Monthly Mass Grids - Ocean

GRCTellus Ocean: Data Processing

The current surface mass change data are based on the RL06.X spherical harmonics from CSR, JPL and GFZ (maximum degree/order: n=60).
The C20 and C30 coefficients are replaced with the solutions from Satellite Laser Ranging via Technical Note TN-14 [Loomis et al., 2020].
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 de-correlation filter has been applied to the data to minimize the effect of correlated errors whose telltale signal are N-S stripes in GRACE(-FO) 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).
Atmospheric pressure/mass changes are restored (based on ECMWF IFS; AOD1B background model correction details here).
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, we recommend to use non-destriped & un-smoothed data, and mask out any ocean areas within 300 km of land points to avoid land leakage and biases. Time series of Global Mean Ocean Mass from spherical harmonic data are available at PO.DAAC; time series based on JPL mascons are available at the NASA Sealevel portal.

The ocean data contain no wavelength shorter than ~300km because of the cutoff at spherical harmonic degree 60. However, the 'bandpass' at longer wavelengths is not a uniform value of 1 because of the gaussian smoother. Although the SAMPLING of all grids is 1 degree in both latitude and longitude (approx. 111 km at the Equator), two neighboring samples are NOT 'independent' because of the postprocessing applied.

GRCTellus Ocean: 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)).
Further details on the GRC Telllus Ocean processing for RL05 is documentd in Chambers and Bonin (2012).

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.