Vertical deformations from homogeneously processed GRACE and global GPS long-term series

Temporal variations in the geographic distribution of surface mass cause surface displacements. Surface displacements derived from GRACE gravity field coefficient time series also should be observed in GPS coordinate time series, if both time series are sufficiently free of systematic
errors. A successful validation can be an important contribution to climate change research, as the biggest contributors to mass variability in the system Earth include the movement
of oceanic, atmospheric, and continental water and ice. In our analysis, we find that if the signals are larger than their precision, both geodetic sensor systems see common
signals for almost all the 115 stations surveyed. Almost 80% of the stations have their signal WRMS decreased, when we subtract monthly GRACE surface displacements from those
observed by GPS data. Almost all other stations are on ocean islands or small peninsulas, where the physically expected loading signals are very small. For a fair comparison, the data(79 months from September 2002 to April 2009) had to be treated appropriately: the GPS data were completely reprocessed with state-of-the-art models. We used an objective
cluster analysis to identify and eliminate stations,where local effects or technical artifacts dominated the signals. In addition, it was necessary for both sets of results to be expressed
in equivalent reference frames, meaning that net translations between the GPS and GRACE data sets had to be treated adequately.
These data sets are then compared and statistically
analyzed: we determine the stability (precision) of GRACEderived,
monthly vertical deformation data to be ∼1.2 mm,
using the data from three GRACE processing centers. We
statistically analyze themean annual signals, computed from
the GPS and GRACE series. There is a detailed discussion
of the results for five overall representative stations, in order
to help the reader to link the displayed criteria of similarity
to real data. A series of tests were performed with the goal
of explaining the remaining GPS–GRACE residuals.