Influence of Hydrology-Related Temporal Aliasing on the Quality of Monthly Models Derived from GRACE Satellite Gravimetric Data

Abstract

Temporal variations of the Earth's gravity field are traditionally represented with a set of spherical harmonic coefficients derived once per month. In practice, however, the gravity field changes continuously (e.g. because of on-going hydrological processes). This discrepancy causes temporal aliasing which manifests itself as errors in the obtained gravity field models. The goal of this study is to quantify the influence of temporal aliasing caused by hydrological processes onto the quality of computed monthly gravity field models; the Zambezi river basin is used as the test area. The GRACE observations are simulated along the actual GRACE orbits as gravitational potential differences. The mass variations obtained by the Least Square adjustment are compared with the "true" values used for data simulations. It is shown that the optimal estimation period is determined by the trade-off between the two conflicting effects of ever-decreasing propagated noise and ever-increasing aliasing. Catchments smaller than roughly 2000 km and estimation periods up to a month produce solutions with no noticeable aliasing. A catchment 3300 km wide has an optimal estimation period of 15 days and the one that is 4400 km large is most accurate if estimated for not more than 11 days. This is under the assumption that aliasing does not corrupt the solution significantly if it is less than half the propagated noise. © Springer-Verlag Berlin Heidelberg 2009.