1 Time Variations of the European Gravity Field, David Crossley, Saint Louis University, Missouri, USA Jacques Hinderer, EOST / IPG, Strasbourg, France Jean-Paul Boy, NASA Goddard Space Flight Center, Maryland, USA

2 Global Geodynamics Project Phase 1 – IUGG SEDI initiative Workshops: Brussels 1997, Munsbach 1999, Jena 2002

3 GGP Stations

4 GGP Satellite Project  CHAMP and GRACE satellite calibration and validation  Provides surface gravity measurements that are independent of satellite observations, compared to other methods that rely on modeling  Goal is to find and interpret coherent seasonal gravity effects using European GGP ground stations

5 GGP and Satellite Missions

6 Ground and Satellite Gravity Contributions

7 Objectives  Use Superconducting Gravimeter (SG) data from the European sub-network of GGP  Compute residual gravity series for each station, July 1997 to December 2001  Combine series spatially into a surface that approximates the length scale of GRACE data over Europe ( km)  Estimate the error in this surface and compare to GRACE accuracy predictions (0.4  gal at 300 km)  Compare with existing CHAMP satellite data  Set up a semi-automatic procedure for producing European surface gravity for the duration of the CHAMP and GRACE missions

8 Simulation of Hydrology Recovery: 5 year model for Manaus, Amazon Basin (Wahr et al., 1998)

9 BE Belgium BR Brasimone PO Potsdam MB Membach MC Medicina ME Metsahovi MO Moxa ST Strasbourg VI Vienna WE Wettzell BH Bad Homberg WA Walferdange GGP Stations – Europe

10 Processing GGP Data  Start with uncorrected ICET 1 minute files  Fix pressure - linear interpolation for gaps  Remove local tide (solid and oceanic components) and local barometric pressure  Fix gaps and remove spikes – replace with linear interpolation  Decimate to 1 hour, remove IERS polar motion

11 Tides, Local Pressure and Polar Motion Removed

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16 Processing Drift and Offsets  Simultaneous Drift and Offset Estimation – VERY IMPORTANT TO DO THIS RIGHT  Offsets need to be checked against station log files  Drift and offsets can differ even between two spheres of a single instrument  Drift and offsets should be consistent between SG residuals and AG measurements

17 Wettzell - Initial Drift and Offsets CD029 Uncorrected GGP 1 minute data from ICET

18 Strasbourg Residuals - SG, AG and Polar Motion

19 Medicina Residuals, SG & AG Corrected for tides, air pressure, polar motion (Romagnoli et al. 2003)

20 SG Drift Estimation (  gal / yr)

21 Station Residuals BE to MO  gal

22  gal Station Residuals PO to WE2

23 Atmospheric Loading Models  Local pressure, admittance –0.3  gal / hPa; includes direct attraction + loading  Global pressure using only surface data, e.g. ECMWF. Includes treatment of oceans as static, IB, or non-IB. Thin atmosphere – no vertical structure (e.g. Boy et al., 1998)  Global (p,T) - as above, but model  (h) as a function of surface (P,T) – perfect gas – to 20 km (e.g. Boy et al., 2002)  Full 3-D vertical structure from actual 3-D meteorological data. Computationally intensive, but important at seasonal periods (Boy et al., 2002).

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27 Atmospheric loading, station ME

28 Combined Daily Residuals

29 Minimum Curvature Surface  For each day fit a minimum curvature surface to all the stations  This surface goes through every station and therefore does no spatial averaging

30 Minimum Curvature Surface, All Stations

31 6-month Samples, all Stations

32 Spatial Smoothing  Insufficient data to reconstruct a smoothed field from spherical harmonic analysis  Insufficient station data to fit a surface directly  Instead, fit an n’th degree polynomial 2-D surface to the minimum curvature surface

33 Comparison of Surfaces Minimum Curvature3 rd Degree Polynomial

34 3 rd Degree Surface – All Stations

35 Reconstruct Gravity at Stations  Select the gravity values on the surface at the stations  Average all stations for each day (mean surface)

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38 Select Central Stations

39 New Surface Using Central Stations

40 Central Stations, 6-Month Intervals

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42 Northward Event 99 Jan 99 Feb 99 Mar 99 May 99 Apr 99

43 Northward Event 99 May 99 Jun 99 Jul 99 Aug 99

44 So far: GGP data shows clear annual signal for 3 years (97- 00), but not for Accuracy at ~0.8  gal, comparable to predicted GRACE (0.4  gal) at 300 km. Atmospheric loading - difference between global (p,T) and local p - affects the mean gravity field at about the 0.5  gal level. No attempt yet to estimate secular changes over Europe

45 Interpretation of Annual Signals  Local:  Instrument effects, thermal anomalies, vegetation, groundwater, surface water, soil moisture  Regional and global:  Atmospheric pressure (3-D) attraction / loading  Ocean circulation, loading  Hydrology  Soil compaction Zerbini S., B. Richter, M. Negusini, C. Romagnoli, D. Simon, F. Domenichini, W. Schwahn (2001) EPSL 192. C. Romagnoli, S. Zerbini, L. Lago, B. Richter, D. Simon, F. Domenichini, C. Elmi, and M. Ghirotti (2003) in press.

46 Annual Signals at Medicina

47 GPS and Gravity at Medicina  GPS (upper), gravity (lower)  Expected anti-correlation  Reasonable admittance - 18 mm vs 6  gal (peak to peak)

48 Continental Water Storage  Model uses estimated precipitation, downwelling radiation and near surface atmospheric conditions  Soil saturates and recharges groundwater that partially recharges surface water; model includes evaporation  Monthly water storage is convolved with a Green’s function to estimate gravity attraction and elastic deformation. van Dam, T., Wahr, J. Milly, C., and Francis O., (2001) J. Geodet. Soc Japan.

49 Global loading results Model results from some GGP stations Includes attraction and loading Note maxima usually in winter

50 Water Storage Statistics

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52 Comment  Residual annual gravity in Europe has a similar amplitude ~ 2.5  gal and phase (probably) as estimated water loading  To proceed further, we need CHAMP and GRACE satellite data

53 Other Possibilities (1) Japan (2) Greenland

54 Japan Ocean Hemisphere Project National Astronomical Observatory, Misuzawa (Esashi, Canberra and Ny-Alesund) Kyoto University (Bandung) National Institute for Polar Research, Tokyo (Syowa)

55 BA Bandung CB Canberra ES Esashi KY Kyoto MA Matsushiro WU Wuhan GGP Stations - W. Pacific

56 Greenland Uplift Post-glacial uplift from 3 different models (a)ICE-3G – Greenland component only (b)HUY2 ignoring ice changes during last 4000 yr (c)ICE-3G - variability outside Greenland J. Wahr, T. van Dam, K. Larson, and O. Francis, (2001) JGR, 106 Kellyville Kulusuk

57 Greenland GPS

58 Greenland AG

59 Greenland Interpretation GPS uplift rate as a function of increasing data length (a)Kellyville multi-day averages (b)Kellyville daily averages (c), (d) same for Kulusuk

60 New Greenland Proposal  Network of 5 new SGs around coast  Should confirm gravity variations to < 1  gal  Tied to AG measurements  Monitored by GRACE

61 New GWR field instrument (requires no He refills)

62 Larger European GGP Array  New sites would be useful in Europe to fill gap between ME and existing stations  e.g. Central France, Denmark, Southern Sweden, Poland

63 Conclusions  GGP database will monitor gravity variations for satellite missions  Both SGs and AGs are required to confirm drift and offsets at the 1  gal level  GPS measurements required to correct for ground vertical deflection; requires > 4 years to define secular trends  Atmospheric loading should be done with full 3-D modeling, as for GRACE  More hydrological studies, including soil moisture and continental water loading, are required.

64 Thanks to: all members of the GGP Support Groups who have dedicated their efforts to maintaining the gravity stations