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)