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Solid Earth An interactive open-access journal of the European Geosciences Union

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https://doi.org/10.5194/se-2017-127
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 4.0 License.
Research article
27 Nov 2017
Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Solid Earth (SE).
Time-variable gravity fields and ocean mass change from 37 months of kinematic Swarm orbits
Christina Lück, Jürgen Kusche, Roelof Rietbroek, and Anno Löcher Institute of Geodesy and Geoinformation, University of Bonn, Bonn, Germany
Abstract. Measuring the spatiotemporal variation of ocean mass allows one to partition volumetric sea level change, sampled by radar altimeters, into a mass-driven and a steric part, the latter being related to ocean heat change and the current Earth’s energy imbalance. Since 2002, the Gravity Recovery and Climate Experiment (GRACE) mission provides estimates of the Earth’s time-variable gravity field, from which one can derive ocean mass variability. However, GRACE has reached the end of its lifetime with data degradation and several gaps during the last years, and there will be a prolonged gap until the launch of the follow-on mission GRACE-FO. Therefore, efforts focus on generating a long and consistent ocean mass time series by analyzing kinematic orbits from other low-flying satellites; i.e. extending the GRACE time series.

Here we utilize data from the European Space Agency’s (ESA) Swarm Earth Explorer satellites to derive and investigate ocean mass variations. We investigate the potential to bridge the gap between the GRACE missions and to substitute missing monthly solutions. Our monthly Swarm solutions have a root mean square error (RMSE) of 4.0 mm with respect to GRACE, whereas directly estimating trend, annual and semiannual signal terms leads to an RMSE of only 1.7 mm. Concerning monthly gaps, our Swarm solution appears better than interpolating existing GRACE data in 13.5 % of all cases, for 80.0 % of all investigated cases of an 18-months-gap, Swarm ocean mass was found closer to the observed GRACE data compared to interpolated GRACE data. Furthermore, we show that precise modelling of non-gravitational forces acting on the Swarm satellites is the key for reaching these accuracies. Our results have implications for sea level budget studies, but they may also guide further research in gravity field analysis schemes, including non-dedicated satellites.


Citation: Lück, C., Kusche, J., Rietbroek, R., and Löcher, A.: Time-variable gravity fields and ocean mass change from 37 months of kinematic Swarm orbits, Solid Earth Discuss., https://doi.org/10.5194/se-2017-127, in review, 2017.
Christina Lück et al.
Christina Lück et al.
Christina Lück et al.

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Short summary
Since 2002, the GRACE mission provides estimates of the Earth's time-variable gravity field, from which one can derive ocean mass variability. Now, that the GRACE mission has come to an end, it is especially important to find alternative ways for deriving ocean mass changes. For the first time we use kinematic orbits of Swarm for computing ocean mass time series. We compute monthly solutions, but also show an alternative way of directly estimating time-variable spherical harmonic coefficients.
Since 2002, the GRACE mission provides estimates of the Earth's time-variable gravity field,...
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