G13A-0623
GOCE Validation by Means of Satellite Altimetry
Whereas state-of-the-art global gravity field models as well as terrestrial gravity data have been proposed for GOCE calibration/validation purposes, satellite altimeter data has heretofore not been considered in this context. Although the numerical and conceptual challenges are manifold, the abundance of satellite altimeter data justifies an attempt to use these data for GOCE validation. We present a new method for the independent validation of the GOCE gravity gradiometer data by means of satellite altimetry. The idea is to rotate the GOCE gravity gradients, which are given in the gradiometer instrument frame, to align them with the satellite altimeter tracks. The corresponding along-track second derivatives from altimetry can be performed using the high along-track precision and resolution and avoids any approximation through gridding. When the GOCE gravity gradients are downward continued, a direct comparison can be applied to individual satellite altimeter tracks or at cross-overs of ascending and descending altimeter tracks, where a linear combination of horizontal derivatives is related to the radial gravity gradient of the disturbing potential. We assess the accuracy of the new method and discuss the associated main problems as well. The latter include: 1) gravity gradient rotation which is affected by the unequal accuracy of the GOCE gradients as well as the large error at long wavelengths; 2) the downward continuation of the GOCE data; 3) orbit errors and sea level variations in the satellite altimeter data; and 4) the along-track double differentiation of the altimeter data which amplifies noise.
G13A-0624
Future missions for observing Earth's changing gravity field: a closed-loop simulation tool
The GRACE mission has successfully demonstrated the observation from space of the changing Earth's gravity field at length and time scales of typically 1000 km and 10-30 days, respectively. Many scientific communities strongly advertise the need for continuity of observing Earth's gravity field from space. Moreover, a strong interest is being expressed to have gravity missions that allow a more detailed sampling of the Earth's gravity field both in time and in space. Designing a gravity field mission for the future is a complicated process that involves making many trade-offs, such as trade-offs between spatial, temporal resolution and financial budget. Moreover, it involves the optimization of many parameters, such as orbital parameters (height, inclination), distinction between which gravity sources to observe or correct for (for example are gravity changes due to ocean currents a nuisance or a signal to be retrieved?), observation techniques (low-low satellite-to-satellite tracking, satellite gravity gradiometry, accelerometers), and satellite control systems (drag-free?). A comprehensive tool has been developed and implemented that allows the closed-loop simulation of gravity field retrievals for different satellite mission scenarios. This paper provides a description of this tool. Moreover, its capabilities are demonstrated by a few case studies. Acknowledgments. The research that is being done with the closed-loop simulation tool is partially funded by the European Space Agency (ESA). An important component of the tool is the GEODYN software, kindly provided by NASA Goddard Space Flight Center in Greenbelt, Maryland.
G13A-0625
The GOCE User Toolbox - GUT - An ESA Effort to Facilitate the Use of GOCE Level-2 Products
The Gravity and Ocean Circulation Explorer - GOCE - satellite mission is a new type of Earth observation
satellite that will measure the Earth gravity and geoid with unprecedented accuracy. The objective of the
GOCE User Toolbox development project is in close collaboration with ESA's High-Level (Level 1 – Level 2)
Processing Facility (HPF) effort to develop algorithms and input and output specification for a GOCE user
toolbox that is required by the general science community for the exploitation of GOCE level 2 and ERS-
ENVISAT altimetry. Combining GOCE geoid models with satellite altimetric observations of the sea surface
height substantial improvements in the modelling of the ocean circulation and transport are foreseen. No
ocean circulation products are planned to be delivered as level-2 products as part of the GOCE project, so a
strong need exists, for oceanographers, to further process the GOCE level-2 geoid and merge it with Radar
Altimetry. The primary requirement of oceanographers is to have access to a geoid and its error covariance
at the highest spatial resolution and accuracy possible, although required resolution depends on application.
For effective use of the geoid data, knowledge of the error covariance is mandatory. Within the ESA
supported forerunner GUTS project, the user requirements for GOCE User Toolbox associated with geodetic,
oceanographic and solid earth applications have been consolidated. The Toolbox functionality and results
will be presented.
http://earth.esa.int/gut/
G13A-0626
EIGEN-5C - the new GeoForschungsZentrum Potsdam / Groupe de Recherche de Geodesie Spatiale combined gravity field model
Global gravity field models play a fundamental role in geodesy and Earth sciences, ranging from practical
purposes, like precise orbit determination, to applications in geosciences, like investigations of the density
structure of the Earth's interior. In this presentation we report on the latest, recently released EIGEN-model,
EIGEN-5C (EIGEN = European Improved Gravity model of the Earth by New techniques) and its associated
satellite-only model EIGEN-5S. The global gravity field model EIGEN-5C is complete to degree and order 360
(corresponding to half-wavelength of 55 km) and was jointly elaborated by GFZ Potsdam and CNES/GRGS
Toulouse. As its precursor EIGEN-GL04C (released in March 2006), this model is inferred from a combination
of GRACE and LAGEOS satellite tracking data with surface gravity data, based on the accumulation of
normal equations. However, this new model presents remarkable changes and improvements compared to its
precursors. EIGEN-5C incorporates a further extended GRACE and LAGEOS data set, covering almost the
entire GRACE period from mid 2002 to end of 2007, but also newly available gravity anomaly data sets for
Europe and Australia. New processing features are the complete reprocessing of the GRACE and LAGEOS
data using the recent RL04 standards and background models by GFZ (combined with the GRACE/LAGEOS
10-days time series derived at GRGS based on nearly identical standards and background models) and a
further extension of the full normal equations (in contrast to block diagonal form) derived from terrestrial data
to a maximum degree and order of 280 (which was restricted to 179 for EIGEN-GL04C). In particular, this
presentation focuses on the inter-comparison of this latest EIGEN model with the recently presented EGM08
model, which was developed by the National Geospatial-Intelligence Agency (NGA) of the USA.
The EIGEN-5C model and its associated satellite-only model EIGEN-5S are available for download at the
ICGEM data base (International Center for Global Earth Models) at GFZ Potsdam via the following URL:
http://icgem.gfz-potsdam.de/ICGEM/
http://icgem.gfz-
potsdam.de/ICGEM/
G13A-0627
Quick-Look Gravity Solutions From GRACE
High-resolution gravity field time-variability estimates – if available with a low latency – serve multiple purposes. For science, it allows the exploration of its potential for assimilation or constraining near-real time geophysical models – as has been proposed by other authors for hydrological applications. For GRACE operations, it provides a quick end-to-end check of the data collection and data processing systems, and minimizes the potential for extended data loss due to mis-configuration of the flight system. The GRACE Science Data System (SDS) element at JPL creates "quick-look" Level-1B (or tracking) data products for the purposes of daily monitoring of the science data quality. The UTCSR element of the GRACE SDS has implemented a "quick-look" Level-2 gravity field product using this data. A moving-window system is implemented, with a window step of one day; window width of 15 days; incorporating GRACE data up to and including "yesterday". This paper describes the architecture and implementation of this (nearly) autonomous data processing sub- system. The data flow, data dependencies and latencies are described. The models generally conform to the GRACE Release-04 standards, though there are certain important exceptions. These exceptions and their significance are discussed. The quick-look products are then compared to the routine GRACE Release-04 products, which are normally available with a 6-week latency.
G13A-0628
Improvements in GRACE Gravity Fields Using Regularization
The unconstrained global gravity field models derived from GRACE are susceptible to systematic errors that show up as broad "stripes" aligned in a North-South direction on the global maps of mass flux. These errors are believed to be a consequence of both systematic and random errors in the data that are amplified by the nature of the gravity field inverse problem. These errors impede scientific exploitation of the GRACE data products, and limit the realizable spatial resolution of the GRACE global gravity fields in certain regions. We use regularization techniques to reduce these "stripe" errors in the gravity field products. The regularization criteria are designed such that there is no attenuation of the signal and that the solutions fit the observations as well as an unconstrained solution. We have used a computationally inexpensive method, normally referred to as "L-ribbon", to find the regularization parameter. This paper discusses the characteristics and statistics of a 5-year time-series of regularized gravity field solutions. The solutions show markedly reduced stripes, are of uniformly good quality over time, and leave little or no systematic observation residuals, which is a frequent consequence of signal suppression from regularization. Up to degree 14, the signal in regularized solution shows correlation greater than 0.8 with the un-regularized CSR Release-04 solutions. Signals from large-amplitude and small-spatial extent events - such as the Great Sumatra Andaman Earthquake of 2004 - are visible in the global solutions without using special post-facto error reduction techniques employed previously in the literature. Hydrological signals as small as 5 cm water-layer equivalent in the small river basins, like Indus and Nile for example, are clearly evident, in contrast to noisy estimates from RL04. The residual variability over the oceans relative to a seasonal fit is small except at higher latitudes, and is evident without the need for de-striping or spatial smoothing.
G13A-0629
Studies of GRACE Gravity Field Inversion Techniques
The geophysical inverse problem using satellite observations, such as GRACE, to estimate gravity change and mass variations at the Earth's surface is a well-known ill-posed problem. Different methods using different basis function (representing the gravity field) for different purposes (global or regional inversion) have been employed to obtain a stable solution, such as Bayesian estimation with prior information, the repro-BIQUUE of variance components and iterative least-squares estimation with simultaneous updating of a prior covariance, and to achieve enhanced spatial resolutions. The gravity field representation methods include spherical harmonics, regional gridded data (including mascons), and various wavelet representations (Poisson wavelets, Blackman band-limited regional wavelets with global representation). Finally, the use of data types (KBR range, range-rate, range-rate-rate) and data-generation methods (e.g., nonlinear orbit determination and geophysical inverse approach, energy conservation principle, etc) could also reflect relative inversion accuracy and the content of signal spectra in the resulting solution. In this contribution, we present results of a simulation experiment, which used various solution techniques and data types to attempt to quantify the relative advantage and disadvantage of each of the techniques.
G13A-0630
Applications of EOF filter methods to GRACE
The Gravity Recovery and Climate Experiment (GRACE) mission is designed to map the Earth's gravity field at monthly intervals with a spatial resolution of about 250 to 500 km. The solutions provided by the GRACE science team demonstrate that a wide variety of surface mass signals can be observed. For this paper we used the RL04 solution developed by the center of space research at the University of Texas at Austin. We use all available monthly gravity solutions and represent the temporal signal as a surface mass layer under the assumption of an elastic lithosphere. GRACE gravity fields solutions are affected by noise which we attempt to suppress by means of an Empirical Orthogonal Filtering technique based on Gaussian smoothed surface mass data. We will discuss various implementations of the EOF filtering technique that have enhanced the spatial resolution by which surface mass anomalies can be retrieved from GRACE. Surface mass solutions with EOF algorithm 1 are based upon a singular value decomposition of smoothed surface mass fields. A later version of the EOF method, called algorithm 2, directly operates on the spherical harmonic coefficient sets grouped by spherical harmonic order m. We obtained surface mass fields with EOF algorithm 1 and compared the vertical loading displacements to a set of independent IGS station data acquired within the GRACE observation window. An optimum was found at an EOF compression level of 3 and a smoothing radius of 5 or 6.25 degrees so that the discrepancy between GRACE and GPS vertical loading signals is around 1.9 mm for 59 IGS stations. In this we assume that temporal gravity field changes occur on frequencies shorter than about 3 years because of the way we pre-process the GPS data and GRACE surface mass estimates. The performance of EOF algorithm 2 was evaluated by means of simulations which demonstrate that it is feasible to obtain a spatial resolution of 250 km. We use this method to estimate small scale surface mass trends for Greenland and found a surface mass trend of 179±25 Gton/yr between 2003 and 2008. Between 2003 and 2005 we observed 121±27 Gton/yr and between 2006-2008 204±25 Gton/yr. This method showed for the first time mass losses above 2000m elevation.
G13A-0631
Wavelet Modeling of the Gravity Field Using Domain Decomposition Methods: an Example Over Japan
Regional, high resolution modeling of the gravity field is of great importance for many geodetic and geophysical applications, allowing to improve our knowledge of the geoid and of the Earth's internal structure. It requires to combine the satellite-derived information with the surface gravity data, considering their respective error characteristics. Here we show how to realize such a combination in a flexible way using wavelets, taking the example of gravity field modeling over Japan, a very challenging area marked by important gravity variations. The gravity potential is expressed as a linear combination of wavelets, which coefficients are obtained by least squares adjustment of the datasets. In order to handle large systems, we apply a domain decomposition method. First, we define sub-domains as subsets of wavelets. Based on the localization properties of the wavelets in space and frequency, we define hierarchical sub-domains of wavelets at different scales. For each scale, blocks sub-domains are defined using a tailored spatial splitting of the area. Secondly, we approximate the sub-domains normals by introducing local approximations of the wavelets at different scales, which leads to build agglomerated values of the data. Finally, we solve the system iteratively. We show an example of application of this method over Japan, where we combine the EIGEN-GL04S spherical harmonics model with a local combined gravity model from land, marine and KMS 2002 altimetry derived gravity data. The obtained hybrid spherical harmonics/wavelet model has a resolution of about 12 km and underlines the presence of significant biases in the local model. This information can be used to improve the local model, and the method may be applied on the corrected datasets to derive an improved hybrid model.
G13A-0632
SLR-determined Low-Degree Geopotential Harmonics and their use With GRACE Data Products
GRACE provides estimates of the temporal variations in the Earth's gravity field with extraordinary precision, but as with any non-synoptic measurement system, the problem of aliasing of high- frequency signals errors is an important concern. In the case of GRACE, errors in the models for the diurnal and semi-diurnal (solid earth and ocean) tides will alias into long-period variations in the low-degree geopotential harmonics. This issue is particularly apparent for the degree-2 zonal harmonic, C20. In addition, after removing the tidal aliases, the GRACE C20 series is still noisier than SLR by about a factor of 3. As a consequence, GRACE data product users typically replace the C20 estimates from GRACE with estimates derived independently (but using consistent modeling) from SLR tracking of several geodetic satellites. For the other low degree harmonics, the GRACE estimates do not appear to suffer from significant tidal aliasing or excessive noise. In this paper, we will discuss the characteristics of the GRACE and SLR time series for the low-degree harmonics. In particular, we discuss the SLR and GRACE estimates for the harmonic terms describing the orientation of the Earth's principal figure axis, C21 and S21, and compare them to the conventional model for the long-term evolution of these terms.
G13A-0633
Earth Oblateness (J2) Variations on Interannual-Decadal Time Scales: Constraints on Anelastic Effects
Observations of J2 began during the 1970s, enabling constraints to be placed on meridional mass transport within the Earth system that significantly predate the GRACE mission. Here, we use SLR solutions with the standard IERS solid Earth tides removed to examine the J2 budget for a 12-year period (1993-2005) during the satellite altimeter era. By accounting for variations arising from ocean mass redistribution and land hydrology using output from the ECCO and LaD models, we isolate J2 changes due to high-latitude land ice (HLLI) that peaked during the global warming that followed the large 1997-98 ENSO. Comparison of the HLLI-inferred global sea level changes with AVISO altimeter data corrected for steric and hydrological effects shows good agreement for an effective ablation latitude of 56 degrees, indicating a substantial contribution from sub-polar glaciers. The close correspondence obtained between residual oblateness and sea level variations argues against the presence of a strong, unmodeled anelastic response to the lunar nodal tide in our J2 data.
G13A-0634
Analysis of Postseismic Effects of the Sumatra 2004 Earthquake: Contribution to GPS and Geoid Height Change From Viscoelasticity
Almost four years have passed since the 2004 earthquake and tsunami offshore Sumatra. Since then, a large amount of measurements have been collected, documenting the postseismic effects of the event. Part of these effects is due to relaxation processes in the crust and upper mantle. We use our slip-model of the earthquake, which is based on coseismic GPS-measurements and embed it into a viscoelastic earth model to compare predicted with observed GPS time series in order to quantify the contribution from viscoelasticity. Furthermore, we use the same modeling technique to compute the co- and postseismic change of the geoid height and compare it to GRACE gravimetry satellite observations.
G13A-0635
Coseismic and Postseismic Gravity Changes Associated with the 2004 Sumatra Earthquake: Comparison between GRACE and SNREI model
GRACE has been providing monthly gravity field data with unprecedented accuracy. The time-varying gravity fields recovered by GRACE reflect mass redistribution on and in the Earth, which caused by, for instance, hydrology cycles and ocean currents. Earthquakes also change the Earth's gravity field slightly. GRACE detected the gravity changes due to the 2004 Sumatra-Andaman earthquake for the first time (Han et al., 2006), and revealed the associated postseismic gravity changes (Ogawa and Heki, 2007). Most of previous studies employed the dislocation theory in a half-space Earth to interpret the coseismic gravity changes observed by GRACE. However the half-space dislocation theory should be strictly limited to near field problems, because the Earth's curvature and stratification can not be ignored for far field problems. The typical spatial resolution of GRACE is a few hundred kilometers. Therefore a dislocation theory in a spherically layered Earth is necessary for the accurate interpretation of the coseismic gravity changes observed by GRACE. In our previous study, we employed a dislocation theory (Sun et al., 2006) with SNREI (a spherically symmetric, non-rotating, perfect elastic and isotropic) Earth model. However the theory lacked the ocean layer and its prediction was not so reliable. In this study, for more accurate calculation, we modified the theoretical calculation by employing a spherically layered Earth model covered with ocean. Practically, we calculated the theoretical gravity changes employing PREM model and a fault slip distribution model estimated from tsunami propagation. Employing GRACE level-2 datasets, the observed gravity changes are calculated as the difference between 4 months average values before and after the earthquake. The theoretical and observed gravity changes basically agreed well. However the amplitude of the theoretical value was about one-half smaller than the observed one. This discrepancy could be interpreted as the effects of postseismic fault slip, which does not included in the theoretical calculation. It also indicates that the postseismic gravity changes mainly due to the afterslip which occurred deeper than 40km. This could be interpreted as an aseismic slip on the slab interface, which is a dominant deformation after recent subduction zone events.
G13A-0636
Post-seismic gravity variation caused by a great earthquake in subduction zone -spectral finite-element approach to consider 3-D viscousity structure
Theoretical models of the viscoelastic relaxation of a spherical earth are derived to model large-scale post- seismic deformation resulting from great earthquakes over decadal time scales. Most existing models of post- seismic deformation do not consider strong lateral heterogeneities in mantle viscosity, in particular in the subducting slab where such events occur. In addition, the self-gravitation effect is often treated only approximately. Both effects become important when observations from space geodetic techniques such as GPS and GRACE are interpreted. In this paper, we present a spectral finite-element approach that allows these two effects to be considered in a rigorous way. In this way, much larger lateral viscosity variations can be handled than by perturbation techniques. We derive interface conditions for an arbitrary shear fault in the form of double-couple forces that are equivalent to a prescribed dislocation, and simulate a relaxation process for an incompressible Maxwell earth with a three-dimensional viscoelastic structure. Computational results are validated for a spherically symmetric model by an independent method based on the inverse Laplace integration, and good agreement is obtained. As an example, we apply this approach to the 2004 Sumatra-Andaman earthquake, and simulate a large-scale post-seismic gravity potential variation by a forward calculation. In the presence of a slab, the secular variation in geoid height change decreases by 30% for wavelengths longer than 500 km, with respect to the case excluding the slab. The effect of the slab can exceed 0.3 mm/yr for short-term variations when the asthenosphere viscosity is 10 to 19 Pa s, which are larger than the observation errors of GRACE. Lateral heterogeneities in viscosity due to a slab should therefore be considered for interpreting observed post-seismic relaxation due to a large thrust event in a subduction zone.
G13A-0637
The significance of secular trends of mass variations derived from GRACE monthly solutions
Since 2002, the Earth's gravity field is globally observed by the Gravity Recovery and Climate Experiment (GRACE) satellite mission. The GRACE monthly gravity field solutions reflect mass variations in the atmosphere, hydrosphere and geosphere. The GRACE data are applied to determine variations with different periodic signatures (e.g., seasonal, short and medium-term), but also long-periodic mass variations and secular trends are derived. As the GRACE monthly solutions always show the integral effect of all mass variations, various analysis methods, filtering techniques and reduction models are used to separate single processes such as glacial isostatic adjustment and hydrological changes. On the one hand this requires, that the reduction models (e.g. from hydrology) have the same quality and resolution as the processes to be addressed. On the other hand, the restricted time span of the measurements may limit the separation of long-periodic and secular signals. Therefore, one has to be cautious with the final interpretation of the results. The filtering of the monthly gravity fields is another candidate that affects the interpretation, as the amplitudes of the signals and their spatial resolution are influenced. We show this on examples like the North American and Fennoscandian uplift areas and selected river basins, where the use of different reduction models from hydrology (such as WGHM, LaDWorld, GLDAS), the selection of the filters or a change of the analyzed time span can lead to misinterpretation of geophysical processes to be considered. It also becomes evident that the determination of trends should be performed together with the determination of periodic components. However, it is only appropriate to include such periodic terms which are really contained in the data, and consequently, which can be detected as significant.
G13A-0638
Gravity Changes Detection from GRACE in China and its Vicinity
After the Gravity Recovery and Climate Experiment mission was successfully launched in March 2002, Three
researcher center£¬a lot of gravity fields of the Earth over its six year life time are released by UTCSR, NASA
in the USA and GFZ in German, which includes the time-variable global gravity field models with the
maximum degree to 120 (RL01) and 60 (RL04) , and 1 month time resolution. The unprecedented precision
of GRACE mission will enable us to detect the time variations of the gravity field related to global
redistributions of water and air mass inside fluid envelops at the surface of the Earth.
The purpose of this paper is to study how to use these products to detect the gravity changes in China and
its vicinity according to the spatial resolution and the required precision for earthquake, and try to analyze
some gravity information for special earthquakes. Firstly, in order to study earthquake, the maximum
truncated degree of the monthly GRACE gravity field models is discussed. Secondly, we use the GRACE
level 2 products to calculate the gravity changes in China mainland and its vicinity with Gaussian smoothing
method, and analyze the reason for the results. Thirdly, we do some experiments on special area and special
earthquakes and get reasonable results.
G13A-0639
The Changing Cryosphere in Alaska: Results and Implications
Changes in the climate system affect the geodetic properties of the planet as a whole; ablation of land ice, in particular, directly impacts the Earth's dynamic oblateness (J2) by redistributing water mass from high latitudes to the global ocean. Dickey et al. (2002) discovered a pronounced geodetic signature of glacial melting beginning in 1997, and showed that it accounted for a substantial portion of a large J2 anomaly that occurred during the last large ENSO event and its aftermath. Melting of Alaskan glaciers made the largest sea level contribution of the regional glacier groupings during this time, coincident with extremely high air temperature anomalies over the region. In this study we consider mass balance changes in Alaskan glaciers during the GRACE era, combining time-variable gravity retrievals with other interdisciplinary data types including output from the most recent hydrology models, to assess further changes of land ice mass balances in this key region and their contributions to ongoing global geodetic and sea level changes.
G13A-0640
GRACE Estimate of Mountain Glacier Contribution to Global Sea Level
Mountain glacier ice melt is the leading contributor to global ocean mass increase (Meier et al., 2007; IPCC, 2007), and may continue to dominate for much of this century (Meier et al., 2007). We present an improved estimate of glacial mass loss that combines ground-based observations of mountain glacier mass trends with estimates of surface mass variations from the GRACE project. The variability of mountain glacier ice mass can be estimated through ground-based measurements; however, the great number of and sometimes remote setting of mountain glaciers precludes comprehensive observations of their mass variations. The strengths of the GRACE project complement the shortcomings of ground-based estimates. While it cannot spatially resolve individual glaciers, GRACE does have the sensitivity to detect the mass loss from some of these glaciers, and enables monthly mass balance estimates. An iterative forward-modeling technique is used to minimize the rms of the difference between the ground-based glacier mass estimates and mass anomaly estimates based on six years of satellite gravimetry. Discrepancies between GRACE mass estimates and ground-based glacier mass balance estimates may indicate that glacier meltwater is re-freezing or entering aquifers within the 300-500km GRACE footprint, and thus not contributing to sea level.
G13A-0641
Ice Melting Versus Tectonic Uplift: Current Mass Balance in Tibet as seen by GRACE
Uplift of the Tibetan Plateau started ~50 m.y. ago as a result of the collision between the Indian and the Eurasian Plates. It now consists of extensive flat terrain with average elevation of 5 km and the world highest mountain range at its southern rim. GPS surveys in 1990s demonstrated the average uplift rate of ~0.8 cm/yr in the plateau and ~1.6 cm/yr in the southern part (Himalaya) [Xu et al., 2000]. The total increase of mass is comparable to those in Fennoscandia where rapid uplift by postglacial rebound (PGR) makes a clear signal of gravity increase in the GRACE time-variable gravity data. On the other hand, the Tibetan Plateau is known as the third pole with the largest store of ice in the low/middle latitude zone. These mountain glaciers have relatively large seasonal changes in thickness, and their total mass is susceptible to recent global warming [Meier, 1984]. In fact, their current retreat rates suggest that 2/3 of the glaciers may disappear by 2050 [Qiu, 2008]. So the Tibetan ice mass loss should be observable with GRACE as a gravity decrease about a half of that observed in southern Alaska. Trend in gravity in Tibet 2002-2008 by GRACE shows only moderate amount of decrease. This would be a mixture of the two opposite factors, i.e. (1) gravity increase due to rapid tectonic uplift, and (2) gravity decrease due to ice melting. In this study, we try to present a self-consistent model by reconciling the mass gain due to uplift and mass loss caused by the glacial retreat. First we modeled the vertical velocity field in Tibet using GPS data, and calculated the gravity increase due to the uplift. Then we subtracted this gravity increase from the gravity trend observed by GRACE, to isolate the gravity signature of ice melting. The inferred mass loss, about a half of the value in Alaska, and 1.5 times as large as those in the Southern Andes, was fully consistent with the glaciologically expected rate of ice melting in Tibet. In order to assess its robustness to tectonic models, we assumed the vertical motions of Moho beneath Tibet in several different ways, i.e. (1) it uplifts coherently with surface, (2) it remains at the same position, and (3) it subsides to partially achieve isostasy, and evaluated the difference in the estimated ice melting rate.
G13A-0642
Acceleration of terrestrial water storage changes from GRACE data
Gravity Recovery and Climate Experiment (GRACE) satellite has been producing scientific results on mass variations since its launch in 2002, particularly land water storage on seasonal and inter-annual timescales as the soil moisture reflects the time integration of fluxes of precipitation, evapo-transpiration and runoff. For example, in Amazon Basin (e.g. Tapley et al., 2004), Alaska glacial melting (e.g. Tamisiea et al., 2005), ENSO precipitation anomalies (Morishita and Heki, 2008), and seasonal land water storage with global hydrological model (Syed et al., 2008). If climate changes have trends of time scale longer than inter-annual, we can expect to see quadratic trends in land water time series now that over six years have passed since GRACEfs launch and the time span is becoming long enough to study such trends, which signify the temporal acceleration in gravity, and hence climatic, changes. To look for such accelerations, we compute time series of equivalent water thicknesses in global land regions from monthly GRACE data of gravity anomaly, and model the changes with quadratic functions in addition to seasonal components. We repeat similar calculations for the GLDAS global hydrological model data as well. We found that the geographic distribution of the quadratic trends shows good agreement between GRACE and GLDAS, prominent in East Africa, East Europe, Ural Mountains, eastern North America and southern South America. Amplitudes of the signals are generally larger in GRACE than the corresponding GLDAS model. We also compare and verify such acceleration terms with trends in meteorological data of precipitation and evapo-transpiration.
G13A-0643
Changes in Terrestrial Water Storage Capacity and Flood Potential Using GRACE and GLDAS
A comparison of GRACE-based terrestrial water storage estimates with time-integrated precipitation observations at a basin scale demonstrates the water retention in a basin relative to the water input. What we see over the GRACE record length are maxima in water storage that fall short of maxima in cumulative precipitation. This suggests some saturation point for a given basin at which additional precipitation must be met by marked increases in runoff or evaporation. Like a bucket that has reached its full volume, these annual saturation periods mark the possible transition to a flood-prone situation. In some basins, these observed storage maxima under precipitation excess vary interannually, making floods more likely in certain years. In order to understand this changing storage capacity, available observations and model outputs of terrestrial water storage, precipitation and temperature are considered over interannual periods.
G13A-0644
Water Level Temporal Variation Analysis at Prata Basin Using GRACE
A comparison between daily in-situ water level time series measured at ground-based hydrometric stations of Agência Nacional de Águas (ANA) with vertically-integrated water height deduced from GRACE geoid (height anomaly) is carried out. The 10-day intervals of GRACE models were computed by Groupe de Recherches de Géodésie Spatiale (CNES/GRGS). The height anomaly was converted into equivalent water height, over the Prata basin for a ~6-year period (July-2002 to May-2008). A correlation around 74 per cent has been detected. This correlation allows defining a local transfer function by adjusting a linear relationship between GRACE-based and in situ observation time-series. The study of the Continuous Wavelet Transform was applied in the hydrometric stations and a time-scale correlation was figured out.
G13A-0645
Comparison of Mass Variations Derived from GRACE Data with Those Obtained by Hydrological Modeling
Thanks to the satellite gravimetry mission GRACE (Gravity Recovery And Climate Experiment) launched in 2002, it is now possible to directly monitor the re-distribution of mass at the vicinity of the Earth's surface and inside the Earth. A number of processing centers (GFZ, CSR, JPL, CNES, etc.) are involved in the processing GRACE data and the distribution of global models of mass variations with a spatial resolution of about 350 km and a temporal resolution of 1 month. An original methodology for GRACE data processing has been recently developed at the Delft Institute of Earth Observation and Space Systems (DEOS), TU Delft. Thanks to consistent implementation of statistically optimal data processing algorithms, the obtained models demonstrate remarkably high spatial resolution, particularly in polar areas, where the amount of information per unit area is relatively large due to convergence of satellite ground tracks at the Earth's poles. On the basis of the computed time series of monthly models, it is also possible to quantify regular mass variations, e.g., the rate of the secular trend as well as the amplitude and phase of the yearly cycle. The major focus of the study is a comparison of mass variations derived from GRACE data with those predicted by hydrological models. To this end, we exploit the hydrological model PCR-GLOBWB, which was forced with forecasted daily meteorological surface fields from the ECMWF ERA-40 reanalysis and operational archive over the period 1990-2006. PCR-GLOBWB evaluates the water balance for the soil column and the open water bodies contained within each 0.5° cell of the global land mass with the exception of Antarctica. The resulting discharge-runoff and direct channel input is routed over the drainage network with consideration of the different behavior of river and lake stretches in terms of storage characteristics. Changes in storage are prescribed by the ECMWF surface fields of precipitation, air temperature and actual evapotranspiration. The land surface parameterization within PCR-GLOBWB considers different sources of sub-grid variability in the soil hydrology and the routing scheme, including soil and vegetation types, soil moisture storage capacity, elevation, fraction open water and channel surface roughness. The model includes the most direct pathways of water within the terrestrial part of the hydrological cycle. In the course of the study, we compare the rate of secular trends observed in selected geographical regions (North India, Pamir mountains, etc.). Furthermore, we analyze mass variations in shallow oceans (in particular in the arctic seas and in the mouth of the Amazon), which can be associated with the maximum discharge (observed or simulated) from the neighboring rivers. The conducted analysis not only allows GRACE-based and hydrological models to be cross-validated, but also stimulates further progress in combining GRACE data processing with hydrological simulations.
G13A-0646
Variations in the total water storage in the major river basins of India from GRACE satellite gravity data
We present an estimate of total water storage variations of the major river basins of India during the period of 2002 to mid 2008 from modelling of time-variable gravity field observed by GRACE satellite by utilising the scheme of Swenson and Wahr, (2002). The largest annual volume change is observed over the upper Ganga basin, followed by the lower Gnaga basin and the Yamuna basin of northern India. Basins of northern India show a declining trend of water storage over this time period, whereas the Godavari basin, the largest basin of central south India, as well as basins in central India show similar seasonal variations but increasing trends. It is interesting to note that these trends are prevalent over a decadal time period of ground water level and therefore the trend observed from GRACE data can be extrapolated backward. If these trends are sustained over a long time period, northern India and Bangladesh will lead to a major water crisis
G13A-0647
Variability of the Antarctic Circumpolar Current derived from GRACE retrievals, model simulations and in-situ measurements
The Gravity Recovery and Climate Experiment (GRACE) provides estimates of the Earth's static and time-variant gravity field. Solutions from various processing centres (GFZ, CSR, GRGS, JPL etc.) enable us to determine mass redistributions on the globe. Given that land signals are generally large compared to anomalies over the ocean, an assessment of the latter requires a particularly careful filtering of the data. We utilized the Finite Element Sea-Ice Ocean Model (FESOM) to develop a filtering algorithm which relies on the spatial coherency of ocean bottom pressure (OBP) anomalies. Taking large-scale circulation patterns into account, the new filter yields an improved representation of OBP (i.e. ocean mass) variability in the filtered GRACE data. In order to investigate the representation of Antarctic Circumpolar Current (ACC) variability in the pattern-filtered GRACE retrievals, an analysis of OBP anomalies in FESOM results and in-situ measurements has been performed. Data from a PIES (Pressure sensor equipped Inverted Echo Sounder) array (36°S-55°S, 2°W-13°E) south of Africa provides bottom pressure recorder data from 2002-2008 for the ACC region. Based on anomalies of OBP gradients between individual instruments, these in-situ measurements give an estimate of the overall transport variability as well as of the movement of ACC fronts and transport redistribution between different sectors of the ACC. The validation of simulated and satellite-derived OBP anomaly gradients against these data yields a measure for the representation of this variability in FESOM and GRACE. Furthermore, model simulations are used to assess the relation between transport variations in individual filaments of the Southern Ocean and total transport variability in this and other sectors of the ACC.
G13A-0648
Improving GRACE Mass Estimates for the Baltic Sea and Validation Using in Situ Measurements
The variation in the sea level of the semi-closed Baltic Sea has been monitored in several complimentary ways. Now GRACE provides a method to directly measure the total mass variability in the Baltic. Using in situ and modelled Baltic data, we show that GRACE is able to recover the variation in the total water mass. We derive sea level surfaces from tide gauge data and estimate steric effects using hydrodynamic models as well as in situ salinity and temperature measurements for their verification. With its areal extent (~400 km x 1000 km) as well as fast temporal variations (hourly to monthly), the Baltic Sea provides a challenging test field for the temporal and spatial resolution of GRACE. We use both the standard monthly GRACE gravity field solutions and regional solutions and compare their capability to recover Baltic water mass variations. Due to spatial averaging, the GRACE mass estimates over the elongated area are contaminated by signals outside the region. The contribution of continental hydrology can be removed using water storage models to estimate mass variations on surrounding land areas. We discuss the processing steps required for the different GRACE solutions to improve the GRACE mass estimates for the Baltic, including mitigation of signal leakage as a result of spatial filtering. The capability of GRACE to recover internal mass redistributions in the Baltic is also investigated. Finally, we discuss the reduction of the Baltic contribution for studying land-uplift signal due to post-glacial rebound.
G13A-0649
Sea Level and Ocean Bottom Pressure Variations From Altimetry, Mercator Model and GRACE Mission in the Argentine Basin
Sea Level Variations (SLV) in the Argentine basin, as already observed from space altimetry missions, show one of the most prominent variances in the oceans. Here we study two major signals in the Argentine basin, the annual and submonthly SLV signals, based on altimetry, various hydrographic data, and the time-variable gravity data from the GRACE mission. We demonstrate that the annual variation is mainly driven by density variations of the water column, that is the steric-SLV. In contrast, the submonthly SLV in the form of the so- called Argentine gyre, a barotropic counterclockwise gyre with a period around 25 days, is mainly mass- induced, that is, mostly produced by mass variations in the region [Fu et al. 2000]. The Argentine gyre signal is well reproduced by the MERCATOR ocean circulation (and bottom pressure) model, unlike the annual signal. We show that now the aliased form (into monthly sampling) is also captured in the GRACE time- variable gravity data after applying appropriated filters; further study awaits higher temporal-resolution GRACE data.
G13A-0650
Gravity measurements on rhyolite domes near the Krafla volcano, North Iceland
Krafla is a central volcano in the Northern Volcanic Zone (NVZ) of Iceland. It began forming about 200 000 years BP, has a caldera, and is transected by a N10°A trending fissure swarm. Krafla's products are mostly basaltic but rhyolite domes have formed around the caldera rims. The Krafla area is both easily accessible and one of the most studied areas in northern Iceland due to the geothermal power plant situated above the caldera's shallow magma chamber and the recent volcano-tectonic episode, the Krafla fires in 1975-1984. Silicic rocks in Iceland are generally associated with central volcanoes. Rhyolite magma can rise due to buoyancy forces as a cryptodome to the surface where it erupts. Due to its high viscosity and resistance to flow it often accumulates and forms a lava dome over the vent. Data on density and volumes are essential for meaningful modelling of the emplacement of cryptodomes and lava domes. Such data is scarce. Therefore a gravity survey was carried out in the Krafla area in the autumn of 2007 to determine the mean bulk density values of rhyolite domes and their approximate mass and volume. Profiles were measured over three formations, ranging in size from Hlíðarfjall (310 m high and 2 km long), formed 60 000 years BP, to Hrafntinnuhryggur (80 m high and 2.5 km long) formed 30 000 years BP, to Hraunbunga (125 m high and 1.8 km long) formed 10 000 years BP. Mean bulk density for each formation was obtained by the Nettleton method and the volumes were calculated. Mean bulk density and volumes obtained were, Hlidarfjall: 1600- 1800 kg/m3, 0.14 ± 0.01 km3; Hrafntinnuhryggur: 1600-1900 kg/m3, 0.021 ± 0.002 km3; Hraunbunga: 1800-1900 kg/m3, 0.040 ± 0.004 km3. The mean bulk densities from rock samples collected at the survey area are in good agreement, supporting these findings. All the formations have low density values, reflecting both low grain density and high porosity. The Nettleton profiles demonstrate that these formations are neither buried by younger volcanic eruptives nor are there roots detected. The three domes studied were therefore emplaced as vent-forming domes.
G13A-0651
LEO Precise Orbit Determination
The two GRACE satellites belong to a series of dedicated satellite missions for the precise high resolution determination of the Earth gravity field and its variation over time. Especially designed for these goals, LEOs (Low Earth Orbiters) circulate in orbits of just 300 to 500 kilometers above the surface of the Earth. Since the launch of these missions high-precision orbit models gained much importance. For this objective a software for precise orbit determination was established, employing a dynamic and reduced-dynamic approach. For the dynamic approach a high quality force field, which contains all relevant conservative forces, was modeled for the orbit integration. Within a least-squares adjustment the orbit is fitted to GPS measurements. For orbital arcs over 24 hours this method yields accuracies of a few meters. To compensate for these deficiencies of the dynamic model pseudo-stochastic parameters are introduced. These parameters can be instanteanous velocity changes (pulses) or piecewise constant accelerations. The resulting reduced-dynamic orbit model exploits both the advantages of the force field and the geometric strength of the observations, which leads to a considerably improved precise orbit determination. For the CHAMP and GRACE satellites, comparisons to orbit solutions of other agencies show accuracies of well below ten centimetres. In summary, this presentation highlights the currently achieved quality of dynamic and reduced-dynamic orbit solutions provided by our new software tool developed at the Technical University Vienna.