Gravity Recovery and Climate Experiment (GRACE): Detection of Ice Mass Loss, Terrestrial Mass Changes, and Ocean Mass Gains
The gravity field of the Earth, caused by the distribution of masses inside and on the surface of the Earth, changes in time due to the redistribution of mass. Such mass fluxes can be due both to natural processes (such as the seasonal water cycle, ocean dynamics, or atmospheric variations), as well as due to human actions, such as the systematic withdrawal of groundwater for human consumption. The ability to measure such changes globally is of great significance for understanding the environmental dimension of sustainability.
KeywordsGravity Field Indian Ocean Dipole Global Precipitation Climatology Project Glacial Isostatic Adjustment Terrestrial Water Storage
- Equivalent water thickness
Since time changes in the gravity field are caused by time changes in mass distributions, equivalent water thickness (“EWT”) is the variable thickness of a thin layer of water (thin relative to both the radius of the Earth and the horizontal scale of the signals) draping the Earth that would correspond to the observed changes in gravity. The conversion from gravitational spherical harmonics to water thickness (and vice versa) is unique and well defined, regardless of what actually causes the gravitational changes. The concept is not used when studying changes in the solid Earth, such as glacial isostatic adjustment or earthquakes.
- Glacial isostatic adjustment (GIA)
Also known as postglacial rebound, it is the viscoelastic response of the mantle and lithosphere to the removal of the great ice sheets that covered parts of the Earth and peaked 21,000 years ago . The deglaciation was essentially complete 6,000 years ago. The lithosphere rises where the ice sheets used to be, but sinks in other locations.
A set of layers at altitudes between approximately 80 and 1,000 km above the Earth’s surface, with electrons and electrically charged atoms. The ionosphere leads to a delay to electromagnetic radiation, which is frequency-dependent and changes with local time and solar activity. The GRACE KBR system uses two frequencies to correct for this path delay.
K-band microwave ranging system measures the distance between the two GRACE satellites using two frequencies, 24 and 32 GHz.
Mass concentrations. The term was coined by Muller and Sjogren  to describe mass concentrations in the lunar nearside, beneath the center of the surface features termed “mare” (pl. “maria”). Today the term “mascons” refers to an alternative method to solve the GRACE gravity fields in terms of distributed spherical caps or point masses, instead of using the spherical harmonic representation.
- Newton’s law of gravitation
It states that every point mass attracts every other point mass with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them: F = G.m1.m2/r2, where m1 and m2 are the masses, r is the distance between them, and G is the universal gravitational constant, G ≈ 6.6738 × 10−11 m3/kg/s2. This law is at the heart of the GRACE measurements, since any specific mass on the Earth is in general at a different distance from the two spacecrafts, causing a slight difference in the gravitational acceleration they impart to the spacecraft, and thus causing a slight but measurable relative acceleration between the spacecrafts.
Is an object, natural (like the Moon) or artificial (each GRACE satellite) that orbits around another large object, in this case the Earth. “Orbits” means that the centripetal acceleration due to the speed of the satellite equals the gravitational acceleration between the satellite and the larger object it orbits around; in this manner the satellite neither falls toward Earth, nor escapes its gravitational pull. In practice, the GRACE satellites do fall slightly toward the Earth while they orbit around it, whereas the Moon slowly increases its distance to the Earth.
- Spherical harmonics
Are a set of functions of latitude and longitude that form an infinite, orthogonal, normalized set of basis functions whose sum, with appropriate scale coefficients, completely describes any other function defined in terms of spherical coordinates. Spherical harmonics satisfy Laplace’s equation, as does the gravitational potential outside the Earth. Laplace’s equation states that the sum of the second derivatives of the gravitational potential with respect to each of the three directions of space at a point must add up to zero if there are no masses at that point.
This work was performed in part at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration, and at the Center for Space Research, University of Texas-Austin. Copyright 2011 California Institute of Technology.
- 2.Awange JL, Sharifi M, Ogonda G, Wickert J, Grafarend EW, Omulo M (2007) The falling Lake Victoria water levels: GRACE, TRIMM and CHAMP satellite analysis of the lake basin. Water Resources Manage. doi:10.1007/s11269-007-9191-y
- 5.Bettadpur S (2007) CSR Level-2 processing standards document for product release 04 GRACE. The GRACE Project. Center for Space Research, University of Texas at Austin, pp 327–742. http://podaac.jpl.nasa.gov/gravity/grace
- 9.Chambers DP (2006a) Observing seasonal steric sea level variations with GRACE and satellite altimetry. J Geophys Res 111 (C3). doi: 10.1029/2005JC002914
- 10.Chambers DP (2006) Evaluation of new GRACE time-variable gravity data over the ocean. Geophys Res Lett 33(17)Google Scholar
- 15.Chen JL, Wilson CR, Tapley BD (2006) Satellite gravity measurements confirm accelerated melting of Greenland ice sheet. Science 313. doi: 10.1126/science.1129007
- 18.Chen JL, Wilson CR, Tapley BD, Blankenship DD, Ivins E (2007) Patagonia icefield melting observed by GRACE. Geophys Res Lett 34(22):L22501. doi: 10.1029/2007GL031871
- 25.Cretaux J-F, Soudarin L, Davidson FJM, Gennero M-C, Berge-Nguyen M, Cazenave A (2002) Seasonal and interannual geocenter motion from SLR and DORIS measurements: comparison wit with surface loading data. J Geophys Res 107. doi: 10.1029/2002JB001820
- 26.de Linage C, Rivera L, Hinderer J, Boy J-P, Rochester Y, Lambrotte S, Biancale R (2009) Separation of coseismic and postseismic gravity changes for the 2004 Sumatra-Andaman earthquake from 4.6 yr of GRACE observations and modelling of the coseismic change by normal-modes summation. Geophys J Int 176:695–714ADSCrossRefGoogle Scholar
- 33.Han S-C, Sauber J, Luthcke SB, Ji C Pollitz FF (2008) Implications of postseismic gravity change following the great 2004 Sumatra-Andaman earthquake from the regional harmonic analysis of GRACE intersatellite tracking data. J Geophys Res 113:B11413. doi: 10.1029/2008JB005705 ADSCrossRefGoogle Scholar
- 42.Knudsen P, Bingham R, Andersen O, Rio M-H (2011) A global mean dynamic topography and ocean circulation estimation using a preliminary GOCE gravity model. J Geodesy. doi: 10.1007/s00190-011-0485-8
- 43.Landerer FW, Swenson SC (2011) Accuracy of scaled GRACE terrestrial water storage estimates. Water Resour Res, in pressGoogle Scholar
- 55.Macrander A, Böning C, Boebel O, Schröter J (2010) Validation of GRACE gravity fields by in-situ data of ocean bottom pressure. In: Flechtner F, Gruber T, Güntner A, Mandea M, Rothacher M, Schöne T, Wickert J (eds) System Earth via geodetic-geophysical space techniques. Springer, Berlin. http://dx.doi.org/10.1007/978-3-642-10228-8_14 Google Scholar
- 57.Maximenko N, Niiler P, Rio M-H, Melnichenko O, Centurioni L, Chambers D, Zlotnicki V, Galperin B (2009) Mean dynamic topography of the ocean derived from satellite and drifting buoy data using three different techniques. J Atmos Ocean Technol. doi: 10.1175/2009JTECHO672.1, http://www.springerlink.com/content/r882426635467007/
- 66.Parker RL (1975) Theory of ideal bodies for gravity interpretation. Geophys J Roy Astron Soc 42(2):315–334Google Scholar
- 77.Riva REM, Gunter BC, Urban TJ, Vermeersen BLA, Lindenbergh RC, Helsen MM, Bamber JL, van de Wal RSW, van den Broeke MR, Schutz BE (2009) Glacial isostatic adjustment over Antarctica from combined ICEsat and GRACE satellite data. Earth Planet Sci Lett 288:516–523. http://dx.doi.org/10.1016/j.epsl.2009.10.013 ADSCrossRefGoogle Scholar
- 79.Rowlands DD, Luthcke SB, McCarthy JJ, Klosko SM, Chinn DS, Lemoine FG, Boy J-P, Sabaka TJ (2010) Global mass flux solutions from GRACE: a comparison of parameter estimation strategies—mass concentrations versus stokes coefficients. J Geophys Res 115:B01403. doi: 10.1029/2009JB006546 CrossRefGoogle Scholar
- 87.Swenson S, Chambers DP, Wahr J (2008) Estimating geocenter variations from a combination of GRACE and ocean model output. J Geophys Res 113. doi: 10.1029/2007JB005338
- 94.Vianna ML, Menezes V V (2011) Double‐celled subtropical gyre in the South Atlantic Ocean: means, trends, and interannual changes. J Geophys Res 116:C03024. doi: 10.1029/2010JC006574
- 95.Vinogradova NT, Ponte RM, Tamisiea ME, Quinn KJ, Hill EM, Davis JL (2011) Self-attraction and loading effects on ocean mass redistribution at monthly and longer time scales. J Geophys Res-Oceans 116. http://dx.doi.org/10.1029/2011JC007037
Books and Reviews
- Dickey J et al (1997) Satellite gravity and the geosphere. US National Research Council, National Academy Press, Washington, DCGoogle Scholar