[1] EGM2008 is a spherical harmonic model of the Earth's gravitational potential, developed by a least squares combination of the ITG-GRACE03S gravitational model and its associated error covariance matrix, with the gravitational information obtained from a global set of area-mean free-air gravity anomalies defined on a 5 arc-minute equiangular grid This grid was formed by merging terrestrial, altimetry-derived, and airborne gravity data Over areas where only lower resolution gravity data were available, their spectral content was supplemented with gravitational information implied by the topography EGM2008 is complete to degree and order 2159, and contains additional coefficients up to degree 2190 and order 2159 Over areas covered with high quality gravity data, the discrepancies between EGM2008 geoid undulations and independent GPS/Leveling values are on the order of ±5 to ±10 cm EGM2008 vertical deflections over USA and Australia are within ±11 to ±13 arc-seconds of independent astrogeodetic values These results indicate that EGM2008 performs comparably with contemporary detailed regional geoid models EGM2008 performs equally well with other GRACE-based gravitational models in orbit computations Over EGM96, EGM2008 represents improvement by a factor of six in resolution, and by factors of three to six in accuracy, depending on gravitational quantity and geographic area EGM2008 represents a milestone and a new paradigm in global gravity field modeling, by demonstrating for the first time ever, that given accurate and detailed gravimetric data, asingle global model may satisfy the requirements of a very wide range of applications

https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/2011JB008916

The development and evaluation of the Earth Gravitational Model 2008 (EGM2008)

Gravity models are powerful tools for mapping tectonic structures, especially in the deep ocean basins where the topography remains unmapped by ships or is buried by thick sediment. We combined new radar altimeter measurements from satellites CryoSat-2 and Jason-1 with existing data to construct a global marine gravity model that is two times more accurate than previous models. We found an extinct spreading ridge in the Gulf of Mexico, a major propagating rift in the South Atlantic Ocean, abyssal hill fabric on slow-spreading ridges, and thousands of previously uncharted seamounts. These discoveries allow us to understand regional tectonic processes and highlight the importance of satellite-derived gravity models as one of the primary tools for the investigation of remote ocean basins.

http://science.sciencemag.org/content/sci/346/6205/65.full.pdf

New global marine gravity model from CryoSat-2 and Jason-1 reveals buried tectonic structure

[1] Three approaches are used to reduce the error in the satellite-derived marine gravity anomalies. First, we have retracked the raw waveforms from the ERS-1 and Geosat/GM missions resulting in improvements in range precision of 40% and 27%, respectively. Second, we have used the recently published EGM2008 global gravity model as a reference field to provide a seamless gravity transition from land to ocean. Third, we have used a biharmonic spline interpolation method to construct residual vertical deflection grids. Comparisons between shipboard gravity and the global gravity grid show errors ranging from 2.0 mGal in the Gulf of Mexico to 4.0 mGal in areas with rugged seafloor topography. The largest errors of up to 20 mGal occur on the crests of narrow large seamounts. The global spreading ridges are well resolved and show variations in ridge axis morphology and segmentation with spreading rate. For rates less than about 60 mm/a the typical ridge segment is 50–80 km long while it increases dramatically at higher rates (100–1000 km). This transition spreading rate of 60 mm/a also marks the transition from axial valley to axial high. We speculate that a single mechanism controls both transitions; candidates include both lithospheric and asthenospheric processes.

/pdf/global-marine-gravity-from-retracked-geosat-and-ers-1-cc0qiwepyy.pdf

Global marine gravity from retracked Geosat and ERS‐1 altimetry: Ridge segmentation versus spreading rate

[1] A digital representation of ocean floor topography is essential for a broad variety of geological, geophysical and oceanographic analyses and modeling. In this paper we present a new version of the International Bathymetric Chart of the Arctic Ocean (IBCAO) in the form of a digital grid on a Polar Stereographic projection with grid cell spacing of 2 × 2 km. The new IBCAO, which has been derived from an accumulated database of available bathymetric data including the recent years of multibeam mapping, significantly improves our portrayal of the Arctic Ocean seafloor.

/pdf/an-improved-bathymetric-portrayal-of-the-arctic-ocean-oeuunqg702.pdf

An improved bathymetric portrayal of the Arctic Ocean: Implications for ocean modeling and geological, geophysical and oceanographic analyses

. This paper presents the ECCO v4 non-linear inverse modeling framework and its baseline solution for the evolving ocean state over the period 1992–2011. Both components are publicly available and subjected to regular, automated regression tests. The modeling framework includes sets of global conformal grids, a global model setup, implementations of data constraints and control parameters, an interface to algorithmic differentiation, as well as a grid-independent, fully capable Matlab toolbox. The baseline ECCO v4 solution is a dynamically consistent ocean state estimate without unidentified sources of heat and buoyancy, which any interested user will be able to reproduce accurately. The solution is an acceptable fit to most data and has been found to be physically plausible in many respects, as documented here and in related publications. Users are being provided with capabilities to assess model–data misfits for themselves. The synergy between modeling and data synthesis is asserted through the joint presentation of the modeling framework and the state estimate. In particular, the inverse estimate of parameterized physics was instrumental in improving the fit to the observed hydrography, and becomes an integral part of the ocean model setup available for general use. More generally, a first assessment of the relative importance of external, parametric and structural model errors is presented. Parametric and external model uncertainties appear to be of comparable importance and dominate over structural model uncertainty. The results generally underline the importance of including turbulent transport parameters in the inverse problem.

/pdf/ecco-version-4-an-integrated-framework-for-non-linear-4sj9o3ygtx.pdf

ECCO version 4: an integrated framework for non-linear inverse modeling and global ocean state estimation

Earth Gravitational Model 2008

Correction to “The Development and Evaluation of the Earth Gravitational Model 2008 (EGM2008)”

The study of the Arctic Ocean has been hampered by incomplete basic knowledge of the basin and its structure. An improved gravity anomaly map (Figure 1) and grid have been developed that complement the new International Chart of the Arctic Ocean [Jakobsson et al., 2008]. This article announces the availability of the Arctic Gravity Project (ArcGP) grid version 2.0 and discusses the genesis of the project.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2008EO320002

New Gravity Field for the Arctic

The TOPEX/Poseidon (T/P) satellite alti- meter mission marked a new era in determining the geopotential constant W0. On the basis of T/P data during 1993–2003 (cycles 11–414), long-term variations in W0 have been investigated. The rounded value W0 = 62636856.0 ± 0.5) m2s−2 has already been adopted by the International Astronomical Union for the definition of the constant LG = W0/c2 = 6.969290134 × 10−10 (where c is the speed of light), which is required for the realization of the relativistic atomic time scale. The constant LG, based on the above value of W0, is also included in the 2003 International Earth Rotation and Reference Frames Service conventions. It has also been suggested that W0 is used to specify a global vertical reference system (GVRS). W0 ensures the consistency with the International Terrestrial Reference System, i.e. after adopting W0, along with the geocentric gravitational constant (GM), the Earth’s rotational velocity (ω) and the second zonal geopotential coefficient (J2) as primary constants (parameters), then the ellipsoidal parameters (a,α) can be computed and adopted as derived parameters. The scale of the International Terrestrial Reference Frame 2000 (ITRF2000) has also been specified with the use of W0 to be consistent with the geocentric coordinate time. As an example of using W0 for a GVRS realization, the geopotential difference between the adopted W0 and the geopotential at the Rimouski tide-gauge point, specifying the North American Vertical Datum 1988 (NAVD88), has been estimated.

S. Kenyon

Papers

The development and evaluation of the Earth Gravitational Model 2008 (EGM2008)

Earth Gravitational Model 2008

Correction to “The Development and Evaluation of the Earth Gravitational Model 2008 (EGM2008)”

New Gravity Field for the Arctic

The geopotential value W 0 for specifying the relativistic atomic time scale and a global vertical reference system