G22A-01 INVITED
The EGM2008 Global Gravitational Model
The development of a new Earth Gravitational Model (EGM) to degree 2160 has been completed. This model, designated EGM2008, is the product of the final re-iteration of our modelling and estimation approach. Our multi-year effort has produced several Preliminary Gravitational Models (PGM) of increasingly improved performance. One of these models (PGM2007A) was provided for evaluation to an independent Evaluation Working Group, sponsored by the International Association of Geodesy (IAG). In an effort to address certain shortcomings of PGM2007A, we have considered the feedback that we received from this Working Group. As part of this effort, EGM2008 incorporates an improved version of our 5'x5' global gravity anomaly database and has benefited from the latest GRACE based satellite-only solutions (e.g., ITG- GRACE03S). EGM2008 incorporates an improved ocean-wide set of altimetry-derived gravity anomalies that were estimated using PGM2007B (a variant of PGM2007A) and its associated Dynamic Ocean Topography (DOT) model as reference models in a "Remove-Compute-Restore" fashion. For the Least Squares Collocation estimation of our final global 5'x5' area-mean gravity anomaly database, we have used consistently PGM2007B as our reference model to degree 2160. We have developed and used a formulation that predicts area-mean gravity anomalies that are effectively band-limited to degree 2160, thereby minimizing aliasing effects during the harmonic analysis process. We have also placed special emphasis on the refinement and "calibration" of the error estimates that accompany our final combination solution EGM2008. We present the main aspects of the model's development and evaluation. This evaluation was accomplished primarily through the comparison of various model derived quantities with independent data and models (e.g., geoid undulations derived from GPS positioning and spirit levelling, astronomical deflections of the vertical, etc.). We will also present comparisons of our model-implied Dynamic Ocean Topography with other contemporary estimates (e.g., from ECCO).
G22A-02
Analysis of Basin-Scale Mass Exchange between the Atlantic/Indian Oceans and the Pacific
It has been known for some time that there is a seasonal exchange of water mass between the Pacific and Atlantic Oceans that is due solely to variations in ocean transports related to wind forcing. This appears as a leading mode in most global barotropic models. In this presentation, we examine this exchange by averaging ocean bottom pressure from numerical models, GRACE, and steric-corrected altimetry for the Atlantic, Pacific, and Indian basins from late-2002 until early 2008, after the near eustatic global mass variation has been removed. As expected, the variations in the Pacific and Atlantic are highly negatively correlated with a dominant seasonal variation. After removing the seasonal variations, we find that the Indian and Atlantic Oceans have interannual variations that are highly correlated. We also find a strong variation in all basins at a period of approximately 1.5-years that is phase locked with a similar variation in the Southern Annular mode. In addition, we find significantly larger interannual variations in the GRACE observations than are predicted by either numerical model. The largest is an exchange of mass from the Atlantic and Indian Oceans to the Pacific that began in late-2006 and appears to be continuing through early-2008.
G22A-03
A Comparison of in situ Bottom Pressure Array Measurements with GRACE Estimates in the Kuroshio Extension
Ocean bottom pressure estimates from GRACE (Gravity Recovery and Climate Experiment) have been
validated by comparisons with an array of in situ bottom pressure measurements. The 600 km by 600 km
array comprised 46 bottom pressure sensors that were part of the Kuroshio Extension System Study (KESS).
Previous validations in other ocean regions have been limited to pointwise bottom pressure measurements
due to the lack of available data. Spatially-averaged monthly-mean bottom pressure over the KESS array is
highly correlated with GRACE bottom pressure estimated at the center of the array. The correlations are
nearly equally high for three standard choices of spatial smoothing radius applied to GRACE estimates, 300,
500, and 750 km. In contrast, pointwise comparisons between GRACE and individual bottom pressure
measurements are high or low in sub-regions of KESS, depending partially upon the local variance of deep
mesoscale eddies whose energetic length scales are shorter than 300 km. KESS is a suitable validation
experiment for the GRACE estimates at monthly scales with 300 to 750 km spatial radius of smoothing.
http://uskess.org
G22A-04 INVITED
Using GRACE Satellite Acceleration Data to Recover Arctic Ocean Tides
Arctic ocean tidal solutions are not constrained by altimetry data because missions such as TOPEX/POSEIDON do not extend to high latitudes. The resulting errors in tidal models alias into the monthly GRACE gravity field solutions at all latitudes. Fortunately, it is possible to use the GRACE inter-satellite ranging data to solve for these tides directly. Five years of GRACE inter-satellite acceleration data are inverted to solve for the amplitude and phase of major solar and lunar tides in the Arctic ocean using a mascon approach. The resulting tidal amplitudes are compared to existing tidal models using in-situ data from coastal tide gauges and deep sea bottom pressure recorders. Simulations were performed to verify that the inversion algorithm works as designed.
G22A-05
Simulation Study to Monitor and Model Individual Sources of Mass Distribution and Transport in the Earth System by Means of Satellites
The ocean and atmosphere together with the solid Earth, cryosphere, and continental water form the main dynamical elements of the Earth's fluid systems. The dynamics of the interior processes and the interactions at the surface of the solid earth with the outer fluid envelope (oceans, atmosphere, cryosphere, continental water) cause changes in potential fields and the shape of the Earth. Time-variable space borne gravity observations are considered, among others, as a means of studying exchanges of mass between the land, ocean and atmosphere. The European Space Agency (ESA) has funded a study to investigate the possibility for monitoring the individual sources of mass distribution and transport in the Earth system from space. The study, which has just recently been concluded, included an extensive investigation of the major fluid mass transport processes and their relation to gravity and geometry. A fluid mass transport model, describing the real world, was developed. The model covers 11 years, with 6 hourly sampling, up to degree and order 180. Four different satellite constellations and orbit geometries were devised to sample the mass variability in time. Synthetic gravity retrievals, with and without realistic error estimates, were produced for each constellation. The gravity retrievals were then used to extract the original continental water storage, oceanic, ice and solid earth mass signal. This study has allowed us to determine the current limitations in satellite gravity for observing surface mass changes. More importantly, the results provide valuable information on issues that are critical to the success of future satellite gravity missions.
G22A-06
Satellite gravity fields for studying the earthquake cycle
We overview what we have learned about the earthquake cycle from the Gravity Recovery And Climate Experiment (GRACE) mission. The large-scale gravity measurements from GRACE satellites were helpful to detect coseismic displacements and postseismic transients from the great Sumatra-Andaman Islands (thrust event; Mw ~9.2) earthquake in December 2004. Large negative gravity change around the Andaman Sea can not be explained without considering dilatation of the crust. The fast relaxation (postseismic) behaviors in the time-series of the estimated coefficients (from May 2003 to April 2007) for localized basis functions are explained better with biviscous viscoelastic flow than with afterslip. A transient viscosity of 5*1e17 Pa s and steady-state viscosity of 5*1e18 to 1e19 Pa s are delineated. The prominent postearthquake positive gravity change around the Nicobar Islands is accounted for by seafloor uplift with much less postseismic perturbation in intrinsic density. We also provide perspectives on new missions including Gravity and Ocean Circulation Explorer (GOCE) and GRACE follow-on with regard to the earthquake cycle. The gradiometer satellite mission GOCE, scheduled for launch in October, is expected to be useful to detect the coseismic deformation caused by great earthquakes such as the 2004 Sumatra-Andaman earthquake within the bandwidth from spherical degrees 25 to 100 after spatio-spectral localization. GRACE follow-on mission is supposed to lower the detection threshold of earthquakes to a magnitude as low as 7.5.
G22A-07
Assessing Human Impacts on the Water Balance of Lake Victoria Using GRACE and Altimeters.
GRACE has provided global estimates of vertically integrated water storage anomalies at monthly intervals with a useful spatial resolution of 300-500 km. Water levels of large lakes are now routinely monitored by satellite altimeters. By combining these datasets, it may be possible to separate the effects of climate and human management on lake levels. During the period 2002-2006, the levels of Lake Victoria dropped about two meters. Water storage changes of Lake Victoria and other large lakes in the East African Rift Valley are compared, and the relative impacts of climate and human management are shown to be of similar magnitude.
G22A-08
Closing the Global Sea Level Rise Budget with GRACE, Argo, and Altimetry Observations
An important goal of climate studies is to determine the relative contribution of steric (heating and salinty) and eustatic (melting ice, runoff) sea level rise and to understand how and why these contributions vary. Concurrent measurements from the Argo array of profiling floats and the GRACE gravity mission, which respectively measure steric and eustatic changes, provide an independent measure of total sea level change. An analysis of the steric and ocean mass components of sea level shows that the sea level rise budget for the period January 2004 to December 2007 can be closed. Using corrected and verified Jason-1 and Envisat altimetry observations of total sea level, upper ocean steric sea level from the Argo array, and ocean mass variations inferred from GRACE gravity mission observations, we find that the sum of steric sea level and the ocean mass component has a trend of 1.5 ± 1.5 mm/year over the period, in agreement with the total sea level rise observed by either Jason-1 (2.2 ± 1.6 mm/year) or Envisat (1.7 ± 1.8 mm/year). This provides verification that the altimeters, Argo buoys, and GRACE are providing consistent results and opens the way to routine monitoring of the major components of sea level rise.