Inversion of GPS and GRACE data to determine regional surface mass changes

We present a methodology to invert a regional set of GPS displacement data to determine surface mass variations at sub-monthly time scale. It is assumed that surface deformation is a result of the Earth's elastic response to surface mass load of hydrology, atmosphere, and ocean. We developed an algorithm to estimate the spectral information of displacements from "regional" GPS data through regional spherical (Slepian) basis functions and apply the load Love numbers to determine the mass load in a spectral domain. We discuss the results from processing 16 years of daily vertical motions from 114 GPS stations in Australia. The GPS-inverted surface mass changes are validated against GRACE data, atmosphere and ocean models, and a land surface model. Seasonal and interannual terrestrial mass variations from GPS inversion are in agreement with GRACE data and water storage model outputs. The GPS recovery compares better with the water storage model around smaller coastal basins than GRACE solutions. The sub-monthly mass changes from GPS mostly agree with atmospheric mass changes in central Australia. The technique is extended to combine global GPS and GRACE data together to determine global mass changes. We will discuss the results from a joint inversion of 2000 GPS and GRACE data to determine global mass variations in the ITRF, particularly emphasising the importance of geocenter motion (degree-1 deformation) in evaluating hydrological mass variation.