GRACE Tellus: Monthly Mass Grids

Monthly Mass Grids - Ocean

GRCTellus Ocean: Data Processing

The ocean data are based on the RL05 spherical harmonics from CSR, JPL and GFZ (maximum degree/order: n=40; this is driven by the current destriping algorithm - see below).
Atmospheric pressure/mass changes removed (based on ECMWF IFS; AOD1B background model correction details here).
The C20 (degree 2 order 0) coefficients are replaced with the solutions from Satellite Laser Ranging [Cheng et al., 2011]. The C20 values derived from GRACE observations have a larger uncertainty than the SLR-values.
The degree 1 coefficients ( geocenter) are estimated using the method from Swenson, Chambers, and Wahr (2008).
A glacial isostatic adjustment (GIA) correction has been applied based in the model from Geruo A and J. Wahr (2013).
A destriping filter (optimized for the ocean areas and different from the land destriping) 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 500 km wide gaussian filter has also been applied to the data; this smoothing yields the best fit of GRACE observations to total SSH (from altimetry) minus steric SSH (Argo data) (Chambers and Bonin, 2012).
A spherical harmonic filter cutoff at degree/order 40 acts as a third filter on the data.
Land leakage correction: Ocean signals are typically weaker than land signals, by factors of 2 and more, on seasonal and interannual time scales. High latitude ocean bottom pressure signals are stronger than low latitude signals. The spatial filters (gaussian, degree 40 cutoff) used to decrease high wavenumber errors also imply that a value at an 'ocean pixel' within at least 500 km of land will include part of that land signal. If that land signal is very large it may overwhelm the ocean signal. To minimize this land signal leakage onto ocean signals, a special iterative procedure is applied here. Please note that the destriping filter can cause correlations over much larger distances.
GLOBAL MEAN OCEAN MASS: Please note that the gridded GRCTellus OCEAN maps are optimized to examine regional OBP variations, but are NOT intended to be spatially averaged to determine global mean ocean mass. As of GRCTellus version [RL05.DSTvDPC1401], the area-weighted global mean is set to zero. To compute global ocean mass from GRACE, one must use non-destriped & un-smoothed data, and should mask out any ocean areas within 300 km of land points to avoid land leakage and biases. A time series of Global Mean Ocean Mass will soon be provided here.

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. While the SAMPLING of all grids is 1 degree in both latitude and longitude (approx. 111 km at the Equator), this does not mean that two consecutive samples are 'independent' precisely because of the smoothing applied. For more details on the current GRCTellus OCN post-processing, see the paper by Chambers and Bonin (2012) .

GRCTellus Ocean EOFR: Data Processing; version [RL05.EOFRvDPC1401]

The 'EOFR' bottom pressure grids are obtained by projecting the data from the regular GRC Ocean grids described above onto the Empirical Orthogonal Functions (EOFs) of the Ocean Model for Circulation and Tides (OMCT), and then reconstructing the OBP variations using the first 15 modes (Chambers and Willis, 2010). This effectively filters out signals in the GRACE data that are inconsistent with the physics and OBP variations in the OMCT ocean model. The EOFR filtered reconstructed bottom pressure fields agree better with radar altimetric sea surface height corrected for steric effects determined from Argo floats. In addition, leakage artifacts and errors around ice sheets and glaciers are reduced significantly, as well as noise in the midlatitudes where OBP variability is lower. Please cite Chambers and Bonin (2012) when using these data.

Error Estimates

The RMS error estimates for version [RL05.DSTvDPC1401] is estimated to be 1.0 cm at mid and low latitudes, and 1.5-2 cm at high latitudes. The RMS error of the EOFR grid is approximately 0.7 cm. Values over neighboring pixels are correlated due to the destriping filter, cutoff at spherical harmonic degree 40, and the 500 km gaussian smoothing. Errors are comparable for the three centers; see Chambers and Bonin (2012) for further details.

GRCTellus Land: Units and Format

The units of the GRACE ocean bottom pressure data and error grids are centimeters of equivalent water thickness. All grids have 360 longitude points (0.5,1.5,2.5,...,359.5), and 180 latitude points (-89.5, -88.5, ..., -0.5, +0.5, ...+89.5). However, missing grid points are not included in the ascii files. The data are provided in

NETCDF, suitable for automatic ingestion into several software packages.
ASCII, a plain text format (compressed with gzip).
GEOTIFF, suitable for GIS-processing tools.
Estimates for measurement and signal leakage errors are also provided (in separate files (ascii) or together with the scaling coefficient file (netcdf)).
DATA PROCESSING and CAVEATS DESCRIPTION FOR OCEAN GRIDS. (PDF, 706 KB) (PDF, 706 KB). Please note that this 'manual' applied to the RL04 processing; the processing for RL05 is slightly different in the destriping filter;

ACKNOWLEDGEMENT and CITATION

When using the GRCTellus OCN fields, please include the folioing 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.

Please also cite (as applicable):

D.P. Chambers. 2012. GRACE MONTHLY OCEAN MASS GRIDS NETCDF RELEASE 5.0. Ver. 5.0. PO.DAAC, CA, USA. Dataset accessed [YYYY-MM-DD] at http://dx.doi.org/10.5067/TEOCN-0N005.

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.

REFERENCES

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.

Cheng, M., J. C. Ries, and B. D. Tapley (2011), Variations of the Earth's figure axis from satellite laser ranging and GRACE, J. Geophys. Res., 116, B01409, doi:10.1029/2010JB000850.

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.