1. The quest for a consistent signal in ground and GRACE gravity time series M. Van Camp – B. Meurers – O. de Viron L. Métivier – O. Francis Special thanks: T. Jahr V. Palinkas S. Stein H. Wziontek W. Zürn

2. Comparisons GRACE-superconducting gravimeters Added value by combining them? Numerous publications : e.g. Abe et al., GJI, 2012 & Crossley et al., GJI, 2012. 2 sources of information on gravity changes : GRACE & Superconducting Gravimeters

3. The reality

4. What GRACE sees

5. What a land-based gravimeter sees 1 point is 1 km x 1 km (in the best case) Point measurement

6. How to compare Gravimeters and GRACE ? In both cases: annual signal (not a surprise in geodesy) 40 nm/s² 4 µGal Here: superconducting gravimeter in Bad Homburg (DE) Coherent between the gravimeters? Coherent between gravimeters and GRACE?

7. To compare annual signals: phasor Maximum within 74 days (for stations experiencing large amplitude) What do we learn? Winters are wetter! Notice: even worse if the sign of underground stations is not inverted 74 days = 25 nm/s²

8. What about Gravimeters & GRACE ? Annual component No agreement on annual : Neither between SGs Neither between SGs and GRACE If agreement : so what?

9. Annual removed: Let us look at the interannual How? - EOF technique : common mode significant for 3 gravimeter series not better than random series Why?

10. 11 on 45 pairs significantly correlated (anticorrelated if underground- surface pairs) Only little common signal What can we do with it? 100 nm/s² 10 µGal BH WA MO ST MC VI MB CO WE PE CO MB WA ST PE BH MO MC VI WE Interannual

11. We find Nothing Is this strange? http://www.cortjohnson.org/wp- content/uploads/2013/04/Complex-System-Gut.jpg Disparate phases & amplitudes Complicated transfer function Similar Forcing (let us assume!) Annual Inter- annual 10 on 45 gravimeter pairs significantly correlated

12. Correcting for local effects? Need to convert gravimeter data to something that we can compare with GRACE : Must know, by other means, the mass of water everywhere around the gravimeter. Must rely on perfect hydrological models, but whats the extra value of comparing gravimeters to GRACE ?

13. Conclusions We find the case that surface gravity and GRACE data are seeing the same signal to be weak Poor correlations between the gravimeters themselves True for the annual period True for the inter-annual periods Possible common, long period climate effects ? Longer time series & More stations needed SGs great for local hydrogeological investigations GRACE great for large scale hydrogeological phenomena More? See Van Camp et al. GJI (in review)