Secular Geoid Rate from GRACE for Vertical Datum Modernization

Abstract

GRACE-derived geoid rates are studied for North-America, where the adjustment of the Earth to ancient ice sheets causes a secular geoid increase up to 1.3 mm/year. These significant geoid changes are of particular interest for establishing a new geoid-based vertical datum in Canada and other high accuracy applications. To quantify the uncertainty of the derived rate of change of the geoid, several methods for GRACE error approximation are studied using: (i) calibrated standard deviations, (ii) a full covariance matrix, and (iii) residuals of least-squares fit of a trend and periodic variations to the time series of spectral coefficients. It is found that the residuals give the largest error estimates, probably because correlated errors are captured better. Furthermore, through maximizing the signal-to-noise ratio, it is found that the Swenson and Wahr (2006) filter of correlated GRACE errors should be applied to coefficients above degree 22 and order 4. Measurement errors are largely longitude independent, with magnitude around 0.06 mm/year. The largest geoid rate uncertainty is estimated in the area of present-day ice melt in Alaska and south of the Great Lakes and south-west of Hudson Bay (over 0.3 mm/year) due to uncertainty in continental water storage. For the creation of a geoid rate model based on GRACE data it is important that efforts are focused on reducing uncertainty in these areas, rather than improving post-processing.