Assessment of point-mass solutions for recovering water mass variations from satellite gravimetry

Previous studies have shown the feasibility of point-mass modellings for deriving terrestrial water storage (TWS) from the harmonic solutions of the Gravity Recovery And Climate Experiment (GRACE) mission at regional scales (e.g., Greenland and Antarctica). However, a thorough assessment of point-mass modelling approaches at the global and river basin levels is still necessary. Therefore, this study's objective is to assess the implementation and performance of the point-mass modelling approaches based on simulations using as inputs the TWS from Global Land Data Assimilation System (GLDAS). First, the approximate solutions of Newton's integral using the Taylor series expansion, such that the zeroth-order approximation is equivalent to the "original point-mass" (OPM) and the third-order approximation to the "improved point-mass" (IPM) modellings are presented. Second, numerical comparisons of the gravitational potential forwarded by the IPM and OPM are carried out at which both approaches show errors smaller than the GRACE uncertainties for the potential differences (similar to 7.6 x 10(-4) m(2)/s(2)). Nevertheless, the spatial patterns of the OPM's errors still assemble the TWS's spatial variations. Finally, simulations showed that considering OPM's deviations from IPM improves the root-mean-square-difference (RMSD) of the inverted TWS up to 50% at the global and basin scales if the edge effects are neglected. After accounting for the edge effects, the IPM solution presented an RMSD of 6.44 mm with an enhancement of about only 20% regarding the OPM. Although the present study confirms the suitability of point-mass approaches for recovering TWS, further investigations regarding its advantages compared to GRACE spherical harmonic synthesis are still necessary.