Massentransporte und Massenverteilungen im System Erde  
    Empirical Orthogonal Functions can decompose the integral GRACE gravity signalEmpirical Orthogonal Functions can decompose the integral GRACE gravity signal  
 

 

Empirical Orthogonal Functions can decompose the integral GRACE gravity signal

The twin GRACE satellites observe the time varying gravity field caused by the sum of all mass variations within the Earth system. It has now been proven that the integral GRACE observations can be separated by means of empirical orthogonal functions derived from geophysical models and that mass variations within the individual subsystems ocean, atmosphere and continental hydrosphere can be distinguished and quantified.

Since March 2002 the two GRACE satellites observe with unprecedented high precision the time varying gravity signal caused by the sum of all mass variations within the Earth system. Since the water mass variation of the continental hydrosphere is the most uncertain component of the global water cycle (see e.g. the 4th assessment report of IPCC, 2007) it is one of the most challenging problems to separate, to identify, and quantify the mass variations of the individual subsystems ocean, atmosphere, and continental hydrosphere.

 

Using empirical orthogonal functions (EOF) derived from geophysical models it has now been proven that the decomposition is possible. The result is by no way obvious, because atmospheric mass variations overlay the two other subsystems, ocean and continental hydrosphere, and these two disjoint subsystems cause a leakage into each other. The ability to separate the individual mass variations was first proven by a closed loop simulation (Schmeer et al., 2008a). Starting with geophysical models, GRACE observations were synthesized, and the separation of these integral observations led to estimated time coefficients (so called Principal components, PC) coinciding with those of the predefined models. The coincidence remains excellent if other geophysical models are used.

 

In a second step the separation was performed with real, unfiltered GRACE data (Schmeer et al., 2008b). EOFs were derived from monthly time series of (i) the ECMWF surface pressure data, (ii) the Ocean Model for Circulation and Tides (OMCT, Thomas, 2002), and (iii) the WaterGAP Hydrology Model (WGHM, Döll et al. 2003). A time series of 67 monthly GRACE gravity field models with atmospheric and oceanic background models reconstructed were taken as input (Flechtner, 2005). For hydrology and atmosphere the reconstruction shows a good agreement with high correlations (88.3 and 95%) between original and the reconstructed PCs (see Figure 1). For hydrology there is a clear phase shift for the annual signal with GRACE observing the extrema about 1 month later than predicted by WGHM. It is also to be investigated why the correlation for the subsystem ocean (not shown) is worse and remains below 50%.

 

Fig. 1 Top Panels: Spatial pattern for the first most dominant modes of the WGHM hydrology (left) and the ECMWF atmosphere (right) .The bottom panels show the time coefficients (principal components) a) derived from the WGHM hydrology (green) and b) estimated with GRACE data (red).
  • Döll, P., F. Kaspar and B. Lehner, A global hydrological model for deriving water availability indicators: model tuning and validation, J. Hydrol. 270 , 2003.
  • Flechtner, F.: AOD1B Product Description Document, GRACE Proj. Doc. JPL 327-750, rev. 2.1, Jet Propul. Lab., Pasadena, Calif., 2005.
  • Schmeer, M., W. Bosch, and M. Schmidt (2008a): Separation of oceanic and hydrological mass variations by simulated gravity observations
  • Schmeer, M., H. Drewes, W. Bosch, and M. Schmidt (2008b) Separation of Mass Signals by Common Inversion of Gravimetric and Geometric Observations (MaSiS), Poster presented at the 2nd SPP1257 Kolloquium, 2008-10-06/08, LVG Munich
  • Thomas, M.: Ozeanisch induzierte Erdrotationsschwankungen - Ergebnisse eines Simultanmodells für Zirkulation und Gezeiten im Weltozean. PhD thesis, University Hamburg, 2001.