∼1.3 mm level. The much larger effect on PPP solutions indicates that satellite clock estimates are sensitive to the OTL model applied. The time series of coordinate differences shows a strong spectral peak at a period of ∼14 days when inconsistent OTL models are applied and smaller peaks at ∼annual and ∼semi-annual periods, for both ambiguity-free and ambiguity-fixed solutions. These spurious coordinate variations disappear in solutions using consistent OTL mod- els. Users of orbit and clock products must ensure that they use OTL coefficients computed in the same reference frame as the OTL coefficients used by the analysis centers that produced the products they use; otherwise, systematic errors will be introduced into position solutions. All modern products should use loading models computed in the CM frame, but legacy products may require loading models computed in the CE frame. Analysts and authors need to document the frame used for all loading computations in product descriptions and papers. [less ▲]

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See detailCorrection to "Topographically induced height errors in predicted atmospheric loading effects"
van Dam, Tonie UL; Altamimi, Zuheir

in Journal of Geophysical Research. Solid Earth (2011)

Figure 1 presented in the paper “Topographically induced height errors in predicted atmospheric loading effects” by van Dam et al., (Journal of Geophysical Research, VOL. 115, B07415, doi:10.1029 ... [more ▼]

Figure 1 presented in the paper “Topographically induced height errors in predicted atmospheric loading effects” by van Dam et al., (Journal of Geophysical Research, VOL. 115, B07415, doi:10.1029/2009JB006810) is incorrect. The correct image is presented here. The corrected image does not alter the conclusion presented in the original paper. [less ▲]

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See detailQuality assessment of GPS reprocessed terrestrial reference frame
Collilieux, Xavier; Métivier, Laurent; Altamimi, Zuheir et al

in GPS Solutions (2011), 15(3), 219--231

The International GNSS Service (IGS) contributes to the construction of the International Terrestrial Reference Frame (ITRF) by submitting time series of station positions and Earth Rotation Parameters ... [more ▼]

The International GNSS Service (IGS) contributes to the construction of the International Terrestrial Reference Frame (ITRF) by submitting time series of station positions and Earth Rotation Parameters (ERP). For the first time, its submission to the ITRF2008 construction is based on a combination of entirely reprocessed GPS solutions delivered by 11 Analysis Centers (ACs). We analyze the IGS submission and four of the individual AC contributions in terms of the GNSS frame origin and scale, station position repeatability and time series seasonal variations. We show here that the GPS Terrestrial Reference Frame (TRF) origin is consistent with Satellite laser Ranging (SLR) at the centimeter level with a drift lower than 1 mm/year. Although the scale drift compared to Very Long baseline Interferometry (VLBI) and SLR mean scale is smaller than 0.4 mm/year, we think that it would be premature to use that information in the ITRF scale definition due to its strong dependence on the GPS satellite and ground antenna phase center variations. The new position time series also show a better repeatability compared to past IGS combined products and their annual variations are shown to be more consistent with loading models. The comparison of GPS station positions and velocities to those of VLBI via local ties in co-located sites demonstrates that the IGS reprocessed solution submitted to the ITRF2008 is more reliable and precise than any of the past submissions. However, we show that some of the remaining inconsistencies between GPS and VLBI positioning may be caused by uncalibrated GNSS radomes. [less ▲]

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See detailVertical deformations from homogeneously processed GRACE and global GPS long-term series
Tesmer, Volker; Steigenberger, Peter; van Dam, Tonie UL et al

in Journal of Geodesy (2011)

Temporal variations in the geographic distribution of surface mass cause surface displacements. Surface displacements derived from GRACE gravity field coefficient time series also should be observed in ... [more ▼]

Temporal variations in the geographic distribution of surface mass cause surface displacements. Surface displacements derived from GRACE gravity field coefficient time series also should be observed in GPS coordinate time series, if both time series are sufficiently free of systematic errors. A successful validation can be an important contribution to climate change research, as the biggest contributors to mass variability in the system Earth include the movement of oceanic, atmospheric, and continental water and ice. In our analysis, we find that if the signals are larger than their precision, both geodetic sensor systems see common signals for almost all the 115 stations surveyed. Almost 80% of the stations have their signal WRMS decreased, when we subtract monthly GRACE surface displacements from those observed by GPS data. Almost all other stations are on ocean islands or small peninsulas, where the physically expected loading signals are very small. For a fair comparison, the data(79 months from September 2002 to April 2009) had to be treated appropriately: the GPS data were completely reprocessed with state-of-the-art models. We used an objective cluster analysis to identify and eliminate stations,where local effects or technical artifacts dominated the signals. In addition, it was necessary for both sets of results to be expressed in equivalent reference frames, meaning that net translations between the GPS and GRACE data sets had to be treated adequately. These data sets are then compared and statistically analyzed: we determine the stability (precision) of GRACEderived, monthly vertical deformation data to be ∼1.2 mm, using the data from three GRACE processing centers. We statistically analyze themean annual signals, computed from the GPS and GRACE series. There is a detailed discussion of the results for five overall representative stations, in order to help the reader to link the displayed criteria of similarity to real data. A series of tests were performed with the goal of explaining the remaining GPS–GRACE residuals. [less ▲]

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See detailSimulation of the time-variable gravity field by means of coupled geophysical models
Gruber, Th; Bamber, J. L.; Bierkens, M. F. P. et al

in Earth System Science Data (2011), 3(1), 19-35

Time variable gravity fields, reflecting variations of mass distribution in the system Earth is one of the key parameters to understand the changing Earth. Mass variations are caused either by ... [more ▼]

Time variable gravity fields, reflecting variations of mass distribution in the system Earth is one of the key parameters to understand the changing Earth. Mass variations are caused either by redistribution of mass in, on or above the Earth's surface or by geophysical processes in the Earth's interior. The first set of observations of monthly variations of the Earth gravity field was provided by the US/German GRACE satellite mission beginning in 2002. This mission is still providing valuable information to the science community. However, as GRACE has outlived its expected lifetime, the geoscience community is currently seeking successor missions in order to maintain the long time series of climate change that was begun by GRACE. Several studies on science requirements and technical feasibility have been conducted in the recent years. These studies required a realistic model of the time variable gravity field in order to perform simulation studies on sensitivity of satellites and their instrumentation. This was the primary reason for the European Space Agency (ESA) to initiate a study on ''Monitoring and Modelling individual Sources of Mass Distribution and Transport in the Earth System by Means of Satellites''. The goal of this interdisciplinary study was to create as realistic as possible simulated time variable gravity fields based on coupled geophysical models, which could be used in the simulation processes in a controlled environment. For this purpose global atmosphere, ocean, continental hydrology and ice models were used. The coupling was performed by using consistent forcing throughout the models and by including water flow between the different domains of the Earth system. In addition gravity field changes due to solid Earth processes like continuous glacial isostatic adjustment (GIA) and a sudden earthquake with co-seismic and post-seismic signals were modelled. All individual model results were combined and converted to gravity field spherical harmonic series, which is the quantity commonly used to describe the Earth's global gravity field. The result of this study is a twelve-year time-series of 6-hourly time variable gravity field spherical harmonics up to degree and order 180 corresponding to a global spatial resolution of 1 degree in latitude and longitude. In this paper, we outline the input data sets and the process of combining these data sets into a coherent model of temporal gravity field changes. The resulting time series was used in some follow-on studies and is available to anybody interested. [less ▲]

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