Combination of gravimetric and altimetric space observations for estimating oceanic polar motion excitations

Abstract

[1] Global dynamic processes cause variations in the Earth's rotation, which are monitored by various geometric observation techniques such as Global Navigation Satellite Systems (GNSS), Satellite Laser Ranging (SLR), and Very Long Baseline Interferometry (VLBI) with millimeter accuracy. The integral effect on Earth rotation of mass displacements and motion is therefore precisely known, but the separation of contributions from particular geodynamic processes remains a challenge. Here we show that the oceanic mass effect on Earth rotation can be derived from both time variable gravity field solutions from the Gravity Recovery And Climate Experiment (GRACE) and sea level anomalies (SLA) observed from satellite altimeter missions. The GRACE solutions require filtering and the application of an ocean mask, whereas the SLA need to be corrected for the steric effect as polar motion is only affected by mass redistributions. We assess the accuracy of our oceanic polar motion excitations by using GRACE and SLA solutions from different processing centers. In addition, we compare polar motion excitations from GRACE, satellite altimeter data and their combinations with excitations estimated from ocean models. We show that the combination of gravimetric and altimetric solutions reduces systematic errors of the individual solutions. The combined solutions are about 2 times more accurate than ocean model results and about 3 times more accurate than the so-called reduced geodetic excitation functions. We anticipate our analysis to be valuable input for improved modeling of oceanic mass redistributions.

The influence of Antarctic ice loss on polar motion: an assessment based on GRACE and multi-mission satellite altimetry

“…3 Gravimetry solutions for monthly polar motion excitation functions for AIS, APIS, WAIS and EAIS: ITSG-Grace2018/TUM (blue), ITSG-Grace2018/TUD (green), CSR RL06M (cyan), JPL RL06M (magenta), LDCmgm90 (black) and altimetry solutions: TUD (red), UL (brown). Note the different ranges of the axis due to the different magnitudes of the signals in the different regions and altimetry solutions and (4) weighted combination of the gravimetry and altimetry solutions based on the least squares adjustment approach described in Göttl et al (2012) to show that via a combination an improved excitation time series can be retrieved. Hereinafter we refer to the results from the latter combination approach as "adjusted solutions".…”

“…These improvements confirm that even due to a simple combination (arithmetic mean) of GRACE and satellite altimetry systematic and random errors can be reduced and the robustness of the geodetic derived Antarctic polar motion excitation functions can be increased. By applying the weighted least squares adjustment approach described by Göttl et al (2012) further improvements can be achieved (RSD: 2 to 19% ). The polar motion excitation function χ EAIS 2 show the highest uncertainties, this phenomenon requires further investigation.…”

Earth Rotation Variations – Long Period

“…For example, the individual effects on the Earth's rotation of changes in the mass distribution of the oceans (e.g., G€ ottl et al, 2012;Jin et al, 2010) and of water stored on land (e.g., Brzezinski et al, 2009;Fernández, 2009;Jin et al, 2010Jin et al, , 2012Seoane et al, 2011) can be studied with GRACE measurements after atmospheric effects are removed from them using a model. For example, the individual effects on the Earth's rotation of changes in the mass distribution of the oceans (e.g., G€ ottl et al, 2012;Jin et al, 2010) and of water stored on land (e.g., Brzezinski et al, 2009;Fernández, 2009;Jin et al, 2010Jin et al, , 2012Seoane et al, 2011) can be studied with GRACE measurements after atmospheric effects are removed from them using a model.…”

Reducing filter effects in GRACE-derived polar motion excitations

“…Since LOD variations are related to the degree-2 spherical harmonic coefficient C 20 , which still suffers from ocean tide model errors within the GRACE data processing (Chen and Wilson 2008), most GRACE Earth rotation studies are restricted to polar motion investigations like this study. Initially, GRACE time variable gravity field models were used to estimate the integral mass-related excitations of polar motion (e.g., Cheng and Tapley 2004;Nastula et al 2007;Chen and Wilson 2008;Gross 2009;Seoane et al 2009; Seoane and Gambis 2012), later they were also used to estimate the individual contributions of the continental hydrosphere, oceans and cryosphere separately (e.g., Jin et al 2010Jin et al , 2012Seoane et al 2011;Chen et al 2012Chen et al , 2013Chen et al , 2017Göttl et al 2012Göttl et al , 2015Göttl et al , 2018Göttl 2013;Adhikari and Ivins 2016;Meyrath and van Dam 2016).…”