Using GRACE Satellite Acceleration Data to Recover Arctic Ocean Tides Bryan Killett 1, John Wahr 1, Shailen D. Desai 2, Dah-Ning Yuan 2, Mike Watkins 2, 1 University of Colorado and CIRES, Boulder, CO, USA 2 Jet Propulsion Laboratory, Pasadena, CA, USA TexPoint fonts used in EMF. Read the TexPoint manual before you delete this box.: AAAAA

Quick Overview Motivation: FES 2004 is primarily based on TOPEX/Poseidon data, which does not extend north of 66° N. Thus, Arctic ocean tides are not well constrained by satellite altimetry. Relative acceleration values between the two GRACE satellites are used to solve for mass concentrations (mascons) on Earths surface. The solution method allows each mascons mass to oscillate at tidal and seasonal frequencies, as well as changing linearly. FES 2004 effects have been subtracted from the acceleration values, so the amplitudes at tidal periods represent errors in FES The mass amplitudes are converted to equivalent cm of water amplitudes.

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

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

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

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

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

GRACE relative accel. due to a mascon not below satellites

Inversion Details Smoothed residual acceleration values were averaged at 5 second intervals when satellites are north of 50° N latitude. 6 million accelerations total over 5 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.

Simulations To test the inversion program, arbitrary mascon amplitudes were created on Earths surface. These mascons have constant values, linear trends and amplitudes at M2 and K1 periods. Next, the accelerations that GRACE would record due to these mascons were calculated using the actual times and positions of the GRACE satellites. Finally, these simulated accelerations were inverted to solve for surface mascon amplitudes using the same algorithm used for real data.

Simulation – Constant Term

Simulation – Secular Trend

Simulation – M2 (Sine)

Simulation – M2 (Cosine)

Simulation – K1 (Sine)

Simulation – K1 (Cosine)

Inversion of Real GRACE Data

Real Data – Secular Trend

Real Data – Annual Amplitude

FES 2004 – M2 Amplitude

Residual M2 Amplitude

FES 2004 – K1 Amplitude

Residual K1 Amplitude

FES 2004 – O1 Amplitude

Residual O1 Amplitude

Conclusion Existing tide models such as FES 2004 have room for improvement. GRACE is a useful tool for recovering tidal signals even at semidiurnal frequencies. Errors in FES2004 arent significantly larger north of 66°N compared to south of 66°N (the TOPEX/Poseidon turning point). Simulations suggest that the large K1 amplitudes at the north pole are not real.

Main grid points are white, and spaced 230 km apart. Supporting grid points are colored differently according to which main grid point theyre associated with. Each main grid point has 8 supporting points.

The red area contains points at which a solution is desired for region #1. In order to properly account for gravity due to mascons in the green area, stack accelerations are loaded from both red and green regions. Mascons in the red AND green regions are then solved for, but only mascons in the red region are saved.

Notice that the red areas do not overlap, while the green areas do overlap. The green overlap areas represent computational waste, but they help reduce edge effects. This region can be larger than the 1 st polar region because GRACEs ground track density is higher at the poles, making polar region computations lengthy.