Towards improved GRACE de-aliasing - first results from dynamical and residual ocean tide analysis

1. Towards improved GRACE de-aliasing - first results from dynamical andresidual ocean tide analysis Wolfgang Bosch1), Detlef Stammer2), Frank Flechtner3), Karl-Heinz Ilk4), Torsten Mayer-Gürr4), Christoph Dahle3), Eifu Taguchi2), R. Savcenko1) E-mail: bosch@dgfi.badw.de • Deutsches Geodätische Forschungsinstitut (DGFI), München • Institut für Meereskunde (IfM), Hamburg • GeoForschungsZentrum (GFZ), Potsdam • Institut für Geodäsie und Geoinformatik (IGG), Bonn

2. Rationale and Objectives Rationale: • Deficiencies of global ocean tide models over shelves and polar oceans known – errors map to GRACE gravity fields . Objectives: Improving GRACE de-aliasing by • Empirical residual tide analysis w.r.t. the state-of-the-art ocean tide model FES2004 by • Solving for long wavelength residuals from GRACE data • Solving short scale residual tides from altimeter data • Enhance hydrodynamic modelling by revising the HAMTIDE model

3. K2 S2 M2 [Tage] Sampling of K2, S2, and M2 by GRACE

4. Alias and Rayleigh Periods for GRACE 1825 d = 5 a: GRACE until now 2726 d = 7.5 a

5. CSR RL04 COS 161d CSR RL04 SIN 161d Analysing the GRACE time series all excl. C20! GRGS COS 161d GRGS SIN 161d GFZ RL04 COS 161d GFZ RL04 SIN 161d

6. Analysis of Residuals (Dec 2006) Gridding (2° x 2°, 4 years)

7. M2 Amplitude from Residuals

8. Empirical tide analysis with Altimetry data • Altimeter satellites have their own alias periods (Smith 2000) • Combination of mission with different sampling allows to overcome some of the obstacles of individual missions • Harmonized and cross-calibrated (common multi-mission crossover analysis of all systems operating simultaneously) • Common residual analysis with TOPEX, TOPEX-EM, Jason-1, ERS-1, ERS-2, ENVISAT, GFO (w.r.t. FES2004) • Least Squares harmonic analysis on a global 15’x15’ grid • Solving for mean, drift, seasonal signal ( Sa, Ssa), long period (Mm, Mf), diurnal(Q1, O1, P1, K1), semidiurnal (M2, S2, K2, N2) and shallow water (M4, MS4, M6)

9. Results for M2

10. Results for S2

11. Residual tides (w.r.t. FES2004) at NW European Shelf

12. Validation with BODC bottom pressure gauges • Left: rms of sea surface heights (after FES2004 correction) • Right: % gain in variance due to residual tide corrections

13. Clear indications that the ocean tide model presently used for GRACE processing can be considerably improved by • Long wavelength structure seen by GRACE data itself (residuals or time series analysis) • Short wavelength resulting from analysis of multi-mission altimeter data • Ultimate goal is to combine both empirical approaches

14. HAMTIDE(Hamburg direct Assimilation Method for TIDE) • HAMTIDE: direct minimization of model deficiency and the inaccuracy data in a least square sense, • Assimilation of altimeter data, tide gauge, and bottom pressure recorder data • Blue: indput data • Red: 102 Pelagic Sites used as standard reference

15. HAMTIDE(Hamburg direct Assimilation Method for TIDE) • HAMTIDE is run with three different spatial resolutions, i.e., 1°, 10’ and 5’. • Right: 10’ run M2 • Comparison with FES2004 (10’ resolution)

16. HAMTIDE applied to strongly nonlinear effects on the NW European Shelf • Rms: 0.8 cm 7.7 cm Fig. 5 Fig. 6

17. See Posters for details of the project Taguchi, E., D. Stammer, W. Zahel, R. Savcenko, and W.Bosch: Toward high resolution dynamic modelling using HAMTIDE Savcenko, R. and W. Bosch: Assessment of errors in global ocean tide models and their impact on GRACE gravity fields