PREVIOUS VERSIONS (7/2007; 8/2006) REMOVED
PLEASE DOWNLOAD *ALL* DATA AGAIN

JUMP TO WHAT IS NEW

WHAT IS 'EQUIVALENT WATER THICKNESS'?

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, whose vertical extent is much smaller than the radius of the Earth.

The mass of the atmosphere is removed during processing using ECMWF fields, so these grids do not reflect atmospheric variability, except for errors in ECMWF.

Similarly, an ocean model is used to remove high frequency, wind and pressure-driven ocean motions. The values of that model are removed 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 the reason we provide OCEAN and LAND grids separately. The GRACE data are the same but the ocean grids have the ocean + atmosphere model added back while the land grids assume the atmosphere model removed was perfect.

Here we present the changes in equivalent water thickness. The method is explained in Wahr et al., 1998. However, the specific processing used here is due to Chambers 2006.

DATA VERSIONS

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). Release 04 is the latest official version from these centers, andi s used here

A document explaining our postprocessing, inclduing destriping, removal of land contamination, degree 1 and (2,0) replacements, and PGR addition, is provided here.

Note that the data provided here have YES been corrected by a PGR model.  Please, see our separate PGR discussion page.

BROWSE IMAGES and NUMERIC DATA

Browse images are provided for all the datasets.

The time-average over the time period 1/2003 to 12/2005 of that set has been removed from the data.

The gridded data and browse images are publicly available at (opens in new window)

ftp://podaac.jpl.nasa.gov/pub/tellus/monthly_mass_grids/chambers-destripe/dpc200711

UNITS and FORMAT

The data are now provided in three formats:

 GEOTIFF, suitable for GIS software
 NETCDF, suitable for automatic ingestion into several software packages.
 ASCII, a plain text format described below.

The simple ASCII format are lines with
longitude  latitude  value
longitude  latitude  value
...

There are 17 header lines preceding the first lat-lon-value data record. The contents are self-describing. For example, file

HDR ATFORMAT015 NHEADER_ROWS=17
HDR FILENAME=GRC_JPL_RL04_DPC_LND_500_200610.txt
HDR VARIABLE=WATER THICKNESS FROM GRACE
HDR UNIT=CMeqH2O
HDR TIME MEAN REMOVED=2003-2006
HDR TIME=200610
HDR LON1=0.5. LAT1=-89.5. DLON=1. DLAT=1. NLONS=360 NLATS=180
HDR CENTER/RELEASE=JPL_RL04
HDR POSTPROCESS=DESTRIPE FILTER, DPC_20070411
HDR POSTPROCESS=NONE
HDR FILTER=GAUSSIAN
HDR FILTERPARAMS WIDTH=500km
HDR FILTERPARAMS MAXDEG=60
HDR MASK=LAND ONLY PIXELS (VZ-2006-11-22-ETOPO5)
HDR INPUT DATA FILE=JPL_RL04_land_500km_200610.txt
HDR FORMAT=f6.1,1x,f5.1,1x,f8.3
HDR DATE=10may2007
   0.5 -89.5    5.078
   1.5 -89.5    5.069
   2.5 -89.5    5.071
   3.5 -89.5    5.063
   4.5 -89.5    5.064
   5.5 -89.5    5.057
   6.5 -89.5    5.058
   7.5 -89.5    5.058

If reading in FORTRAN 77, for example,

REAL GRC(360,180), RLON1, RLAT1, GRC1
INTEGER ILON1, ILAT1
OPEN (LUN, ...
DO k=1, 17
  READ (LUN, *, END=499 ..., ERR=...)
END DO DO k=1, 360*180
  READ (LUN, *, END=499 ..., ERR=...) RLON1, RLAT1, GRC1
  ilon1  = (RLON1 - 0.5) + 1
  ilat1  = (RLAT1 - 89.5) + 1
  GRC(ILON1, ILAT1)  = GRC1
END DO
499 CONTINUE
  ...
END

TIME SPANS OF EACH MONTHLY SOLUTION

'Monthly' is used somewhat loosely: please see the table of actual data days used for each 'monthly' solution. It is important to note that the GFZ solutions (not given here) use slightly different days for the same approximate months. The difference is due to data editing.

TIME AVERAGE REMOVED FROM MONTHLY SOLUTIONS

Each monthly grid here represents the difference the masses for that month, and the average over 2003-2005. If you compare against other data or model, it is critical that anomalies from the same time-average be compared; this is simply done by removing the average over the chosen period from any set of grids, including those provided here.

CITATION

When using these data, please include this phrase in the acknowledgements

GRACE data were processed by D. P. Chambers, supported by the NASA Earth Science REASoN GRACE Project, and are available at http://grace.jpl.nasa.gov

and cite:

Chambers, D.P.: Evaluation of New GRACE Time-Variable Gravity Data over the Ocean. Geophys. Res. Lett., 33(17), LI7603, 2006.

REFERENCES used above:

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

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.

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, 32,205–30,229, 1998.