Temporal gravity field models inferred from GRACE data

The GRACE (Gravity Recovery and Climate Experiment) mission is designed to monitor temporal variations in the fluid mass at the surface of the Earth. 113 satellite-only geopotential solutions to degree and order 50 were computed every 10 days for the period 29 July 2002 through 9 December 2005. These solutions were obtained using a processing strategy, background model and solution stabilization in particular, which was different from the ones used by the GRACE project; however, the general parameter recovery method is the same. A temporal and spatial resolution of 10 days and approximately 666 km (spherical harmonics up to degree 30) is achieved without significant streaking effects in the maps and good continuity across the solutions. The EIGEN-GL04S gravity field model, which is based on exactly 2 years of GRACE and LAGEOS data in order to properly average the seasonal effects, is used as the mean field to which all 10-day solutions are compared in order to infer temporal variations. After conversion to equivalent water height, the maps can be used to evaluate seasonal and linear variations in water mass storage. The large Sumatra earthquake of 26 December 2004 shows up clearly in the geoid maps, and they can be used to better understand the underlying geophysical mechanism. The linear trend in the solutions over 3.5 years is calculated, which represents mass gain (i.e., more snow or water) or loss (e.g., melting glaciers).