Abstract:

We examine low degree gravitational variations of C21, S21, and C20 observed by the Gravity Recovery and Climate Experiment (GRACE) satellites during the first 3 years of this gravity mission. The GRACE observations are compared with independent estimates from accurately measured Earth rotational changes and predictions from atmospheric, oceanic, and hydrological models. The 35 GRACE constrained monthly gravity solutions, covering the period April 2002 to June 2005, show strong seasonal variability in the C21, S21, and C20 time series, and generally agree with Earth rotation-derived changes and geophysical model estimates, in particular in S21. We evaluate potential effects on these low degree gravitational changes from the ocean pole tide, which is not applied in the constrained solution, and also potential effects from the upcoming improvement in the solid Earth pole tide. If these effects are corrected, GRACE observed C21 and S21 would agree significantly better with estimates from the Earth rotation and geophysical models, suggesting we may anticipate major improvements in the determination of these low degree gravitational changes in the undergoing GRACE reprocessing.