Time variability of the Earth's gravity field: Hydrological and oceanic effects and their possible detection using GRACE

The GRACE satellite mission, scheduled for launch in 2001, is designed to m ap out the Earth's gravity field to high accuracy every 2-4 weeks over a no minal lifetime of 5 years. Changes in the gravity field are caused by the r edistribution of mass within the Earth and on or above its surface. GRACE w ill thus be able to constrain processes that involve mass redistribution. I n this paper we use output from hydrological, oceanographic, and atmospheri c models to estimate the variability in the gravity field (i.e., in the geo id) due to those sources. We develop a method for constructing surface mass estimates from the GRACE gravity coefficients. We show the results of simu lations, where we use synthetic GRACE gravity data, constructed by combinin g estimated geophysical signals and simulated GRACE measurement errors, to attempt to recover hydrological and oceanographic signals. We show that GRA CE may be able to recover changes in continental water storage and in seafl oor pressure, at scales of a few hundred kilometers and larger and at times cales of a few weeks and longer, with accuracies approaching 2 mm in water thickness over land, and 0.1 mbar or better in seafloor pressure.