# SLR, GRACE and Swarm Gravity Field Determination and Combination
## 3 Results and Discussion
A reduction in the scatter can be observed after the launch of LARES in 2012. The monthly Swarm solutions exhibit even greater scatter than the SLR solutions and additionally seem to over-estimate the seasonal variation compared to GRACE. After 2014, the Swarm and the SLR results match well in trend  
Figure 11: Monthly mass estimates for entire Greenland (see masks in Figure 6). GRACE and Swarm gravity fields were truncated at degree 6 to match the resolution of SLR gravity fields. Spherical harmonic coefficient C\({}_{20}\) was excluded from the comparison. Figure 5.0 Monthly mass estimates for the Amazon basin. GRACE and Swarm gravity fields were truncated at degree 6 to match the resolution of SLR gravity fields. Spherical harmonic coefficient C\({}_{20}\) was excluded from the comparison. and in phase. During 2014, the Swarm results are impaired by high ionosphere activity, non-optimal GPS receiver settings, and in the first half of 2014 by reduced observation sampling [75]. Consecutively, mass trends were determined for the coastal regions of Greenland with strong mass loss, the region of the inland ice sheet, where a weak gain in mass is observed by GRACE, and the coast of West Antarctica. The masks used are provided in Figure 5.1 The separation of Greenland into regions of mass gain and regions of mass loss was done based on Figure 5.2 The mask of Antarctica and its glacial sub-basins is derived from Horwath and Dietrich [82]. Starting with the first monthly estimates from SLR in 1995, we fit deterministic models including bias, trend and seasonal variations within 5-year periods to integrated mass estimates of either SLR or GRACE monthly gravity field models summed up over the coastal areas (Figure 13, left) or the inland ice sheet (Figure 13, right) of Greenland, or the coast of West Antarctica (Figure 14). In case of SLR, the trend estimation is based either on the unconstrained degree 6 monthly gravity field solutions or on the regularized degree 10 solutions (compare Figures 8 and 9 for the corresponding spatial plots). For comparison, the original degree 60 GRACE gravity fields were truncated at degrees 6 or 10 and also mass trends in 5-year intervals determined. Figure 5.3 Masks defining inland and coastal regions of Greenland (**left**) and the coast of West Antarctica (**right**). The resolution of the masks is \(1^{\circ}\). Figure 5.4 Monthly mass estimates in Greenland from GRACE and SLR, and best fitting trends for 5-year periods for the region of mass loss along the coast (**left**) or the inland region of mass gain (**right**). Each 5-year period is centered around zero. The SLR trend estimates reveal good agreement with GRACE (truncated at the corresponding degree) for the time periods where a direct comparison is possible. However, due to signal leakage at the very low resolution of SLR, the absolute mass loss is drastically underestimated. Truncated at degree 6 or 10, due to the very limited spatial resolution, neither GRACE nor SLR are able to trace the inland mass gain observed by GRACE at higher resolution (Figure 13, right). Prior to the start of GRACE, no distinct mass trend within Greenland can be observed. Since the start of the highly sensitive GRACE observations, the mass loss at the coast is accelerating within the time period covered by Figure 5.5 This fact has already been reported, e.g., by [PERSON] [83] and recently by [PERSON] et al. [84], and is visible in SLR and GRACE estimates alike. In case of the degree 6 SLR solutions, the five year trend estimates are 2000-2004: \(-\)21.5 \(\mathrm{Gt}\)/year, 2005-2009: \(-\)43.6 \(\mathrm{Gt}\)/year, 2010-2014: \(-\)63.1 \(\mathrm{Gt}\)/year. As explained above, due to leakage, these values are drastically underestimated compared to the degree 60 GRACE solutions 2003-2004: \(-\)179.8 \(\mathrm{Gt}\)/year, 2005-2009: \(-\)184.4 \(\mathrm{Gt}\)/year, 2010-2014: \(-\)302.7 \(\mathrm{Gt}\)/year (no GIA corrections applied).