GRACE MONTHLY MASS GRIDS - OCEAN
THE DESTRIPING FILTERS and OVERALL PROCESSING
IN THE LAND AND OCEAN GRIDS ARE DIFFERENT,
AND HAVE BEEN OPTIMIZED FOR LAND OR OCEAN APPLICATIONS.
Please download ALL MONTHS from these new RL05 solutions,
and discard previous versions in order to work with a consistent time series
Two different sets of ocean grids derived from GRACE were originally offered here. Both were computed by Don P. Chambers (U. South Florida).
The first set, labeled 'dpc20120822' is purely GRACE data to which a number of filters described below are applied.
The second set, labeled 'dpceofr20120822' is obtained by projecting the first set onto the Empirical Orthogonal Functions (EOFs) of an ocean model as described below.
A third set, labeled 'dpc20120330' is similar to 'dpc201012' but uses RL05 GRACE data. At this writing, not all months of data based on RL05 have been issued.
We have concluded that the RL05 grids are so much more accurate than the RL04 versions ('dpceofr201012' and 'dpc201012'), that we have declared the RL04 obsolete. Furthermore, the current RL05 grids with optimized post processing are more accurate than those offered a couple of months ago with the same processing we had applied to RL04.
OCEAN DATA PROCESSING version dpc20120822
- The ocean data are based on spherical harmonics from either the U. Texas Center for Space Research CSR RL5.0, Jet Propulsion Lab. JPL RL5.0 or the Geoforschungzentrum Potsdam GFZ RL 5.0
- The degree 2 order 0 coefficients are those derived from Satellite Laser Ranging by Cheng and Tapley (2004), rather than those observed by GRACE
- The degree 1 coefficients are those derived by Swenson, Chambers, and Wahr (2008)
- A postglacial rebound signal in the data has been removed according to the latest revision by Geruo A (U. COlorado, dated 9/2012) of the model of Paulson et al, 2007.
- A destriping filter has been applied to the data, to minimize the effect of an error whose telltale signal are N-S stripes in GRACE monthly maps.
- A gaussian filter with 500 km half width has also been applied to the data. We only offer this smoothing (in the past we offered three smoothing radii) because this smoothing turned out to yield the best fit to altimetry minus Argo data (Chambers and Bonin, 2012)
- A spherical harmonic filter cutoff at degree 40 acts as a third filter on the data.
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Both the de-striping and gaussian filters cause a leakage of land signals onto ocean areas.These maps are not intended to be averaged in order to determine global mean ocean mass, but instead, to examine regional OBP variations. To compute global ocean mass one must use non-destriped maps, and must stay away a certain distance from the coasts to avoid leakage by the gaussian filter.
- LAND DECONTAMINATION: Ocean signals are typically weaker than land signals, by factors of 2 or 3. This is true both on seasonal and interannual time scales. High latitude ocean signals are stronger than low latitude ocean signals. The spatial filters (gaussian, degree 40 cutoff) used to decrease high wavenumber error also imply that a value at an 'ocean pixel' within, say, 200 km of land, will include part of that land signal. If that land signal is very large it may overwhelm the ocean value. A special iterative procedure is applied here to minimize this leakage from land signals onto ocean signals (but not the converse). The destriping filter causes correlations over much larger diistances.
See the paper by Chambers and Bonin (2012) for more details on the post processing.
The ocean data contain no wavelength shorter than ~1,000km because of the cutoff at spherical harmonic degree 40. However, the 'bandpass' at longer wavelengths is not a uniform value of 1 because of the gaussian smoother. All these filters attenuate signal even at wavelengths longer than 1000km.
The SAMPLING of all grids is 1 degree in both latitude and longitude (approx. 111 km at the Equator), but that does not mean that two consecutive samples are 'independent' precisely because of the smoothing applied.
OCEAN DATA PROCESSING version dpceofr
The 'eofr' version of the grids is obtained by projecting the data from the grids described above onto the Empirical Orthogonal Functions (EOFs) of the Ocean Model for Circulation and Tides (OMCT), then 'reconstructing' using the first 10 modes (Chambers and Willis, 2010). As a result, the reconstructed data filter out signals in the GRACE data which are inconsistent with the physics embodied in the ocean model. Chambers and Willis found that this reconstructed data set better fit radar altimetric sea surface height corrected for steric effects using Argo float data.
When you use these data, please cite the article by Chambers and Bonin (2012).
ERROR ESTIMATES
The overall global RMS error estimate for the destriped, 500km values is 1.0 cm at mid and low latitudes, and 1.5-2cm at high latitudes. The error of the EOFR values is approximately 0.7cm.
Values over neighbouring pixels remain correlated due to the destriping filter, the filter cutoff at spherical harmonic degree 40, and the 500 km gaussian filter.
The errors are comparable for the three centers, although, for the Arctic ocean, the CSR and JPL solutions have more energy than the GFZ solution, the former are more consistent with data from a bottom pressure recorder.
See Chambers and Bonin (2012) for further details
UNITS and FORMAT
TIME SPANS OF EACH MONTHLY SOLUTION
'Monthly' is used somewhat loosely: please see the TABLE OF ACTUAL DATA DAYS used for each 'monthly' solution, and any warnings included there.
TIME AVERAGE REMOVED FROM MONTHLY SOLUTIONS
Each monthly grid here represents the difference in the masses for that month, and the average over Jan 2003 to Dec 2010. If you compare against other data or model, it is critical that anomalies from the same time-average be compared. This is simple to do: for example, if using the 2004-2006, average these grids over 1/2004 to 12/2006, and subtract that one average grid from all others (including those at times outside this range).
BROWSE IMAGES and NUMERIC DATA
Browse images are provided for the whole time span, for at least one dataset (CSR, or GFZ).
The time-average over the time period 1/2003 to 12/2007 of all datasets have been removed from the data (note that in previous versions we removed the 2003-2005 or 2003-2006 time-average).
The ocean gridded data and browse images are available here
The ocean gridded EOFR data and browse images are available here
ACKNOWLEDGEMENT and CITATION
When using these data, please include the phrase below in the acknowledgements
GRACE ocean data were processed by Don P. Chambers, supported by the NASA MEASURES Program, and are available at http://grace.jpl.nasa.gov
and cite :
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. and J. K. Willis: A Global Evaluation of Ocean Bottom Pressure from GRACE, OMCT, and Steric-Corrected Altimetry. J. of Oceanic and Atmosph. Technology, vol 27, pp 1395-1402.DOI: 10.1175/2010JTECHO738.1, 2010 (if appropriate)
If you use RL05 ocean data, please cite
Chambers D.P and J.A. Bonin: Evaluation of Release-05 GRACE time-variable gravity coefficients over the Ocean. Ocean Sci. Discuss., 9, 2187-2214, 2012. http://www.ocean-sci-discuss.net/9/2187/2012/
If you encounter any problems with the data, please contact the person listed at bottom right.
REFERENCES used above:
Chambers, D.P.: Evaluation of New GRACE Time-Variable Gravity Data over the Ocean. Geophys. Res. Lett., 33(17), LI7603, 2006
Chambers, D. P: Observing seasonal steric sea level variations with GRACE and satellite altimetry, J. Geophys. Res., 111 (C3), C03010, 10.1029/2005JC002914, 2006.
Chambers D.P. and J. K. Willis: A Global Evaluation of Ocean Bottom Pressure from GRACE, OMCT, and Steric-Corrected Altimetry. J. of Oceanic and Atmosph. Technology, vol 27, pp 1395-1402.DOI: 10.1175/2010JTECHO738.1, 2010
Cheng, M. and Tapley, B.D.: Variations in the Earth's oblateness during the past 28 years, J. Geophys Res v109, B9, 2004
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.
LAST UPDATE: 2012-12-10 V.Zlotnicki
Contributors: DPC, A.Thevenin.
