The topic of AGIA, Antarctic Glacial Isostatic Adjustment, is to improve Antarctic ice-mass balance by assimilation of GRACE and SLI data to 3D viscoelastic earth model and joint gravity field inversion.
Ice-mass balance estimates from satellite gravimetry are highly contaminated by mass redistribution in the Earth’s mantle associated with glacial-isostatic adjustment (GIA). The principle idea of project AGIA is to include diverse constraints on past glacial changes that induce GIA, and on contemporary ice-mass balance, along with their uncertainties, into a three-dimensional viscoelastic earth model and perform a joint inversion of GRACE gravity fields to separate the components of mass change. This is done by the adjoint assimilation method. As a final result, predictions of eustatic and GIA-induced sea-level change for the GRACE observation period will be delivered.
Improved prediction of GIA-induced uplift at GPS stations of the POLENET project, Antarctica
Uplift rates (mm/a) presently available for POLENET survey stations in West Antarctica. Blue and green contours indicate the GIA predictions. Predicted elastic uplift rates (mm/a) are shown in brackets (Bevis et al. 2009, dx.doi.org/10.1029/2009GC002642)
As a contribution to the POLENET project, which is dedicated to recovering campaign-based and continuous GPS uplift measurements in West Antarctica, predictions of the GIA-induced surface deformation were made. The prediction relied on a glacial history, which was adapted in terms of the ice volume to reconcile with the present-day GRACE gravity-field observations induced by GIA over Antarctica. However, a comparison with first campaign-based GPS data of a few years revealed rather poor agreement between the GIA prediction and the GPS observation (Figure 1). It was suggested that part of this discrepancy can be attributed to the campaign-based sampling of the GPS data, which hampers a proper reduction of seasonal variations, and, presumably even more problematic, of the interannual mass variations.
Joint inversion for present-day ice-mass change and GIA in North America
Trends in the GRACE gravity fields were analyzed with regard to the Northern Hemisphere GIA signal. A first sensitivity analysis of the GRACE trends with respect to earth and load model parameters was undertaken and optimal parameters were estimated (Figure 2). These investigations will be compared with the results based on the assimilation technique, which will be implemented and applied in the next phase of the project. It is planned to summarize the results for the SPP1257 special issue in fall 2010. Also, the improved prediction of long-term trends in the gravity field will be provided to the SPP1257 partners.
Figure 2: Rate of geoid-height change over North America from GRACE (left) and model adjusted to the GRACE observation (right) including GIA and present-day ice-mass changes (contribution to SPP1257 special issue planned).
Understanding the satellite signals
- Identification of potential inconsistencies between the GIA signals constrained with GRACE and complementary data
- Temporal linear trend of the Earth’s gravity field provided as background field for improved GRACE solution processing
Steady-state and long-term processes
- Improvement of accuracy of GIA models and consequently contemporary ice-mass balance estimates from GRACE
- Delivery of constraints on the mantle viscosities beneath GIA-dominated regions
- Improved separation of long-term signals
- Delivery of eustatic and GIA-induced sea-level variations for the years 2002 to 2012
- Providing new constraints on the freshwater fluxes to the ocean
- Combined GRACE and ICESat estimate of snow density variations in high-accumulation areas of Antarctica and Greenland
- VILMA: Development and Validation of a Three-dimensional Viscoelastic Lithosphere and Mantle Model, Dr. Klemann
- ANTARCTIC-IMB: The Antarctic Ice Sheet Mass Balance from Satellite Geodesy and Modelling, Prof. Dr. R. Dietrich
- GREENLAND-ISE: Assessing the Current Evolution of the Greenland Ice Sheet, Prof. R. Dietrich
- FIGO: Fingerprints of ice melting in geodetic GRACE and ocean models, Prof. Dr. J. Kusche
- Bevis, M.; Kendrick, E.; Smalley, R.; Dalziel, I.; Caccamise, D.; Sasgen, I.; Helsen, M.; Taylor, F.W.; Zhou, H.; Brown, A.; Raleigh, D.; Willis, M.; Wilson, T.; Konfal, S. (2009), Geodetic measurements of vertical crustal velocity in West Antarctica and the implications for ice mass balance, Geochemistry Geophysics Geosystems (G3), 10, Q10005, dx.doi.org/10.1029/2009GC002642.
- Sasgen, I., Martinec, Z. and Bamber, J., (2010a), Combined GRACE and InSAR estimate of West Antarctic ice-mass loss. J. Geophys. Res. – Earth Surface, dx.doi.org/10.1029/2009JF001543, in press.