The impact of GRACE, GPS and OBP data on estimates of global mass redistribution

In an effort to improve the estimates of global surface mass variations, an approach has been developed in which monthly gravity fields from the Gravity Recovery and Climate Experiment (GRACE) are combined with Global Positioning System (GPS) site displacements and modelled ocean bottom pressure (OBP) data. The motivation for this combination stems from the notion that while GRACE monthly fields provide excellent results for the mid- to high-degree spherical harmonics coefficients, they are currently unable to determine the lowest degree coefficients as accurately. In addition, the GRACE monthly fields do not deliver estimates of the geocentre motion (i.e. degree 1 terms), which are needed when comparing GRACE solutions with ground-based measurements. These ground-based measurements are normally given in a reference frame whose centre of mass does not coincide with the centre of mass of the whole earth system, resulting in non-zero degree 1 coefficients. Through loading theory, large-scale mass variations can be derived from globally distributed GPS site displacement vectors and modelled OBP values; however, both measurement types have their own limitations and do not have homogeneous coverage over the globe. To assess the impact that these errors would have on current and future real-data combinations with GRACE monthly fields, a sensitivity study was conducted. A range of combinations were explored in which the spatial distribution and quality of the GPS and OBP data sets were varied. The results show that significant improvements to the GRACE monthly gravity fields, in particular at the low degrees, can be achieved when these solutions are combined with present-day GPS and OBP products. An idealized scenario was also investigated to identify the specific shortcomings and limitations of a real data combination scenario. A description of the methodology, assessment criteria and results of the sensitivity study will be presented, along with a discussion of the overall findings and future potential of this work.