Mass Variation Signals in GRACE Products and in Crustal Deformations from GPS: A Comparison

Geophysical surface mass variations are reflected both in gravity field variations and in load deformations of the solid Earth . These two signatures may be observed by GRACE and by GPS, respectively. This article reports about a comparison between both. Concerning GPS-derived deformations, a meaningful geophysical interpretation requires both homogeneously processed observations and a stable realization of the terrestrial reference system. Here we use results from a reprocessing of a global GPS network with consistent use of the latest processing and modelling strategies. This reprocessing includes the estimation of low-degree deformation terms. We directly compare them to respective GRACE results and find good agreement. Our main results concern the comparison of site displacement time series obtained from GPS, on the one hand, and from GRACE gravity variations converted to load deformations, on the other hand. We do this comparison both for the GRACE background models of short-term variations and for the final monthly GRACE solutions. For vertical deformations, we find good agreement. In contrast, the agreement is poor for the horizontal directions. The differences between GPS and GRACE contain some components which appear to have large-scale correlated patterns in space and seasonal patterns in time. More detailed analyses indicate that residual errors in the GPS solutions are likely the dominant cause of these differences. Analysing internal deformations of regional subnetworks is a way to circumvent some of the large-scale systematics of the GPS solution. Indeed, regional analyses show reasonable agreement between GPS and GRACE even in the horizontal components. Overall, our results demonstrate the progress and challenges of combining independent satellite geodetic observations within the Global Geodetic Observing System.