The unexpected signal in GRACE estimates of $$C_{20}$$ C 20

For science applications of the gravity recovery and climate experiment (grace) monthly solutions, the grace estimates of $$c_{20}$$ (or $$j_{2}$$ ) are typically replaced by the value determined from satellite laser ranging (slr) due to an unexpectedly strong, clearly non-geophysical, variation at a period of $$sim $$ 160 days. this signal has sometimes been referred to as a tide-like variation since the period is close to the perturbation period on the grace orbits due to the spherical harmonic coefficient pair $$c_{22}/s_{22}$$ of s2 ocean tide. errors in the s2 tide model used in grace data processing could produce a significant perturbation to the grace orbits, but it cannot contribute to the $$sim $$ 160-day signal appearing in $$c_{20}$$ . since the dominant contribution to the grace estimate of $$c_{20}$$ is from the global positioning system tracking data, a time series of 138 monthly solutions up to degree and order 10 ( $$10times 10$$ ) were derived along with estimates of ocean tide parameters up to degree 6 for eight major tides. the results show that the $$sim $$ 160-day signal remains in the $$c_{20}$$ time series. consequently, the anomalous signal in grace $$c_{20}$$ cannot be attributed to aliasing from the errors in the s2 tide. a preliminary analysis of the cross-track forces acting on grace and the cross-track component of the accelerometer data suggests that a temperature-dependent systematic error in the accelerometer data could be a cause. because a wide variety of science applications relies on the replacement values for $$c_{20}$$ , it is essential that the slr estimates are as reliable as possible. an ongoing concern has been the influence of higher degree even zonal terms on the slr estimates of $$c_{20}$$ , since only $$c_{20}$$ and $$c_{40}$$ are currently estimated. to investigate whether a better separation between $$c_{20}$$ and the higher-degree terms could be achieved, several combinations of additional slr satellites were investigated. in addition, a series of monthly gravity field solutions ( $$60times 60$$ ) were estimated from a combination of grace and slr data. the results indicate that the combination of grace and slr data might benefit the resonant orders in the grace-derived gravity fields, but it appears to degrade the recovery of the $$c_{20}$$ variations. in fact, the results suggest that the poorer recovery of $$c_{40}$$ by grace, where the annual variation is significantly underestimated, be affecting the estimates of $$c_{20}$$ . consequently, it appears appropriate to continue using the slr-based estimates of $$c_{20}$$ , and possibly also $$c_{40}$$ , to augment the existing grace mission.