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    A magnifying glass for GRACE: de-correlated spherical harmonic solutionsA magnifying glass for GRACE: de-correlated spherical harmonic solutions  
 

 

A magnifying glass for GRACE: de-correlated spherical harmonic solutions

Analyzing more than 7 years of GRACE data has provided a completely new view on the Earth's changing gravity field and on the processes causing these changes. A new algorithm has been developed which allows reducing noise ('stripes') in GRACE solutions, and identifying signals of smaller scale than thought possible before.

Using data provided by the gravity recovery and climate experiment (GRACE) twin-satellite mission, scientists from various disciplines have been able, for the first time, to observe directly the redistribution of mass in the world's ocean, the mass balance of the Greenland and Antarctica ice sheets, water stock changes in the Amazon and many other areas, and the co- and post-seismic gravity effects associated with large seismic events such as the December 2004 Sumatra-Andaman Earthquake.

 

However, a significant problem that users of monthly GRACE gravity field solutions face is the presence of correlated and resolution-dependent noise in the provided spherical harmonic coefficients. The reason for this peculiar characteristic is GRACE's mission geometry in connection with potential limitations in current analysis strategies. The GRACE A and B twin-satellites fly in a single orbital plane, and the inter-satellite ranging observable used in gravity modeling translates into a distinct along-track sensitivity.

 

The new algorithm for anisotropically de-correlating ("de-striping") GRACE errors accounts for this along-track sensitivity pattern (Kusche et al., 2009). Depending on the application (e.g. whether one is interested in deriving monthly mass changes or rather inter-annual or even secular trends), variable additional smoothing can be applied. Tests have shown that the signal to noise ratio of the obtained maps of surface mass variability (large signal in the Amazon vs. low signal in the Sahara) is much better than with Gaussian filtering applied.

 

Fig. 1: WRMS of monthly GRACE solutions, as seen through 3 different versions of the new anisotropic de-correlation algorithm. Left: medium smoothing (DDK2 filter). Top right: Less smoothing (DDK3 filter), Bottom right: More smoothing (DDK1 filter). 58 GFZ RL04 GRACE monthly solutions, covering the time period 09/2002-08/2007 (missing months are 12/2002, 01/2003, 06/2003, 01/2004) have been used.

De-correlated monthly GFZ RL04 gravity models can be downloaded now from the web-site icgem.gfz-potsdam.de of the International Centre for Global Earth Models (ICGEM), (click on "Models from Dedicated Time Periods). Alternatively, grids of geoid height change or surface mass variation expressed as equivalent water column change can be obtained directly (click on "Calculation Service").

 

The impact of improved resolution of GRACE models through de-correlation is subject to further research activity in the project STREMP of the DFG priority program SPP1257 (Fenoglio et al., 2009).

 

Publications:

  • Fenoglio-Marc L., Rietbroek R., Grayek S., Becker M., Kusche J., Stanev E (2009) Water mass variations on seasonal and interannual time scales in the Mediterranean and Black Sea, submitted
  • Fenoglio-Marc L., Rietbroek R., Grayek S., Becker M., Kusche J., Stanev E (2009) Water mass variations on seasonal and interannual time scales in the Mediterranean and Black Sea, submitted
  • Kusche J. (2007) Approximate decorrelation and non-isotropic smoothing of time-variable GRACE-type gravity field models, Journal of Geodesy, 81:733-749. doi:10.1007/s00190-007-0143-3
  • Kusche J., Schmidt R., Petrovic S., Rietbroek R (2009) Decorrelated GRACE time-variable gravity solutions by GFZ and their validation using a hydrological model, Journal of Geodesy, DOI 10.1007/s00190-009-0308-3