Title: Exploring the link between Earth's gravity field, rotation and geometry in order to extend the GRACE-determined terrestrial water storage changes to non-GRACE times
Session: B.4 - Progress in Hydrological Applications
First Author: Hans-Peter Plag
Presenter: Hans-Peter Plag
Co-Authors: R. S. Gross
Abstract: Geodetic observations of Earth's time-variable gravity field, rotation and shape are intrinsically interlinked by that fact that the time variations of these quantities are caused by processes in one and the same unique Earth system. Although the signals induced by the Earth system processes in each of these three main fields of geodesy appear with different spatial characteristics and against a different noise spectrum, in principle, all three areas should allow the extraction of information on the Earth system processes. On time scales of weeks to decades, main signals in all three areas originate in mass relocation in atmosphere, ocean, cryosphere, and terrestrial hydrosphere, i.e. the 'fluid' envelop of the solid Earth. At these time scales, interaction of the solid Earth with its fluid envelop is to a high degree linear (the main non-linearities are in atmosphere and ocean and not impacting the interaction with the solid Earth), and signals in one area should be proportional to similar signals in the two other areas (although the proportionality may depend on spatial and temporal scales). Therefore, the recent period of highly accurate simultaneous observations of all three areas provides the unique basis for calibration of forward models and inversion methods which then could be used to extrapolate the models and/or inversions to the period before GRACE, when only accurate observations of time-variable shape and rotation were available. Likewise, the calibrated models and inversions could be used to extrapolate to post-GRACE times in case an immediate follow-on mission would not be available. The simultaneous period can also be use to establish realistic error estimates for models and inversions based on observations of time-variable rotation and shape only.
Considering the complex interactions of surfical mass relocation with the three geodetic areas and the requirement of mass conservation in the water cycle, we expect that only sufficiently sophisticated and comprehensive Earth system models will be able to make full use of the geodetic observations as constraints for the modeled mass relocation in the global water cycle. Such models, once calibrated with the full observational database of the last six years, will also provide the basis to assess the degree to which inversions of the geodetic observations can provide realistic estimates of mass relocation in the water cycle. In our presentation we will consider theoretical aspects of the model development and outline a road map towards a system model with assimilation of the geodetic observations.