GRACE Science Team Meeting

A.2.1 to A.2.2 - Intercomparisons
(Convener: )

Towards combined global monthly gravity field solutions
(A. Jäggi, U. Meyer, G. Beutler, M. Weigelt, T. van Dam, T. Mayer-Gürr, J. Flury, F. Flechtner, C. Dahle)

Towards an ensemble model for GRACE time-variable gravity fields
(C. Sakumura, S. Bettadpur, S. Bruinsma)


Signal and error content in different Release-05 monthly solution series, with applications to polar regions
(M. Horwath; A. Horvath; R. Pail; N. Peterseim)

Title: Towards combined global monthly gravity field solutions
Presenter: Jäggi, Adrian
Co-Authors: U. Meyer, G. Beutler, M. Weigelt, T. van Dam, T. Mayer-Gürr, J. Flury, F. Flechtner, C. Dahle

Abstract: Currently, official GRACE Science Data System (SDS) monthly gravity field solutions are generated independently by the Centre for Space Research (CSR) and the German Research Centre for Geosciences (GFZ). Additional GRACE SDS monthly fields are provided by the Jet Propulsion Laboratory (JPL) for validation and outside the SDS by a number of other institutions worldwide. Although the adopted background models and processing standards have been harmonized more and more by the various processing centers during the past years, notable differences still exist in terms of signal and noise and the users are more or less left alone with a decision which model to choose for their individual applications. This procedure seriously limits the accessibility of these valuable data.

Combinations are well established in the area of other space geodetic techniques, such as the Global Navigation Satellite Systems (GNSS), Satellite Laser Ranging (SLR), and Very Long Baseline Interferometry (VLBI). Regularly comparing and combining space-geodetic products has tremendously increased the usefulness of the products in a wide range of disciplines and scientific applications. Therefore, we propose in a first step to mutually compare the large variety of available monthly GRACE gravity field solutions, e.g., by assessing the signal content over selected regions, by estimating the noise over the oceans, and by performing significance tests. We make the attempt to assign different solution characteristics to different processing strategies in order to identify subsets of solutions, which are based on similar processing strategies. Using these subsets we will in a second step explore ways to generate combined solutions, e.g., based on a weighted average of the individual solutions using empirical weights derived from pair-wise comparisons. We will also assess the quality of such a combined solution and discuss the potential benefits for the GRACE and GRACE-FO user community, but also address minimum processing requirements to be met by each analysis centre to enable a meaningful combination (either performed on the solution level or, preferably, on the normal equation level).

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Title: Towards an ensemble model for GRACE time-variable gravity fields
Presenter: Sakumura, Carly
Co-Authors: C. Sakumura, S. Bettadpur, S. Bruinsma

Abstract: Precise measurements of the Earth's time-varying gravitational field from the NASA/DLR GRACE mission allow unprecedented tracking of the transport of mass across and underneath the surface of the Earth and give insight into secular, seasonal, and sub-seasonal variations in the global water supply. Several groups produce these estimates and while the various gravity fields are similar, differences in processing strategies and tuning parameters result in solutions with regionally specific variations and error patterns. This study examined the spatial, temporal, and spectral variations between the different gravity field products and developed an ensemble gravity field solution from the products of four such analysis centers. The solutions were found to lie within a certain analysis scatter regardless of the local relative water height variation, and the ensemble model is clearly seen to reduce the noise in the gravity field solutions within the available scatter of the solutions.

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Title: Signal and error content in different Release-05 monthly solution series, with applications to polar regions
Presenter: Peterseim, Nadja
Co-Authors: M. Horwath; A. Horvath; R. Pail; N. Peterseim

Abstract: The Release-5 monthly solutions by the three centers of the GRACE Science and Data System are an enormous improvement with respect to the previous Release 4. Meanwhile, previous assessments have revealed different noise levels between the solutions by CSR, GFZ and JPL, and also different amplitudes of interannual signal in the solutions by GFZ as compared to the two other centers. These differences appear to be partly related to processing choices concerning the maximum spherical harmonic degree (60 or 90) and the question whether certain orbit parameters are estimated prior to, or together with, gravity field parameters. Following previous discussions, CSR and GFZ have kindly provided experimental solutions (additional to the official ones) that use modified choices related to the above-mentioned parameterization questions.

We compare these different series with respect to their signal and noise content. In particular we investigate systematic differences in the amplitudes of signal or noise common to the solutions, which are indications of dampening effects due to stabilizing parametrization choices. We put a special focus on polar signals. Concerning the question for an optimal maximum degree we suggest that for resolving large polar ice mass changes, it would be benefitial to provide gravity field variations even beyond degree 90.

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