Earth rotation and Earth gravity field from GRACE observations. Lucia Seoane, Christian Bizouard, Daniel Gambis

Transcription

1 Earth rotation and Earth gravity field from GRACE observations Lucia Seoane, Christian Bizouard, Daniel Gambis Observatoire de Paris SYRTE, 61 av. de l'observatoire, 7514 Paris

2 Introduction Gravity field changes now observed with unprecedented accuracy thanks to the satellite mission GRACE (and LAGEOS) changes of the inertia moments of the system «Earth» independent from any model matching Matching Confrontation Earth rotation irregularities

3 Data and method Euler-Liouville equation in the equatorial plane : Κ χ G = χ s (I) (II) + χ motion Geodetic excitation inertia term (I ω) relative angular momentum Both terms of this equation are compared using : EARTH ROTATION MONITORING : GRAVITY FIELD OBSERVATIONS : χ G χ s

4 Polar motion excitation from gravity field χ 1 m ass = M R e e 1.98 C ' 1 + k 3 C A 2 21 χ m ass 2 = M R e e S ' 1 + k 3 C A 2 21 M e and R e respectively the s and mean radius of the Earth, C and A : the principal inertia moments of the Earth and k 2 = -.31: the degree 2 load Love number. Non tidal atmospheric and oceanic models to be included into C 21 and S 21 variations for a comparison to Earth rotation data. Only for GRGS solution degree 2 coefficients given by LAGEOS satellites.

5 The gravity field observations Analysis of GRACE data from February 23 to Mars 26 carried out by 4 Centres : the Centre for Space Research (CSR) the GeoForschungsZentrum (GFZ) the Jet Propulsion Laboratory (JPL) the Groupe de Recherche en Géodésie Spatiale (GRGS/CNES) combination with LAGEOS Data

6 GEODETIC POLAR MOTION EXCITATION χ χ + iχ = p + G = 1 2 i σ. p c p = x i y is the observed polar motion. σ c is the Chandler period (433 days)

7 MOTION PART REMOVED FROM THE GEODETIC EXCITATION Gravimetric Excitation only reflect s redistribution removing the motion part of the excitation associated with the atmospheric winds and ocean currents: χ m a s s G = χ G χ m o tio n a tm χ m o tio n o c e a n Motion part : atmospheric angular momentum of the National Centre for Environmental Predictions oceanic angular momentum from ECCO model (JPL/IERS Special Bureau for the Oceans)

8 Comparison of geodetic and gravimetric excitation including atmosphere and oceanic effect 18 χ 1 s Complex correlations χ Grav /χ /χ Geod 12 6 JPL CSR GFZ JPL GRGS 6 12 CSR EOP GRGS GFZ Amplitude phase Year χ 2 s JPL CSR EOP GRGS Standard deviation ratio χ Grav /χ Table 2. Correlation coefficients /χ Geod CSR GFZ JPL GRGS χ GFZ χ Year

9 1 CSR and EOP coherence 1 GFZ and EOP coherence 1 JPL and EOP coherence Coherence CSR and EOP coherence phase Coherence GFZ and EOP coherence phase Coherence JPL and EOP coherence phase degrees degrees degrees Comparison of geodetic and total gravimetric excitation : coherence Coherence GRGS and EOP coherence degrees GRGS and EOP coherence phase

10 Comparison of geodetic and gravimetric excitation : conclusion Global agreement of polar motion excitation with the (2,1) stokes coefficient determined by GRACE Better agreement forχ 2 s, especially for seasonal variations. For χ 1 s too much power (15 %).

11 RESIDUAL EXCITATION Removing atmospheric and oceanic s term from gravimetric/geodetic excitation functions residual excitations reflecting mostly hydrological s redistribution as far as the main effect is well modelled Coherence study between the excitation function from GPS polar motion and the excitation from gravity field

12 14 15 RESIDUAL EXCITATION (II) χ 1 r χ 1 s (hydrology) HAM χ 2 r χ 2 s HAM 7 JPL 12 JPL 35 CSR EOP 6 CSR EOP 35 GRGS 6 GRGS 7 GFZ Year 12 GFZ Year

13 RESIDUAL EXCITATION Complex correlations χ Grav/Hydro /χ Ampli -tude /χ Geod CSR GFZ JPL GRGS Hydrology phase % significance 95% significance CSR GFZ total after removal of the geodetic excitation total after removal of the geodetic excitation Standard deviation ratio χ Grav/Hydro /χ Table 2. Correlation coefficients /χ Geod CSR GFZ JPL GRGS Hydro χ χ % significance JPL % significance GRGS total after removal of the geodetic excitation total after removal of the geodetic excitation

14 1 CSR and EOP coherence 1 GFZ and EOP coherence 1 JPL and EOP coherence Coherence Coherence Coherence CSR and EOP coherence phase GFZ and EOP coherence phase JPL and EOP coherence phase degrees degrees degrees RESIDUAL EXCITATION : coherence Coherence GRGS and EOP coherence degrees GRGS and EOP coherence phase

15 RESIDUAL EXCITATION : summary Overall correlation drops but : Coherence maintained in seasonal band, more strongly in the retrograde seasonal band (no interpretation) (magnitude :.7, delay < 5 days) Systematic phase lag drift systematic error in the gravimetric data? in the geodetic polar motion excitation? In the latter case : poor modelling of the motion term. Atmospheric wind and oceanic currents not sufficient? Bad modeled? The JPL and EOP excitations well compare with the hydrological excitation (ability of GRACE to track global hydrological excitation?)

16 Conclusive remarks Significant differences between GRACE solutions CSR, GFZ, JPL but some common features in the seasonal bands GRGS solution seems to better reproduce highfrequency part (monthly) of the EOP excitation Possibility to track poor modeling of the motion term? Mission and analysis still in progress. Wait and see.

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18 The gravity field solutions are residuals : centre models CSR RL4 GFZ RL4 GRGS Mean Field GIF22a EIGEN_GL4 EIGEN_GL4 Solid Earth Tides IERS Convention 23 IERS Conventions 23 IERS Conventions 23 Ocean Tide FES-24 (Lefèvre 25) FES-24 (Lefèvre 25) FES-24 (Lefèvre 25) Pole Tide IERS Conventions 23 IERS Conventions 23 IERS Conventions 23 Ocean Pole tide Desai's model, 22 Desai's model, 22 Desai's model, 22 Non Tidal Atmospheric and Oceanic Model ECMWF atmospheric model + OMCT baroclinic ocean model (Tho, 22) ECMWF atmospheric model + OMCT baroclinic ocean model (Tho, 22) 3D pressure field of ECMWF + MOG2D barotropic ocean model (Carrère and Lyard, 23)

19 Amplitude and phase of the seasonal variations Annual Semi-annual Excitation CSR Amp. 5±2 Phase deg. 324±18 Amp. 3±2 Phase deg. 345±27 GFZ 5±3 323±29 1±3 339±26 χ 1 s GRGS 3±2 19±37 3±2 319±43 OBS 8±1 342±1 1±1 23±59 CSR 22±2 287±5 5±2 34±23 GFZ 21±3 29±7 7±3 25±23 χ 2 s GRGS 19±2 292±7 5±2 14±29 OBS 29±2 3±4 9±2 43±14 Time origin for the phase : 1/1/24

20 Comparison of geodetic and gravimetric excitation (II) 12 EOP 12 CSR GFZ 15 JPL

21 References Bettadpur, S. (23), Level-2 Gravity Field Product User Handbook, The GRACE Proj., Univ. Of Tex., Austin. Chen J.L., Wilson C.R., Tapley B.D. and Ries J.C. (24) Low Degree Gravitational Changes from GRACE: Validation and Interpretation. Geophys. Res. Lett., 31, L2267. Bourda G. (24) Rotation Terrestre et Champ de Gravité. Doctoral Thesis of Paris Observatory. Lemoine J.M. and Biancale R. (27), Private Communication. Desai, S. (22), Observing the pole tide with satellite altimetry. Jour. Geophys. Res. Vol. 17. J. Ries and D. Chambers (26), Impact of Geodetic Improvements on Satellite Altimetry. International IAG/FIG Symposium Global Reference Frames 26, Munich (Germany).

22 Polar motion excitation from gravity field (I) χ s ~ (I 13 + i I 23 ) / (C-A) C, A main inertia moments of the Earth I 13 = - M R e2 C 21 I 23 = - M R e2 S 21 where C 21, S 21 are the (2,1) spherical harmonics coefficients of the geopotential determined by GRACE