## 3 Results
### MERRA-2 GDB (SSR Trends) and Its Evaluation
#### 3.2.1 Evaluation of MERRA-2 GDB (SSR Trends)
The analysis using GEBA stations, which provide a relatively good spatial coverage over contiguous world areas, such as Europe, southern and eastern Asia or Australia, enables us to draw conclusions about systematic agreement/disagreement. Thus, it is found that in Europe, most stations (80.8%) show qualitative agreement, although there is a tendency of more frequent disagreement in coastal European regions. Besides, an absolute agreement, namely for all GEBA stations, is found over India and Australia. On the other hand, in Japan, most stations (86.1%) exhibit disagreement, while a mixed pattern (54.5% agreement) is found over Central Asia. Note that BSRN stations enable the coverage of North America, which was not possible with GEBA stations, where a mixed pattern appears, with agreement/disagreement in 5/5 (i.e., 50/50%) stations. It is interesting to note that in the present analysis, when using the values of the 22 selected BSRN stations, an agreement in the sign of GDB trends is found at 50% of stations, whereas using the total number (70) of BSRN stations, but without applying the strict availability criteria in the computation of SSR trends, agreement is found for more, i.e., 60%, stations. On the other hand, the corresponding percentage of agreement for all (1099) GEBA stations is 63.4%, i.e., equal to the percentage of 232 stations. It is known that the MERRA-2 reanalysis products are better for years after 2000, because of the assimilation of modern state-of-the-art satellite data, such as Moderate Resolution Imaging Spectroradiometer (MODIS) and Multi-angle Imaging SpectroRadiometer (MISR), in the MERRA-2 Reanalysis System, resulting in more homogeneous time series [102]. Thus, the post 2000 period, namely 2000-2017, was selected in order to further and more thoroughly examine the quality of the MERRA-2 GDB. Additionally, in order to even more thoroughly examine the robustness of the MERRA-2 GDB, an additional analysis was undertaken to examine if MERRA-2 fluxes also provide a successful closure of the solar radiation budget. To this aim, specific GEBA/BSRN stations that provide agreement with MERRA-2 in terms of GDB were first selected. Subsequently, it was examined if for these stations the change of the MERRA-2 Outgoing Solar Radiation (OSR) at TOA agrees or not with the corresponding \(\Delta\)(OSR) from CERES-EBAF satellite data, for the period 07/2000-06/2017. Successful closure for changing Earth SW radiation budget exists for those locations for which both \(\Delta\)(SSR) and \(\Delta\)(OSR) qualitatively agree with corresponding reference surface and satellite measurements. The signs of computed MERRA-2 \(\Delta\)(SSR) and \(\Delta\)(OSR), and the corresponding signs of GEBA/BSRN \(\Delta\)(SSR) and CERES-EBAF \(\Delta\)(OSR) are compared in Figure 10. At the surface, MERRA-2 \(\Delta\)(SSR) agrees with 62.5% of GEBA (145 out of 232) stations, and 31.8% BSRN (7 out of 22) stations. Then, the sign of \(\Delta\)(OSR) between MERRA-2 and CERES was compared only for those stations providing agreement at the surface. In case of the 145 GEBA stations, qualitative agreement was found at TOA for 62.5% of them, which is encouraging. Respectively, for the 7 BSRN sites, agreement was found for 57.1% of them, which is a somewhat lower percentage than for GEBA. The obtained results for the comparison of MERRA-2 solar fluxes at the Earth's surface and TOA demonstrate that MERRA-2 cannot reasonably reproduce the changing SW radiation budget in the post 2000 period. The agreement/disagreement between the MERRA-2 and stations GDB has been also examined over specific sub-periods because, as reported in Section 1, the GDB undergoes decadal-scale changes. The selected sub-periods and the associated GDB patterns, according to the literature [11] based on station measurements, are: 1980-1985 (dimming), 1986-2000 (brightening), 2001-2010 (less brightening), and 2011-2020 (brightening & dimming).