GRACE Related Sites
Center for Space Research (CSR)
GeoForschungs-
Zentrum Potsdam (GFZ)
GRACE Tellus
JPL PO.DAAC
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Overview
The Gravity Recovery and Climate Experiment (GRACE)
satellite mission was launched jointly by NASA and its German counterpart DLR
in March 2002 to map the Earth's gravity field at approximately monthly
intervals. In general, the largest time-variable gravity signals result
from changes in the distribution of water and snow stored on land.
GRACE thus provides global observations of changes in total water
storage, averaged over scales of a few hundred km and greater.
Decription of Datasets
Gravity field data for the GRACE project are produced by
the Center for Space Research (CSR) at the University of
Texas and by GeoForschungsZentrum Potsdam (GFZ) in Germany.
In addition, the Jet Propulsion Laboratory (JPL)
produces a validation product.
Each field consists of a set of spherical harmonic
coefficients, called Stokes coefficients, describing the
Earth's gravity field, averaged over a period of
approximately one month. From each monthly solution,
a time-mean (static) field is removed. Gravity fields
used at this site are truncated at degree 70.
This website is intended to allow the user to perform
basic data analysis on the publicly available Level-2
GRACE data. Computations are performed in real-time,
using parameters specified by the user. End products
are either maps or time series, and may be downloaded
as ascii files.
Data Analysis Methods
Units
The gravity field data shown here are expressed in units of millimeters of
equivalent water thickness.
The relationship between changes in the geopotential and changes in the
associated surface-mass density is described in Wahr J.,
M. Molenaar M, 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 (B12): 30205-30229, 1998.
Smoothing
After the data are converted to mass units, they are
smoothed in the spectral domain using a Gaussian filter
[Wahr et al., 1998]. Smoothing of GRACE data is
necessary due the presence of noisy short-wavelength
spectral coefficients. A Gaussian is used for
simplicity.
Time Series
Time series are computed for either an individual point
(i.e. the Gaussian-averaged value at that point) or for
a specific basin. Background on creating regionally averaged
surface-mass anomalies from GRACE data can be found in
Swenson, S, and J. Wahr, Methods for inferring regional
surface-mass anomalies from satellite
measurements of time variable gravity,
J. Geophys. Res., 107 (B9),
2193, doi:10.1029/2001JB000576, 2002.
Basin boundaries are defined by
the
TRIP dataset, described in Oki, T. and Y. C. Sud,
Design of Total Runoff Integrating Pathways (TRIP)
- A global river channel network. Earth Interactions,
2, 1998.
Error Estimates
Two error estimates are provided here. The first
is the calibrated error estimates provided by the GRACE
processing centers. Alternatively, monthly error
estimates are computed using the method described
in Wahr, J., S. Swenson, V. Zlotnicki, and I. Velicogna,
Time-Variable Gravity from GRACE: First Results,
Geophys. Res. Lett., 31, doi:10.1029/2004GL019779, 2004.
C_20
The current solutions for C_20 have been replaced
with a
solution derived from Satellite Laser Ranging (SLR).
For details, see
Technical Note 5.
This work is supported by the NASA Research, Education, and
Applications Solution Network (REASON)
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