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University of Colorado Real-Time GRACE Data Analysis Site

* High Resolution Fields Now Available *

GRACE Related Sites

Center for Space Research (CSR)

GeoForschungs-
Zentrum Potsdam (GFZ)

GRACE Tellus

JPL PO.DAAC

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.

  • C_10, C11, S11

    • Estimates of degree one gravity field coefficients have been added to the solutions. For details, see Swenson, S., D. Chambers, and J. Wahr, Estimating Geocenter Variations from a Combination of GRACE and Ocean Model Output, J. Geophys. Res. - Solid Earth, 113, B08410, doi:10.1029/2007JB005338, 19 August 2008.

  • Updates

    • "Destriped" gravity field coefficients are now available. These fields are denoted by "DS". Correlated errors responsible for the meridional striping apparent in high spatial resolution gridded fields have been removed from the original coefficients via the filter described in Swenson, S., and J. Wahr, Post-processing removal of correlated errors in GRACE data, Geophys. Res. Lett., 33, L08402, doi:10.1029/2005GL025285, 2006.
    Time series using these fields have been scaled to account for the effect of the filter on the underlying signal. The scaling is derived from simulations where the filter is applied to a model, and the filtered and true model results are compared. The scale factor is simply the multiplicative factor that gives the minimum least-squares difference between the filtered and true model time series. Gaussian smoothing is often unnecessary when using the filtered fields to construct basin averaged time series.

    This work is supported by the NASA 'Making Earth Science Data Records for Use in Research Environments (MEaSUREs) Program'

    © 2006 University of Colorado