ALBERT

Description: Global coupled climate models are in continuous need for evaluation against independent observations to reveal systematic model deficits and uncertainties. Changes in terrestrial water storage (TWS) as measured by satellite gravimetry missions GRACE and GRACE-FO provide valuable information on wetting and drying trends over the continents. Challenges arising from a comparison of observed and modelled water storage trends are related to gravity observations including non-water related variations such as, for example, glacial isostatic adjustment (GIA). Therefore, correcting secular changes in the Earth's gravity field caused by ongoing GIA is important for the monitoring of long-term changes in terrestrial water from GRACE in particular in former ice-covered regions. By utilizing a new ensemble of 56 individual realizations of GIA signals based on perturbations of mantle viscosities and ice history, we find that many of those alternative GIA corrections change the direction of GRACE-derived water storage trends, for example, from gaining mass into drying conditions, in particular in Eastern Canada. The change in the sign of the TWS trends subsequently impacts the conclusions drawn from using GRACE as observational basis for the evaluation of climate models as it influences the dis-/agreement between observed and modelled wetting/drying trends. A modified GIA correction, a combined GRACE/GRACE-FO data record extending over two decades, and a new generation of climate model experiments leads to substantially larger continental areas where wetting/drying trends currently observed by satellite missions coincide with long-term predictions obtained from climate model experiments.

Description: Today, the analysis of the Earth’s time-variable gravity field plays a key role in Earth system research. The GRACE observables provide an almost direct measurement of the mass that is redistributed at or near the surface of the planet. Yet, important questions such as closing the sea level budget from GRACE, altimetry and steric data still pose a challenge, even after 9 years of GRACE. Promising approaches have been developed that combine multisensor data and/or model output, e.g. to estimate ocean warming/cooling, calibrate hydrological models or to improve geo-centre motion estimates. However, in the view of the authors, the biggest challenge in climate applications such as sea level studies is the problem of signal separation. This problem involves separating signal and noise stemming from the measurement systems and/or background modelling, as well as separating mass flux patterns originating from different or the same compartments of the Earth system (the leakage problem in ice sheet mass balance from GRACE, sea level rise partitioning, teleconnections in the hydrological cycle, trends in continental hydrology vs. GIA). Here we will first review the state of the art in addressing the signal separation problem after 10 years of GRACE. Then, we will discuss the potential of a number of candidate schemes for future gravity missions that were investigated in the recent ESA-funded Next Generation Gravity Mission (NG2) study.