Surface Mass Loading Estimates from GRACE and GPS

Abstract

In this paper we discuss the properties of an Empirical Orthogonal Filter approach applied to monthly GRACE gravity solutions which are used to estimate surface mass thickness maps. The vertical loading effect caused by this surface mass is also visible in IGS data acquired within the GRACE observation window. The comparison of GRACE to GPS allows us to constrain the Empirical Orthogonal Filter compression level to 3 and a smoothing radius to 4 or $5^\circ$ whereby the discrepancy between both datasets for vertical loading signals is around 1.9 mm for 59 IGS stations. The comparison assumes that temporal gravity field changes occur on frequencies shorter than about 3 years because of the way we preprocess the GPS data and GRACE surface mass estimates. We notice that EOF filter residuals do exhibit the presence of systematic geophysical effects which are not represented in the first few EOF modes. The EOF filter residuals contain for instance S ₂ ocean tide errors that show up at a period of 161 days, also we observe a semi-annual hydrology signal which is coherent with GLDAS model predictions. New is the presence of an unexplained inter-annual signal in the GRACE derived surface mass estimates at EOF mode 4 and onward. Possible explanations for this residual signal are discussed.