Arctic Ocean Tides from GRACE Satellite Accelerations

1. Arctic Ocean Tides from GRACE Satellite Accelerations Bryan Killett University of Colorado and CIRES, Boulder, CO, USA TexPoint fonts used in EMF. Read the TexPoint manual before you delete this box.: AAAAA

2. The Tidal Potential VT

3. The Tidal Potential VT

4. The Tidal Potential VT

5. Tides for order m = 2 • ~12 hr periods. • “Semi-diurnal.” • Largest tides. Adapted from Dr. Sylvain Paris

6. Tides for order m = 1 • ~24 hr periods. • “Diurnal.” • Medium tides. Adapted from Dr. Sylvain Paris

7. Tides for order m = 0 Adapted from Dr. Sylvain Paris

8. Diurnal Tidal Spectrum Adapted from Desai (1996)

9. GRACE NASA/courtesy of nasaimages.org.

10. GRACE relative accel. due to a mascon directly below satellites Relative acceleration > 0 B A MASCON

11. GRACE relative accel. due to a mascon directly below satellites B A MASCON

12. GRACE relative accel. due to a mascon directly below satellites Relative acceleration < 0 B A MASCON

13. GRACE relative accel. due to a mascon directly below satellites B A MASCON

14. GRACE relative accel. due to a mascon directly below satellites Relative acceleration > 0 B A MASCON

15. GRACE relative accel. due to a mascon not below satellites

16. Motivation • FES2004 is primarily based on TOPEX/Poseidon data, which doesn’t extend north of 66°N. Thus, Arctic ocean tides aren’t well constrained by satellite altimetry. • The GRACE orbit goes up to 89°N. • Relative acceleration values between the two GRACE satellites are used to solve for “mass concentrations” (mascons) on Earth’s surface. The solution method allows each mascon’s mass to oscillate at tidal and seasonal frequencies, as well as changing linearly. • FES2004 effects have been subtracted from the acceleration values, so the amplitudes at tidal periods represent errors in FES 2004. The mass amplitudes are converted to equivalent “cm of water” amplitudes.

17. Inversion Details • Smoothed residual acceleration values are averaged at 5 second intervals when satellites are north of 50° N latitude. • 7 million accelerations total over 7 years. • A constant offset, secular trend and amplitude/phase at seasonal and tidal periods are simultaneously solved for at each mascon. • Mascons are ~230km apart; 1200 mascons cover the area north of 50° N latitude. • Mascons are modeled as point masses for speed.

18. Tides are NOT Point Masses

19. Simulation Input – M2 Sine Coef.

20. Simulation Output – M2 Sine Coef.

21. Simulation Error – M2 Cos. Coef.

22. Simulation Input – K1 Sine Coef.

23. Simulation Output – K1 Sine Coef.

24. Simulation Error – K1 Cos. Coef.

25. Resolution Test

26. Inversion of Real GRACE Data

27. Non-tidal parameters

28. Non-tidal parameters

29. FES 2004 – M2 Amplitude

30. Residual M2 Amplitude

31. M2 – Diff. of Two 3.5yr Solutions

32. FES2004 M2 Deg90 Trunc. Error

33. Change in FES2004 M2 Amp.

34. FES 2004 – K1 Amplitude

35. Residual K1 Amplitude – 5 yrs

36. Residual K1 Amplitude – 7 yrs

37. K1 – Diff. of Two 3.5yr Solutions

38. FES2004 K1 Deg90 Trunc. Error

39. Change in FES2004 K1 Amp.

40. Original GRACE Power Spectrum

41. Noise Reduction for Accelerations Used in the Inversion

42. Original GRACE Power Spectrum

43. Noise Reduction for Accelerations NOT Used in the Inversion

44. Original FES2004 Power Spectrum

45. Conclusions • GRACE-derived corrections are: large where FES2004 is large, not generally larger north of 66°N, and much larger than truncation errors.

46. Conclusions • GRACE-derived corrections are: large where FES2004 is large, not generally larger north of 66°N, and much larger than truncation errors. • GRACE-derived corrections to FES2004 reduce the variance of accelerations not used in the inversion, so they can improve GRACE processing but can’t currently improve tide gauge predictions, probably due to short-scale effects that GRACE can’t resolve.

47. Conclusions • GRACE-derived corrections are: large where FES2004 is large, not generally larger north of 66°N, and much larger than truncation errors. • GRACE-derived corrections to FES2004 reduce the variance of accelerations not used in the inversion, so they can improve GRACE processing but can’t currently improve tide gauge predictions, probably due to short-scale effects that GRACE can’t resolve. • Two independent estimates agree on a ~1cm noise floor for the GRACE-derived corrections.

48. Conclusions • GRACE-derived corrections are: large where FES2004 is large, not generally larger north of 66°N, and much larger than truncation errors. • GRACE-derived corrections to FES2004 reduce the variance of accelerations not used in the inversion, so they can improve GRACE processing but can’t currently improve tide gauge predictions, probably due to short-scale effects that GRACE can’t resolve. • Two independent estimates agree on a ~1cm noise floor for the GRACE-derived corrections. • FES2004 amplitudes are too large in the oceans north of 50°N for the tides M2 , K1 , O1 , P1 .