International Combination Service for Time-variable Gravity Field Solutions (COST-G)

We propose to assemble an international team for gravity field modelling in the context of the GRACE and GRACE-FO missions. These missions are dedicated to determining gravity field variations, hence mass transfers on the Earth at a daily to monthly periodicity.
This work should help promoting and developing the recently established gravity field service COST-G (International Combination Service for Time-variable Gravity Field Solutions) currently being installed under the umbrella of the International Association of Geodesy (IAG) as the product center of the International Gravity Field Service (IGFS) for time-variable gravity fields.
The aim of the team members will be to join their efforts for gravity field modelling in order to generate combined time-variable models and consequently to establish improved reference models like it has been demonstrated by the EGSIEM project of the European Commission.
Scientific rationale and goals
The NASA/DLR GRACE satellite mission provided us during 15 years (2002-2017) with a vision of the mass changing Earth of unprecedented accuracy. The inter-satellite K-Band ranging of micrometer-precision was able to detect mass redistributions and provide mass variation estimates in the Earth system. The prime products achieved were monthly gravity field models which give mainly large scale variations (> 300 km) of continental water storage and ice melting. Monthly GRACE global gravity solutions in spherical harmonic expansion were routinely calculated independently by different research teams such as GFZ (Dahle et al. 2012), CSR (Bettadpur 2012), JPL (Watkins and Yuan 2012), CNES (Bruinsma et al. 2010), TUG (Mayer-Gürr et al. 2012a), UBERN (Meyer et al. 2012) and used for numerous applications in geodesy, hydrology, climatology, geophysics, oceanography… as illustrated in figure 1 (more than 2000 papers are related to the GRACE mission).

Figure 1: Overview of various geophysical areas of application (Ilk et al., 2004)

Because of the different processing standards and strategies used by the different Analysis Centers (AC), the individual solutions exhibit discrepancies up to 20% in terms of mass increase or loss which makes it difficult for users to select the best model according to their study.
That is why UBERN initiated in 2015 a combination action in the framework of the European Commission H2020 program. The EGSIEM (European Gravity Service for Improved Emergency Management, see project primary goal was to combine monthly gravity field models from 4 ACs: AIUB, GFZ, GRGS, TUGRAZ and to provide an improved combined test solution over two years (see figure 2).

Figure 2: Mitigating the noise in Earth mass variations. The EGSIEM map combines noisier individual solutions, expressed in mm of equivalent water height (EWH)

The success of this initiative leads us now to pursue and to formalize this approach, in order to deliver the best gravity products for applications in Earth and environmental science research. The goal is to promote the recently established “International Combination Service for Time-variable Gravity Field Solutions” (COST-G) of the International Gravity Field Service (IGFS), under the umbrella of the International Association of Geodesy (IAG).
This new service will continue the EGSIEM effort and standardize gravity derived mass transport products, improve the quality, robustness and reliability of individual solutions and enable hydrologists, glaciologists, oceanographers, geodesists and geophysicists to take full advantage of one well-defined, consolidated time variable gravity product.
A thorough preparation is needed to achieve these goals. It will consist of:
– Developing the synergy between international teams working on gravity field modelling
– Improving and homogenizing the modelling adopted by the Analysis Centers (AC)
– Providing combined reference solutions by the Combination Center (CC)
– Assessing the reference solutions by a Validation Center (VC)
– Organizing dissemination by a dedicated webmaster (WM)
The ambition is to set up a combination service infrastructure which will improve the actual standards and turn it into an operational mode that will enable the use of GRACE and GRACE-FO mass redistribution data for monitoring hydrological events such as floods or droughts for instance.
In order to be credible it is fundamental to focus also on the validation procedures that are necessary for providing the user community with high quality validated and reliable water storage products.
Ultimately COST-G will provide consolidated time-variable global gravity models in terms of spherical harmonic coefficients and thereof derived grids by combining solutions from individual analysis centers as well as validation criteria which will be made available through dedicated web-interfaces.