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    Terrestrial gravity field variations from superconducting gravimeters for GRACE validationTerrestrial gravity field variations from superconducting gravimeters for GRACE validation  
 

 

Terrestrial gravity field variations from superconducting gravimeters for GRACE validation

Terrestrial gravity observations from superconducting gravimeters (SG) contain fundamental information on mass transfer especially related to hydrology. They offer the unique opportunity to supplement and validate the time-dependent gravity field variations derived from the GRACE satellite mission.

 

The project TASMAGOG focuses on Europe with its dense and long-term network of SGs within the Global Geodynamics Project (GGP). The consistent combination of SG observations with repeated absolute gravity measurements provides gravity time series of highest accuracy and long-term stability.

 

For comparison with temporal GRACE gravity field solutions, the same reductions applied to GRACE data have to be taken into account for the terrestrial data: Earth and ocean tides, atmospheric effects (3D), polar motion signal, and non tidal ocean mass variations are reduced in the terrestrial gravity observations.

 

The first essential result is the consistence of the long-term variations of the SG gravity residuals at several central European SG sites (Fig. 1), despite distances of several 100 km. Thus, regionalisation is permitted. Second, the SG residuals and the gravity changes from the satellite mission GRACE are in general agreement. The same is found for the gravity variations derived from the global hydrological model WGHM.

 

This confirms the hypothesis that the gravity variations are mainly caused by large-scale hydrological variations. The combined gravity data set will be used as a constraint for hydrological model simulations.

Figure 1: Comparison of terrestrial gravity residuals (monthly mean) from SG observations with monthly GRACE solutions using different filtering and with gravity variations derived from modelled changes in continental water storage (WGHM) for stations Moxa (after local hydrological reduction), Bad Homburg, and Medicina. The contribution of vertical deformation due to loading of global hydrology not observable by satellites was added to the GRACE data for the comparison.

In the case of subsurface stations or considerable local topography, as e.g. for Moxa/Germany, where a major part of the topography is above the gravimeter level, the attraction effect of local hydrology strongly depends on the local topography. The separation of local and large-scale hydrological impacts in SG observations, where necessary, is crucial for the comparison with satellite-derived gravity data. It is shown that even for a hydrological challenging situation such as Moxa local hydrology-induced effects can be successfully modelled and reduced (Fig. 2, Naujoks et al., 2009). By combination of a local hydrological model for a small catchment area with a high-resolution 3D gravity model the reduction of the local hydrological effect leads to a seasonal signal in the order of 35 - 40 nm/s², which was masked before.

Figure 2: Gravity residuals with and without reduction of the local hydrological effect estimated from a local model for Moxa.

Weise, A., C. Kroner, M. Abe, J. Ihde, G. Jentzsch, M. Naujoks, H. Wilmes, H. Wziontek, 2009. Gravity field variations from superconducting gravimeters for GRACE validation. J. Geodynamics, 48, 325-330, doi:10.1016/j.jog.2009.09.034.

 

Naujoks, M., C. Kroner, A. Weise, T. Jahr, P. Krause, S. Eisner, 2009. Evaluating local hydrological modelling by temporal gravity observations and a gravimetric 3D model. Geophys. J. Int., subm.