GIA Requirements for Future Gravity Satellite Missions – Implications After More Than 7 Years of GRACE Data

Holger Steffen(1), Riccardo Riva(2), Wouter van der Wal(2) and Bert Vermeersen(2)
(1)Dept. Geosciences, University of Calgary;
(2)DEOS, TU Delft

Analysis of data of the Gravity Recovery and Climate Experiment (GRACE) satellite mission has clearly identified the long-term mass changes in regions of Glacial Isostatic Adjustment (GIA) such as North America, Fennoscandia, Greenland and Antarctica. After more than 7 years, the determined trends are quite robust. Nevertheless, efforts are needed to isolate the GIA signal from the integral effect of processes in the atmosphere, cryosphere, hydrosphere and geosphere, e.g. hydrological contributions in North America and Fennoscandia and ice mass changes in Greenland and Antarctica have to be removed. This involves high demands on the analysis method, the filtering technique and the reduction as well as combination with other models and data, and there is much debate on which approach is the best.

In Fennoscandia, the GRACE solution fits well to results from terrestrial measurements. The uplift centre is located in the Bothnic Gulf, and the uplift area comprises the Scandinavian Peninsula and Finland. A still unknown contribution arises from hydrology, which cannot be sufficiently separated. This also applies to GIA over Northern America. Recent global hydrology models that have been used for reduction of the hydrological effect, however, have to be revised, as they already show non-negligible differences among each other.

To improve knowledge of GIA for Fennoscandia and Northern America, a prolongation of the gravity time series is definitely necessary as it will improve the separability of secular trends both in GIA and hydrology. In our view, using a follow-on mission with identical performance parameters or a new-technology mission with improvements in resolution and accuracy will allow to distinguish the GIA process from hydrological mass variations. A clear GIA signal will then give insight in Earth's 3D structure, tectonic processes and topographic changes.

Over Greenland and Antarctica, satellite gravity measurements are of crucial importance to determine the mass balance of the ice sheet. However, certainly for Antarctica this requires an accurate knowledge of ongoing GIA, which is hampered by the lack of field evidence to accurately constrain the Antarctic glacial history. In the satellite era, new constraints on GIA can be derived from the combination of gravity measurements with additional datasets, such as altimetry (e.g., ICESat and CryoSat) and GPS measurements. Moreover, on short (years to decades) timescales, GIA is a linear process, while ice sheet mass balance is characterized by periodic signals and possibly experience significant deviations form the secular trend.

Therefore, we think that the main requirements for gravity missions over Antarctica are represented by the continuity of the observations in order to isolate periodic signals and deviations from the secular trend and by the highest possible resolution in order to facilitate the combination with local measurements as those provided by altimetry and GPS.