## 3 Results
### MERRA-2 GDB (SSR Trends) and Its Evaluation
#### 3.2.1 Evaluation of MERRA-2 GDB (SSR Trends)  
All Stations  
The scatterplot comparison between the SSR (anomaly) trends (W/m\({}^{2}\)) of the MERRA-2 and GEBA/BSRN stations is shown in Figure 6. On average, the MERRA-2 SSR trends correlate weakly with the corresponding ones of GEBA/BSRN stations, yielding average R values equal to 0.32/0.30. The computed RMSE values are equal to 12.97/8.37 W/m\({}^{2}\) for GEBA/BSRN, while the computed biases (MERRA-2 GDB--stations GDB), equal to \(-3.99\)/\(-3.57\) W/m\({}^{2}\) for GEBA/BSRN, show an overall underestimation of MERRA-2. The number of matched data pairs of Figure 6 lying in the first and third quartiles (positive-positive and negative-negative, i.e., with both reanalysis and stations brightening and dimming, respectively) are 17 and 29 for MERRA-GEBA (corresponding to 50 and 12.5% of the total number of points for the MERRA-GEBA scatterplot) and 6 and 5 for MERRA-BSRN (corresponding 27/23% of the total number of points for the MERRA-BSRN scatterplot). These results indicate that MERRA-2 Reanalysis yields the same sign of GDB in 63 and 50% of GEBA and BSRN stations. In order to also examine the similarity/dissimilarity between the inter-annual variation of MERRA-2 and GEBA/BSRN stations' SSR anomalies, the averaged SSR anomalies of all stations and the corresponding MERRA-2 pixels over the total period for each network, i.e., 1980-2017 for GEBA and 1992-2020 for BSRN, were computed and are given in Figure 7. The number of stations that were available in every month along with the 12-month moving averages is also shown. The overall correlation coefficient between MERRA-2 and GEBA is satisfactory (R equal to 0.8), while MERRA-2 shows systematically larger/smaller positive SSR anomalies than GEBA before/after the year 2000, leading to an average bias (MERRA-2--stations) equal to 1.25 and \(-1.93\) W/m\({}^{2}\) before and after 2000 (Figure 7a). Based on GEBA stations, a statistically significant brightening, i.e., positive \(\Delta\)(SSR), equal to 3.96 W/m\({}^{2}\) is found, versus a smaller (\(\Delta\)(SSR) = 0.32 W/m\({}^{2}\)) and not statistically significant brightening according to MERRA-2. About similar conclusions are drawn from the corresponding comparison between MERRA-2 and BSRN (Figure 7b). The two time-series of SSR anomalies over the period 1992-2020 correlate well (R equal to0.7), with positive (2.4 W/m\({}^{2}\))/negative (\(-\)0.18 W/m\({}^{2}\)) biases before/after 2000. Based on BSRN stations, a not statistically significant brightening, i.e., positive \(\Delta\)(SSR), equal to 1.59 W/m\({}^{2}\) is found, versus a statistically significant dimming (\(\Delta\)(SSR) = \(-\)2.04 W/m\({}^{2}\)) according to MERRA-2. At Station Level  
The GDB (\(\Delta\)(SSR)) for the GEBA/BSRN stations (the time period is different for each station) and the MERRA-2 pixels containing these stations are shown in Figure 8. These results enable the assessment of the GDB derived from stations and reanalysis, as well as the comparison between them. Apart from the computed trends, their statistical significance at the 95% level estimated applying the non-parametric Mann-Kendall test, is also presented in the Figure 8. The stations GDB results reveal many positive and statistically significant trends over Europe and East Asia, opposite to negative statistically significant trends over India. For GEBA stations, the positive trends vary from 0.24 to 42.09 W/m\({}^{2}\), and the negative trends from \(-\)0.28 to \(-\)42.79 W/m\({}^{2}\), whereas the corresponding ranges for BSRN stations are 0.31-15.3 and \(-\)(0.41-20.46) W/m\({}^{2}\). It should be noted that independently of their sign, overall, statistically significant trends are found for 167 out of the 232 (i.e., for 72%) GEBA stations and for 103 out of the 232 (i.e., for 44.4%) of the corresponding MERRA-2 pixels. Respectively, statistically significant are the trends for 10 out of the 22 (45.4%) BSRN stations and for 9 out of the 22 (40.9%) corresponding MERRA-2 pixels.