The official GRACE & GRACE-FO Science Data System continuously releases monthly gravity solutions from three different processing centers:
- JPL (Jet Propulsion Laboratory)
- GFZ (GeoforschungsZentrum Potsdam)
- CSR (Center for Space Research at University of Texas, Austin)
There's just one Earth gravity field, and all centers start off with essentially the same GRACE(-FO) Level-1 instrument observations - so why are there different gravity solutions and mass change data products? And - perhaps of most interest to Level-3 (gridded data product) users - which fields should be used for an analysis?
Why are there multiple different solutions?
Deriving month-to-month gravity field variations from GRACE(-FO) observations requires a complex inversion of relative ranging observations between the two formation-flying GRACE(-FO) spacecraft, in combination with precise orbit determination via GPS and various corrections for spacecraft accelerations not related to gravity changes. Many parameter choices and solution strategies are possible, and have been explored by the GRACE(-FO) science data system and other processing teams. The differences in the resulting Level-2 gravity fields have helped to better understand the characteristics as well as pros/cons of the various approaches, and ultimately led to overall data product improvements and convergence across different centers.
Additionally, different 'flavors' of data products exist. Several centers (e.g., JPL, CSR, GFZ) provide mass change maps based on traditional spherical harmonic basis functions, based largely on the computational efficiency of the parameterization, and because the satellite sensitivity is dependent on the spatial wavelength of the mass variations which is implicit in the harmonic basis function. However, unconstrained harmonic solutions from GRACE(-FO) have typically suffered from poor observability of east-west gradients, resulting in “N-S stripes” that are conventionally removed via empirical smoothing and/or “destriping” algorithms. Although quite effective, especially for larger spatial scales, the destriping also removes some real geophysical signal along with the stripes, and the size, shape, and orientation of the signals strongly affect the effectiveness of destriping. To overcome these limitations of spherical harmonics solutions, GRACE(-FO) mass concentration (mascon) solutions have been developed (e.g., Watkins et al., 2015), which allow for convenient application of a priori information derived from near-global geophysical models to prevent striping in the solutions. The resulting mass change solutions suffer less from leakage errors than harmonic solutions, and do not necessitate empirical filters to remove north-south stripes, lowering the dependence on using scale factors to gain accurate mass estimates. Ocean bottom pressure (OBP) time series derived from the mascon solutions compare better (increased correlation, reduced RMS) with in situ data than do spherical harmonic solutions. Over land, mascon solutions
typically have greater resolution for smaller spatial regions, in particular when studying secular signals.
Which solution to use?
Use Case 1: Hydrology, Ice Mass and Ocean Bottom Pressure mass change applications
For most users in these categories, the state-of-the-art GRACE(-FO) mascon gridded data products are recommended due to the advantages described above. Several Mascon solutions are available: JPL, CSR and GSFC. Please also refer to the GRACE(-FO) Level-3 User Handbook for more details and specific use case examples.
We also make Level-3 grids available derived from the classic spherical harmonic gravity solutions. These grids employ (empirical) post-processing filters to reduce errors and improve the signal-to-noise ratios. These Level-3 mass change grids (available for Land and Ocean domains) are fundamentally consistent with mascon grids at larger spatial scales, but may require more customized averaging kernels for smaller study regions (i.e., smaller than approx. 2 million square kilometers). A use case for Level-3 GRACE(-FO) 'spherical harmonic' grids is to assess differences across different data centers.
Use Case 2: gravity field analysis, sensitivity studies, solid Earth processes (e.g., earthquakes, glacial isostatic adjustment, elastic vertical deformation)
The GRACE(-FO) mascon data sets are expressed in terms of surface mass change (typically: in units of water-height-equivalent), and assume an elastic loading response of the solid Earth (i.e., corrections for GIA have already been made). To study non-elastic Earth system processes, or to rigorously assess gravity fields changes without making a priori assumptions about about their origins or error character, it is best to use the Level-2 spherical harmonic gravity field coefficients, and make any required elastic or non-elastic corrections and analyses in this domain.
Contact & Help
Please make sure to look at the Level-2 and Level-3 Handbooks for further details. If you have additional questions about GRACE-(FO) and associated data products, please please use the Feedback tool [https://grace.jpl.nasa.gov/about/feedback/] to contact JPL's GRACE(-FO) data & processing team.