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GRACE MONTHLY MASS GRIDS
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WHAT IS 'EQUIVALENT WATER THICKNESS'?
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The observed monthly changes in gravity are caused by monthly changes in mass. The mass changes can be thought of as concentrated in a very thin layer of water at the surface, whose thickness changes. In reality, much of the monthly change in gravity is indeed caused by changes in water storage in hydrologic reservoirs, by moving ocean, atmospheric and cryospheric masses, and by exchanges among these reservoirs. Their vertical extent is measured in centimeters, much smaller than the radius of the Earth or the horizontal scales of the changes, which are measured in kilometers. Some changes in gravity are caused by mass redistribution in the 'solid' Earth, such as that following a large earthquake, or that due to glacial isostatic adjustment; in those cases the concept of 'equivalent water thickness' does not apply, even though it is possible to compute the quantity.
The mass of the atmosphere is removed during processing using ECMWF fields, so these grids do not reflect atmospheric variability over land or continental ice (Greenland, Antarctica), except for errors in ECMWF.
An ocean model is used to remove high frequency, wind and pressure-driven ocean motions during processing. The resulting gravity fields would not reflect ocean variability if the model were perfect. To use these results over the oceans, the GRACE solutions provided here have the monthly averaged ocean model grids added back. This is one of the reasons we provide OCEAN and LAND grids separately. Other reasons include removal of land signals from ocean grids, scaling of land grids, and more.
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Data available here are changes in equivalent water thickness. The basic method is explained in Wahr et al., 1998. The land and ocean grids are processed differently.
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SPHERICAL HARMONIC DATA VERSIONS
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GRACE is a first-of-a-kind mission, so not surprisingly, revisions to the data processing are more frequent than for more mature satellite measurements.
Three centers are part of the GRACE Ground System and generate level 2 data (spherical harmonic fields): CSR (U. Texas / Center for Space Research); GFZ (GeoForschungsZentrum Potsdam); and JPL (Jet Propulsion Laboratory). Their output include spherical harmonic coefficients of the gravity field and of the dealiasing fields used to compute them.
The data we offer here are based on those produced by these data centers (the 'official' releases of the GRACE Project), and have additional, postprocessing steps, summarized in the pages linked to below. Other data centers also offer GRACE-derived mass grids, and they can be found here.
At present (10/2012) we offer data only from the latest version: Release 05 from CSR, JPL and GFZ. Data from RL05 are up to date, except that data for 2002 have not yet been produced. RL05 is more accurate, several tests have proven that. Those spherical harmonics are
input to our postprocessing steps. We call our OCEAN version from RL05 is 'dpc20120822' and our LAND version from RL05 is 'scsv201209'.
The post processing has now been optimized for RL05 spherical harmonics. Notice that these are different version numbers than those we gave to the grids produced soon after RL05 data became available
Please download ALL MONTHS from these new solutions
and discard previous versions in order to work with a consistent time series.
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DEGREE 2 ORDER 0 coefficients
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Spherical harmonic coefficients of (degree,order) (2,0) sometimes
disagree with those from satellite laser ranging (SLR), which are more reliable. The grids
provided here have the (2,0) coefficients replaced by those from SLR..
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DEGREE 1 coefficients
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GRACE cannot retrieve spherical harmonic coefficients of degree 1, proportional to the position of the Earth's geocenter relative to an Earth-fixed reference frame. An estimate of these coefficients based on Swenson et al (2008) is used here.
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POST-GLACIAL REBOUND (GLACIAL ISOSTATIC ADJUSTMENT)
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The data provided here have YES been corrected with a PGR model . Specifics are described in our separate page. PGR discussion.
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EARTHQUAKES
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Large earthquakes, such as those off the West Coast of Northern Sumatra (Indonesia) on 12/24/2004; Northern Sumatra on 3/25/2005; Southern Sumatra on 9/12/2007; offshore Maule, Chile on 2/27/2010; and near the East Coast of Honshu, off Tohoku, Japan on 3/11/2011 cause sufficient displacements of the Earth's lithosphere to generate a change in gravity acceleration which GRACE measures. All the earthquakes mentioned above had magnitude 8.5 or higher (http://earthquake.usgs.gov/earthquakes/world/10_largest_world.php). It would be incorrect to interpret the changes in gravity they cause as changes in physical 'equivalent water thickness'. As opposed to the case of Post Glacial Rebound, for which we have a model good enough to 'correct' the GRACE data with, we have NOT corrected the data offered here to remove the signals due to large earthquakes. Users should be weary of signals in the vicinity of these earthquakes.
The ocean grids computed by reconstructing the GRACE signals using the EOFs of an ocean model have filtered out most of the earthquakes signals.
A user can remove approximately the signal due to an earthquake following the approach of de Linage et al (2009); see their equation 3.
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DESTRIPING and SMOOTHING
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A source of error, whose telltale signature are N-S stripes is present in the data after the previous steps. Swenson and Wahr (2006) observed a peculiar property of the spherical harmonic coefficients associated with the striping, and designed a class of filters to remove the problem.
The GRACE satellites fly at over 400 km altitude. The gravity field weakens with altitude, and short wavelengths attenuate more than longer ones. As a consequence it is necessary to smooth short wavelengths to recover the set of masses on the Earth surface that cause the gravity field seen by GRACE at its altitude. To reduce this source of noise, a spatial averaging smoother (a gaussian here) is applied here.
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MONTHS WITH LOWER ACCURACY
Users need to be aware that the monthly grids have higher errors when the orbit is near exact repeat. Such months include July to December 2004. Another source of increasing error is lack of data in a particular month
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Information and data for OCEAN grids
Information and data for LAND grids
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REFERENCES used above:
Chambers, D.P. and J.A. Bonin: Evaluation of Release 05 time-variable
gravity coefficients over the ocean. Ocean Science 8, 859-868, 2012.
www.ocean-sci.net/8/859/2012
Chambers, D. P: Observing seasonal steric sea level variations with GRACE and satellite altimetry, J. Geophys. Res., 111 (C3), C03010, 10.1029/2005JC002914, 2006.
Cheng, M. and Tapley, B.D.: Variations in the Earth's oblateness during the past 28 years, J. Geophys Res v109, B9, 2004
de Linage C., L. Rivera, J. Hinderer, J.-P. Boy, Y. Rogister, S. Lambotte and R. Biancale: Separation of coseismic and postseismic gravity changes for the 2004 Sumatra–Andaman earthquake from 4.6 yr of GRACE observations and modelling of the coseismic change by normal-modes summation. Geophys. J. Int. (2009) 176, 695–714, doi: 10.1111/j.1365-246X.2008.04025.x
Swenson, S. C. and J. Wahr, Post-processing removal of correlated errors in GRACE data, Geophys. Res. Lett., 33, L08402, doi:10.1029/2005GL025285, 2006.
Swenson S.C , D. P. Chambers, and J. Wahr: Estimating geocenter variations from a combination of GRACE and ocean model output. J Geophys. Res.-Solid Earth, Vol 113, Issue: B8, Article B08410. 2008.
Wahr, J., M. Molenaar, and F. Bryan, Time-variability of the Earth's gravity field: Hydrological and oceanic effects and their possible detection using GRACE, J. Geophys. Res., 103, 32,20530,229, 1998.
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LAST UPDATE: 2012-10-29 V.Zlotnicki
Contributors: DPC, A.Thevenin.
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