# SLR, GRACE and Swarm Gravity Field Determination and Combination
## 3 Results and Discussion
Along the coasts of Greenland and close to the coast of West Antarctica, the SLR gravity fields indicate significant mass loss over the ocean, but this has to be expected taking the effect of leakage into account (Figure 3). Figure 6: Global trends 2010–2014 in EWH (**left**) and amplitude of annual EWH variations (**right**) derived from GRACE gravity fields truncated at d/o 10. Figure 7: Global mass trends (in equivalent water height) as observed by SLR between 2010 and 2014 (**left**) and amplitude of annual variations (**right**). Top: unconstrained monthly gravity fields up to degree 6, bottom: constrained monthly gravity fields up to degree 10. In the following, time-series of mass change within certain regions are studied to demonstrate the accordance of SLR and Swarm with GRACE (both truncated at degree/order 6 corresponding to the unconstrained SLR solutions). SHC of degree 1 and C\({}_{20}\) are removed, the latter because the GRACE estimates of C\({}_{20}\) suffer from temporal aliasing [13] and accelerometer instrument noise [80] (the effect of C\({}_{20}\) alone on mass change in Greenland sums up to about 100 G in the time period from 2000 to 2018). No re-scaling to compensate for signal leakage is applied and no corrections for GIA and seasonal variations due to snow mass are applied. For a complete treatment of ice mass loss as derived from GRACE data, we refer to the literature in this field (e.g., [4]). While GRACE observations cover the period 2002-2017, earlier estimates of mass change have to be based on observations of the geodetic SLR satellites. After the end of the GRACE mission,  
Figure 8: Trends in EWH as observed by SLR in the Arctic region during 5-year periods between 1995 and 2014. **Top**: unconstrained monthly gravity fields up to degree 6, **bottom**: constrained monthly gravity fields up to degree 10. Figure 9: Trends in EWH as observed by SLR in the Antarctic region during 5-year periods between 1995 and 2014. **Top**: unconstrained monthly gravity fields up to degree 6, **bottom**: constrained monthly gravity fields up to degree 10.  
mass estimates are based either on SLR or on Swarm observations. The good agreement of SLR- and GRACE-derived seasonal variations in the Amazon basin is obvious in Figure 10, while Swarm tends to slightly overestimate the seasonal variation. The evolution of the Greenland mass is shown in Figure 11. Again, the long-term agreement of all three observation types is very good. SLR does not reveal significant mass loss in Greenland in the pre-GRACE era. Obviously, the GRACE mission was launched just in time to observe the onset of ice loss in Greenland. After acceleration in the ice mass loss, the GRACE mass estimates almost level out in the time period from 2014 to 2016. SLR tends to slightly overestimate the Greenland mass loss between 2014 and 2017, while, after 2017, even a small mass gain is visible in both the SLR and Swarm derived mass estimates. The reduction of the SLR- and Swarm-derived mass loss therefore is time-shifted by about three years compared to GRACE. [PERSON] et al. (2016) also observe diverging mass trends for Greenland but state that, compared to the independent time-series of mass change derived by the Ice-sheet Mass Balance Inter-comparison Exercise (IMBIE) for Greenland and Antarctica (2016), in the long term, these deviations of SLR average out. The inter-annual variability is larger in the monthly SLR estimates than observed by GRACE at this low spherical harmonic degree, and even larger in the monthly Swarm solutions. While the accuracy of the GRACE mass estimates at this very low resolution can considered to be constant in the time-period shown, because temporal variations in noise due to the changing satellite environment and technical problems mainly are manifest in the weakly determined high degree/order SHC, the quality of the SLR-derived estimates depends on the station network that was even more sparse in the 1990s than it is today, and on the number of satellites contributing to the monthly solutions.