The GRACE/GRACE-FO satellite missions enabled numerous studies of the global hydrological cycle, extreme events like floods and droughts, as well as ice mass melt in polar regions related to climate change. These studies are based on the monthly gravity field models derived from the GRACE/GRACE-FO data. The temporal and spatial variability of the Earth’s gravity field is related to global processes of mass transport. To derive the monthly gravity field models, the satellites act as test masses in a free-fall experiment. The exact observation of the satellites’ orbits allows to reconstruct the gravitational forces acting on the satellites and to draw conclusions on the global mass distribution and its temporal variations.
Dr. Ulrich Meyer, Universität Bern
Although the observation principle is rather simple, the correct separation of the different forces affecting the satellites’ motion requires a wealth of additional information. In the end, we are interested in the gravitational pull of the ice and continental water masses at the Earth’s surface, and background models are needed to quantify, e.g., the tidal or weather-induced atmosphere and ocean mass variations with periods shorter than the monthly resolution of the derived global gravity fields. In most cases, the errors of these background models are insufficiently known, and the analysis centers, which are responsible for the generation of the gravity field models from GRACE/GRACE-FO data, have developed very diverse approaches for noise mitigation. Consequently, not one but several time-series of monthly gravity fields are available, that mainly differ by their signal-to-noise ratio.
The differences between the individual time-series have an impact mainly on the analysis of small-scale transport processes, where in the case of GRACE/GRACE-FO satellite gravimetry data, ‚small_scale‘ refers to spatial scales below approximately 500 km. Taking a closer look at, e.g., the mass variations in the Danube river basin, we observe a good agreement of the long-term climatology derived from the different time-series, while the spread of the individual monthly solutions, which carry the essential information on hydrological extremes, is rather large.
To support the scientific user community of the GRACE/GRACE-FO gravity field data, the Combination Service for Time-Variable GravityGravity is also known as the force of gravity or mass attraction. Gravity is the force that two or more bodies exert on each other due to their mass. The best-known gravitational force is the Earth's gravitational pull. It causes bodies on earth to f... fields (COST-G) of the International Association of Geodesy (IAG) has started its operation in the summer of 2019. At the Astronomical Institute of the University of Bern, the COST-G Analysis-Center-Coordinator performs quality control of the individual time-series of monthly gravity field models, assessing their signal content, and generates combined models applying a statistically robust combination method. At the GeoForschungsZentrum Potsdam the combined models are then transformed into global maps of water- and ice-mass variations and provided to the scientific user community via the GravIS web portal.
Beside the generation of consolidated and validated global maps of mass variations, COST-G also aims at providing a platform for the discussion and exchange among the analysis centers of the GRACE/GRACE-FO data. Annual meetings have been organized at the International Space Science Institut (ISSI) in Bern, since 2022 also supported by the ISSI Beijing. The extension of the COST-G combination to include also the very active analysis centers in China is imminent, and will make COST-G a truly global initiative.