B.1 - Progress in Interdisciplinary Applications
(Moderator: John Ries)

   Thomas Gruber (12)
   David Salstein (12)
   Svetozar Petrovic (12)
   Jean Dickey (12)
   C. K. Shum (12)
   Yan Ming Wang (12)
   Michael Schmidt (8)
   David McAdoo (5)
   Richard Gross (12)
   Jolanta Nastula (12)
   Erricos C. Pavlis (10)
   Mark Tamisiea (12)
   Discussion (29)

Thomas Gruber (12)
Presenter: Thomas Gruber
Co-Authors: L. Zenner; Th. Trautmann; J. Wickert, F. Flechtner; D. Stammer
Title: Improved De-Aliasing for Gravity Field Modelling with GRACE

Abstract: Within the German Research Foundation priority programme "Mass Transports and Mass Distribution in the Earth System" a joint project about improved de-aliasing for gravity field modelling with GRACE (and other missions) was proposed by the authors. The proposal was accepted to a large extent and the project will start in early 2007.

The work proposed addresses one of the main remaining problems in the GRACE data analysis, which is the atmospheric and oceanic de-aliasing. From the current perspective, this is one of the limiting factors for the GRACE gravity field performance and for all follow-on studies. The de-aliasing can be improved in two ways. (1) The atmospheric and oceanic pressure fields required for the de-aliasing shall not be regarded as error free. For this purpose, extensive analysis of the key parameters by comparison with other model data or with independent observations will be done. Outcome of this work will be representative error measures for the required parameters, which further on will be taken into account for the computation of the de-aliasing products in order to get error measures for them, too. (2) The complete sampling and aliasing model shall be better understood. For this a mathematical model will be developed taking into account the frequency spectra of all known mass effects, the orbit characteristics of a mission as well as possible gaps in the observation time series. The goal is to determine with this tool optimal time series, which guarantee a uniform space-time sampling for a mission.

The paper gives an overview of the project, summarizes the project goals and provides some first ideas about the envisaged approaches in order to reach these goals.

David Salstein (12)
Presenter: David Salstein
Co-Authors: R.M. Ponte, K. Cady-Pereira
Title: Uncertainties in atmospheric surface pressure fields from global analyses

Abstract: Operational analyses from the U.S. National Centers for Environmental Prediction (NCEP) and the European Centre for Medium-Range Weather Forecasts (ECMWF) provide, among other quantities, atmospheric surface pressure. Such fields are especially useful for data processing and interpretation of gravity satellite missions like GRACE (Gravity Recovery and Climate Experiment).

Quantitative use of these pressure analyses requires knowledge of their uncertainties. To assess these uncertainties, we compare ECMWF and NCEP analyses of surface pressure to each other and to an extensive set of surface barometric observations for the period 2001-05, both land- and ocean-based. The analysis surface pressure at model grid points is reduced to the station topography height and interpolated to determine equivalent surface pressure at the station location; it is also reduced to sea level for other comparisons. Differences between each analysis and the observations are very similar, indicating that both analyses fit the data equally well. Such differences, which include both bias and variance about the bias, are largest over parts of the Southern Ocean, and over Asia, central Africa, western South America, and Antarctica.

The mean difference between the NCEP and ECMWF analyses changes only slightly over the period of study, though it decreases in some regions. More importantly, such differences are substantially smaller than those found by previous studies for an earlier period (1993-95), suggesting recent improvements in both analyses. The largest differences between the NCEP and ECMWF analyses are found over high latitudes, particularly over Antarctica, and in the respective winter hemisphere. Much of the differences occur on rapid timescales, though with a significant portion also at timescales longer than 1-2 months in some regions.

Svetozar Petrovic (12)
Presenter: Roland Schmidt
Co-Authors: R. Schmidt, J. Wčnsch, F. Barthelmes, M. Rothacher
Title: Morphology of time-variable gravity from GRACE and global hydrology models

Abstract: Exploitation of GRACE-based surface mass anomalies in geoscientific applications requires the signal separation of the integral gravity observations performed by satellites, i.e. the detection of individual contributions of gravity change (in space and time) which can then be used as the input for quantitive modelling of specific processes of interest (e.g. hydrology, oceanography, isostatic adjustment, etc.). To achieve this, characterizations of the morphology of the individual processes are needed, which are then to be traced in the GRACE data.

This contribution focuses on hydrology. We present first results of such characterizations inferred from different state-of-the-art global hydrology models. Common features are compared with recent time series of monthly GRACE-only gravity models. Especially an approach based on the application of Empirical Orthogonal Functions (EOF) combined with the detection of periods offers promising results. In this way it is possible to find strong periodic components representative on a global scale or for individual catchment areas.

Jean Dickey (12)
Presenter: Jean Dickey
Co-Authors: Steven L. Marcus and Florian Seitz
Title: Insights from GRACE: Climate Change in Siberia and Northern Canada

Abstract: Discharges from the land masses surrounding the Arctic Ocean form a key component of its freshwater budget, which in turn plays a critical role in regulating its internal dynamics. Especially during summer, runoff from the Ob, Yenisei and Lena basins in Siberia, and the Mackenzie basin in Canada, leads to the formation of a cold Arctic halocline near the surface which strongly impacts the processes of vertical mixing and sea ice formation. To understand the impacts of global change on these processes, therefore, it is critical to gain an accurate assessment of freshwater inputs to the Artic Ocean and their changes as the climate warms.

Due to its high orbital inclination and low altitude, the GRACE constellation provides comprehensive geographical coverage, on length scales sufficiently small to resolve the individual catchment basins that provide a major source of freshwater to the Arctic Ocean. Here we explore the impact of GRACE data on tracking changes in Arctic freshwater runoff and its sources, using comparisons with estimated river discharge and net precipitation to form water mass budgets for the individual basins listed above. Changes on both seasonal and interannual timescales will be investigated.

C. K. Shum (12)
Presenter: C. K. Shum
Co-Authors: S. Han, and C. Jekeli
Title: GRACE Validation and GIA Studies Using Regional Solutions

Abstract: We report our progress on our investigation related to GRACE data product validation using regional solutions, with our science focus on the potential improvement of GIA modeling, with enhanced constraints on ice sheet mass balance and its role in southern ocean sea level rise. In this paper, we will discuss current and ongoing science results include observation of ocean tides underneath Antarctic ice shelf, Amazon and Congo hydrologic mass fluxes, quantification of the coseismic deformation following the Sumatra-Andaman earthquake, and studies of the Laurentia GIA.

Yan Ming Wang (12)
Presenter: Yan Ming Wang
Co-Authors: D Roman and J Saleh
Title: Improving Regional Geoid by optimal Combination of GRACE Gravity Model and Surface Gravity Data

Abstract: For regional geoid determination, surface gravity data and satellite gravity models are combined. The surface gravity data are given in a limited area, at the same time, global gravity models, such as the GGMs from GRACE mission, are available in spherical harmonic series up to certain degree and order. Errors in both data sets have different characteristics. Satellite gravity models are accurate at long wavelengths while surface gravity data contains more reliable short wavelengths of the gravity field. Methods for optimal combination of both data sets have been proposed but with little application.

In this paper, we apply the Kleusberg and Vanicek method for the combination. Since the computations are local, the effect of the non-orthogonality is invested numerically. To avoid the effect non-orthogonality, Fourier analysis is used for combination in spectral domain. The combination results are compared with the NGS' GPS/leveling data. Results will be discussed and conclusions will be drawn.

Michael Schmidt (8)
Presenter: Michael Schmidt
Co-Authors: C.K. Shum
Title: Multi-Resolution Representation of the Gravity Field from Satellite Data

Abstract: In this contribution, we determine a regional multi-resolution gravity model based on time-variable scaling coefficients from GRACE data. Traditionally in satellite gravity recovery problems, the global gravity field of the Earth has been modeled as a spherical harmonic expansion. Spatio-temporal gravity fields from GRACE data are usually computed for fixed time intervals, like one month or 10 days. Since the Earth's gravity field shows heterogeneous structures over the globe, a multi-resolution representation (MRR) seems to be an appropriate candidate for an alternative spatial modeling.

The MRR means basically the approximation of a signal under different resolution levels applying low- and band-pass filters, here realized by spherical isotropic functions. For the temporal variations, we introduce one-dimensional series expansions such as Fourier series and B-spline expansions.

Consequently, we establish a four-dimensional geopotential model of tensor product type. We apply this approach over selected regions, such as the Amazon basin. Furthermore, we address combination strategies with GOCE data and comparisons with GPS time series.

David McAdoo (5)
Presenter: David McAdoo
Co-Authors: C.A. Wagner, S.B. Luthcke
Title: Co- and Post-Seismic Gravity Signals of Earthquakes in GRACE Inter-satellite Range Rate Data

Abstract: We have analyzed nearly coincident passes of GRACE Level 1B, dual-one-way K-Band (KBR-1B) inter-satellite range-rate measurements data to detect gravity changes associated with very large earthquakes. In addition to the 'raw' KBR-1B range-rate data we have used residual range rates derived (Luthcke et al., GRL, 2005) with GGM02C(120) mean field, GOT00 tide and atmospheric gravity removed. For these residual rates Level 1B accelerometer observations are used to remove or reduce effects of surface forces. Calibration parameters for GRACE A and B accelerometers were re-estimated by Luthcke et al., GRL, 2005. We have confined our analyses to KBR data collected within +/- 4 months of September 19, 2004 when GRACE's slowly changing orbital track repeated every 61 orbital revolutions in 4 days. Exploiting this circumstance, we have stacked (and averaged) virtually co-linear passes of data in the vicinity of known large earthquakes that occurred in the last half of 2004. Then we differenced these averages before and after the quakes in order to detect possible gravity changes associated with them. We detect (cf., Han et al., 2006) a strong co-seismic signal from the great Sumatra-Andaman (SA) mega-thrust quake of Dec 26, 2004 with an amplitude of 0.7 micron/sec and a surprisingly large spatial extent of at least 1400 km which exceeds that estimated by a simpler more confined dislocation model. However, the full extent of the gravity or range-rate change for the SA quake is difficult to estimate due to other apparent signals in the region likely associated with hydrologic mass flux in Southeast Asia or unmodeled regional shallow sea tides. Our analyses also suggest that significant post-seismic gravity change occurred in the month following the SA quake. We are also able to detect a significant gravity (range-rate) change signal associated with other large earthquakes in late 2004. One example is the moment magnitude Mw 8.1 earthquake on December 23, 2004.

Richard Gross (12)
Presenter: Richard Gross
Title: Degree-2 Harmonics of the Earth's Mass Load Estimated from GRACE and Earth Rotation Data and Models of Surficial Geophysical Fluids

Abstract: A fluid, mobile atmosphere and oceans surrounds the solid Earth and upon its land surface lays a continually changing distribution of ice, snow, and ground water. The changing distribution of mass associated with the motion of these surficial geophysical fluids changes both the Earth's gravitational field and, by changing the inertia tensor of the Earth, also changes its rotation. Changes in the Earth's rotation have been measured for more than a century and GRACE is now measuring changes in the Earth's gravitational field at monthly intervals. To assess the quality of the GRACE measurements, the degree-2 mass load coefficients determined from them are compared with those determined from Earth rotation measurements from which the effects of tides, winds, and currents have been removed as well as with those determined from atmospheric, oceanic and hydrologic models. Good agreement is found between these disparate estimates of the degree-2 mass load, particularly at seasonal frequencies.

Jolanta Nastula (12)
Presenter: Rui Ponte
Co-Authors: R.M. Ponte; D.A. Salstein
Title: Comparison of polar motion excitations derived from GRACE and from analyses of geophysical fluids

Abstract: Three different sets of degree-2, order-1 harmonics of the gravity field, derived from GRACE data processed at the GeoforschungsZentrum (GFZ), Jet Propulsion Laboratory (JPL), and Center for Space Research (CSR), are used to compute the polar motion excitation functions chi1 and chi2. The GFZ and JPL excitations and the CSR chi2 excitation compare well for the most part with the geodetically observed excitation after removal of effects of oceanic currents and atmospheric winds, with the JPL series yielding the best comparison. The agreement is considerably better than that obtained with previous GRACE data releases and indicates major improvements in the quality of the latest data. The levels of correlation with the geodetic observations and the amount of variance explained in those series are comparable to, but still lower than, those obtained independently from available models and analyses of the atmosphere, ocean, and land hydrology. Future improvements in GRACE data quality should deliver tighter constraints on the effects of mass loading on the excitation of polar motion at monthly time scales.

Erricos C. Pavlis (10)
Presenter: Erricos C. Pavlis
Title: GRACE-enabled monitoring of geophysical signals with SLR

Abstract: GRACE products have so far improved knowledge of the long-wavelength part of the static gravitational field of Earth by more than two orders of magnitude and for the time, monitored at monthly intervals the temporal variations up to degree and order (90, 90). These achievements enabled parallel improvements in our understanding of the Earth System as well as specific individual components. Our initial proposal focused on the improvement of the long-wavelength static and temporal components of the gravitational field, and its effect on the precise definition and realization of the Terrestrial Reference Frame (TRF). In particular, we focused our work on the precise measurement and monitoring of variations in the center of mass of Earth ('geocenter'), a component that is not observable from GRACE, from Satellite Laser Ranging (SLR) observations to the two LAGEOS and ETALON satellite pairs. As a 'freebee', we were also able to measure with a 5-10% accuracy the Lense-Thirring effect predicted by General Relativity--GR, on the LAGEOS node, due to Earth's rotating mass.

Variations of the geocenter due to mass redistribution in the Earth System cause changes in the degree-one terms of the spherical harmonic series used to describe the gravitational field. Although quite small in magnitude, only a few millimeters for the dominant annual and semiannual variations, failure to account for these results in a biased definition of the 'origin' of the TRF, and limits the accuracy of the TRF to a few millimeters at the epoch of definition with rapid deterioration as we move away in time from that epoch. This is unacceptable for Earth System Science goals at present and even more so for future goals set by NASA, NRC, etc. The precise monitoring of the geocenter and its variations over increasingly shorter intervals is a priority goal in order to meet the requirements of the Global Geodetic Observing System (GGOS) of 1 mm for the definition of the TRF origin at epoch and the 0.1 mm/y limit in its temporal evolution. Such stringent requirements are dictated by the geophysical signals that we are trying to observe with confidence, such as sea level variations, which by all estimates range from 1 to 3 mm/y on a global scale.

In summarizing the objectives of our proposed research, we would like to point out the close relationship between the areas we are focusing on. The coupling of static and temporal gravitational signals at very long wavelengths with the determination of very precise orbits for the geodetic satellites and the establishment and maintenance of the TRF results in the extremely synergistic character of the proposed research effort. It allows for the direct application of GRACE products, their enhancement and extension, their calibration with independently derived products, and the development of precise orbits and products from geodetic satellites, while it enables their use for very demanding tests of gravitational theories. This extends the benefits from GRACE products in scientific areas that were not part of the initial mission design.

Mark Tamisiea (12)
Presenter: Mark Tamisiea
Co-Authors: J. L. Davis; E. M. Hill
Title: Observationally-Derived Estimate of Glacial Isostatic Adjustment

Abstract: Understanding the ongoing response of the solid Earth and oceans to glacial isostatic adjustment (GIA) is a key component in interpreting mass variations observed from GRACE in many polar regions, such as Greenland and Antarctica. Uncertainties in the ice sheet history and 3-D viscosity structure of solid Earth complicate the numerical prediction of GIA. Uniquely parameterizing the ice model is not possible, given the limited number and spatial distribution of sea-level histories and end moraines. Moreover, most GIA predictions are currently generated using spherically-symmetric Earth models. To minimize the impact of model shortcomings, we have developed a data assimilation approach that combines GRACE and GPS data together with expected covariances of these signals across North America derived from forward models to generate a new GIA estimate. This technique has the advantage of allowing the estimated GIA fields to be further constrained by the observations without having to determine the impact on the initial model parameters (ice history and viscoelastic Earth structure). Since this approach was introduced at the 2006 Spring AGU Meeting, further work has yielded significantly improved agreement between the GIA estimate and the GRACE rates over Canada, while preserving the much improved fit to GPS observations. We will describe the improvements made to the technique and implications for the combination of additional geodetic data types.

Discussion (29)




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