The quest for a consistent signal in ground and GRACE gravity time series M. Van Camp – B. Meurers...
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Transcript of The quest for a consistent signal in ground and GRACE gravity time series M. Van Camp – B. Meurers...
The quest for a consistent signal in ground and GRACE
gravity time series
M. Van Camp – B. Meurers – O. de VironL. Métivier – O. Francis
Special thanks:T. JahrV. PalinkasS. SteinH. WziontekW. Zürn
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
The reality
What GRACE sees
What a land-based gravimeter “sees”
1 point is 1 km x 1 km (in the best case)
Pointmeasurement
How to compareGravimeters 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?
To compare annual signals: phasorMaximum 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
What about Gravimeters & GRACE ?
Annual component
No agreement on annual :
Neither between SGs Neither between SGs
and GRACE
If agreement : so what?
Annual removed:Let us look at the interannual
How?- EOF technique : common mode significant for 3 gravimeter series not better than random series
Why?
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
BHWA
MO
ST
MC
VI
MB
CO
WEPECO
MB
WA
ST
PE
BH
MO
MC
VI
WE
Interannual
We find NothingIs this strange?
http://www.cortjohnson.org/wp-content/uploads/2013/04/Complex-System-Gut.jpg
Disparate phases &
amplitudesComplicated transfer function
Similar Forcing(let us
assume!)
Annual
Inter-annual
10 on 45 gravimeter
pairs significantly correlated
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 what’s the extra value of
comparing gravimeters to GRACE ?
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)