Signal and Noise in GRACE observed surface mass variations

E.J.O. Schrama1, B. Wouters1, D.A. Lavallée2

(1) TU Delft, The Netherlands, (2) University of New Castle, UK

E-mail: e.j.o.schrama@tudelft.nl

Related Publications:

Ernst J.O. Schrama, Bert Wouters and David A. Lavallée, Signal and Noise in Gravity Recovery and Climate Experiment (GRACE) observed surface mass variations, Vol. 112, B08407,

doi:10.1029/2006JB004882, 2007

Ernst J.O. Schrama and Pieter N.A.M. Visser (2007), Accuracy assessment of the monthly GRACE geoids based upon a simulation, Journal of Geodesy 81, 67-80, DOI 10.1007/s00190-006-0085-1

Kusche J, Schrama E.J.O., (2005) Mass redistribution from global GPS timeseries and GRACE gravity fields: inversion issues. JGR solid earth, Vol 110, B09409, doi:10.1029/2004JB003556.

Outline

Filter design + tuningA. GRACE CSR RL04 solutions are converted into surface mass gridsB. Empirical Orthogonal Functions are used to separate signal and noise

contained in the Gaussian smoothed surface mass gridsC. Result depends on an EOF approximation level “M” and a smoothing

radius “t”D. Optimal choice of “M” and “t” follows from observed deformations at

selected set of GPS stations within the IGS.

Test 1: Degree variance spectra A. Signal, EOF residual, GGM02C formal errors, B. Background model error tides and air pressure

Test 2: Residual analysis A. S2 tide and 180 day hydrology in EOF residualsB. Auto-covariance functions from EOF residuals are compared to

formal covariance functions based on a GSFC GRACE covariance matrix

6.25 deg, CSR RL04, 43 months

51.4%

9.5%

12.6%

EOF variances

EOF signal

EOF residual

0.85*GGM02C

FES2004-GOT00.2

NCEP-ECMWF

Equivalent water height degree spectrum at 6.25 degree smoothing

EOF signal

EOF residual 0.85*GGM02C

FES2004-GOT00.2

NCEP-ECMWF

Geoid height degree spectrum at 6.25 degree smoothing

Surface mass on 3 EOF modes and 6.25 degree smoothing radius

10 32 100 mm

GPS validation• IGS vertical loading within GRACE window• Accept only those IGS stations where rms of

difference relative to GRACE < 3 mm, and where the correlation is greater than 0.5

For the 59 remaining stations we find:

GSFC covariance matrix for July 2003 (Frank Lemoine)

Formal error for the EQWH map from GRACE covariance matrix

mm7.5 degree smoothing

Residual EQWH signal in EOFs 4 and up

CM6.25 degree smoothing

S2 (161 days)

180 days GRACE 180 days GLDAS

Residual signal in EOF 4 + up:

- S2 tide errors- Semi-annual hydrology signal

cm

Auto-Covariance functionsderived from EOFs 4 and up at 173E 0N

6.25 degree smoothing

W-E

S-N

Conclusions

• EOF filter method– 3 EOF modes and 6.25 degree smoothing– Synthetic EOF error = 0.85 * Formal error– Tide and atmosphere pressure errors are 3 to 5 times smaller

compared to formal GGM02C errors

• GPS validation– 3 modes 5.00 or 6.25 degree smoothing – 59 IGS stations calibration set

• Residual signal in EOFs 4 and up– S2 tide errors (161 days) identified– Semi-annual hydrology (180 days) recognized– Auto covariance still reveals North South striping that is not

fully explained by the formal covariance matrix