Static and Time-Variable Gravity from GRACE Mission Data

Based on the GRACE mission data, a new era of static and time-variable gravity models with unprecedented resolution and accuracy have been generated by the GRACE Science Data System teams. In general, the spatial resolution of the field from pre-CHAMP satellite only models of about 1000 km can be increased by a factor of 5 – 6 thanks to the micrometer-precise K-band intersatellite link. The currently obtained gain in accuracy reaches one to two orders of magnitude, compared to the most advanced combination gravity pre-CHAMP models, but is still one order of magnitude away from the projected GRACE baseline accuracy.In this article we highlight the advances in gravity recovery with GRACE, based on recent results from GFZ Potsdam for a new GRACE-only medium-wavelength gravity model, called EIGEN-GRACE03S, a new combined high-resolution model complete up to degree and order 360, called EIGEN-CG03C, and the derivation of time-variable gravity signals from monthly GRACE-only gravity models.Evaluation of EIGEN-GRACE03S and EIGEN-CG03C shows that both models benefit in its long-to-medium wavelength part from an extended data base for GRACE, an augmented processing of the GRACE data as well as a meanwhile more complete and homogeneous compilation of surface data. The progress in resolution and accuracy with respect to earlier GRACE-based gravity models is moderate but visible at the level of 1 – 2 percent for standard comparisons.The derivation of time-variable gravity signals from a time series of 16 monthly GRACE-only gravity solutions reveals the mission’s sensitivity to hydrology-induced surface mass variations. The annual-varying signal on global and regional scales can be resolved down to spatial scales of a few hundred kilometers and the estimates are well above the assumed error level of the GRACE gravity solutions. Observable discrepancies with respect to the signal amplitudes, phases and spatial distribution indicate the potential contributions from GRACE to hydrological modelling, but also reveal systematic errors in the GRACE monthly fields.