## 3 Results
### Evaluation of MERRA-2 SSR
#### 3.1.1 All Stations
According to Table 1, R and relative percent bias do not show significant seasonal changes, apart from slightly (by up to 0.1) lower R values in boreal spring and summer, especially for GEBA, and better in winter and autumn. This is not valid for R computed with deseasonalized SSR anomalies, which is best in spring (and autumn for BSRN). On the other hand, BIAS and RMSE have a remarkable seasonal range, up to about 18/8.5 W/m\({}^{2}\) for bias and GEBA/BSRN, respectively, and up to about 17/8 W/m\({}^{2}\) for RMSE and GEBA/BSRN, respectively, being largest in summer due to the largest absolute amounts of SSR. Similar findings for RMSE and bias were found by [38], who used GEBA and CMA station SSR measurements, and by [39], who used GEBA and BSRN station measurements, but for shorter time periods. Besides, the relative RMSE doesn't exhibit significant seasonal changes (they are less than 7/1.9 W/m\({}^{2}\) for GEBA/BSRN respectively). The obtained metrics are generally better for the comparison with BSRN than GEBA station measurements, which is encouraging, given the higher quality of this network [88]. In order to more effectively assess the seasonal performance of MERRA-2 SSR, and given the systematic 6-month shift of extreme seasons between the two hemispheres, the seasonal variation of main statistical metrics, namely R, RMSE (in absolute  
\begin{table}
\begin{tabular}{l||c c c c c} \hline \multicolumn{1}{c||}{**MERRA-2-GEBA**} & **Annual** & **DJE** & **MAM** & **JIA** & **SON** \\ \hline \hline N & 197,302 & 49,067 & 49,501 & 49,357 & 49,377 \\ \hline \hline slope & 1,04 (0.47) & 1,05 (0.46) & 0.92 (0.51) & 0.85 (0.44) & 1,02 (0.46) \\ \hline BIAS & 24.25 & 15.49 & 26.37 & 33.52 & 21.58 \\ \hline BIAS(\(\Sigma\)) & 15.3 & 15.5 & 14.1 & 15.6 & 16.5 \\ \hline RMSE & 36.07 & 27.62 & 38.65 & 44.58 & 30.95 \\ \hline RRMSE(\(\gamma\)) & 22.8 & 27.7 & 20.7 & 20.7 & 23.7 \\ \hline R & 0.95 (0.61) & 0.96 (0.56) & 0.88 (0.65) & 0.85 (0.60) & 0.95 (0.59) \\ \hline mean,GEBA & 158.36 & 99.78 & 168.39 & 215.50 & 130.85 \\ \hline \hline mean,MERRA-2 & 182.61 & 115.27 & 213.26 & 249.03 & 152.42 \\ \hline id & 1099 & 1099 & 1099 & 1099 & 1099 \\ \hline MERRA-2-BSRN & **Annual** & **DJE** & **MAM** & **JIA** & **SON** \\ \hline N & 112,299 & 2807 & 2826 & 2500 & 2826 \\ \hline slope & 0.99 (0.50) & 0.96 (0.51) & 1.00 (0.50) & 0.99 (0.46) & 0.98 (0.53) \\ \hline BIAS & 10.31 & 5.84 & 10.46 & 14.45 & 10.51 \\ \hline BIAS(\(\Sigma\)) & 5.9 & 4.1 & 5.5 & 6.8 & 6.8 \\ \hline RMSE & 26.12 & 22.32 & 27.15 & 29.88 & 24.51 \\ \hline RRMSE(\(\Sigma\)) & 15.1 & 15.9 & 14.4 & 14.1 & 15.9 \\ \hline R & 0.97 (0.62) & 0.98 (0.61) & 0.95 (0.63) & 0.96 and % terms) and bias (in absolute and % terms) were also computed separately for the two hemispheres and they are given in Figure 4. The results of this figure were obtained using SSR fluxes except for R, which is computed using deseasonalized SSR anomalies (the corresponding R using absolute SSR fluxes is given in Figure S3). As shown in Figure 4, there are high bias and RMSE values in summer in both hemispheres. Nevertheless, it must be taken into account that these values correspond to the highest summer values of SSR. In contrast, the highest relative bias (17.9%) and RRMSE (30.9%) values are in winter in both hemispheres (Figure 4c,e). Similarly, the R values computed using deseasonalized SSR anomalies are lowest in winter (0.52). These results indicate that the MERRA-2 SSR perform worst in the cold period of the year. The different performance of the reanalysis SSR products with seasons may be due to the seasonally varying deficiencies in the reanalysis cloud coverage, aerosol optical depth or water vapor [39; 47]. Figure 3: Scatterplot comparison between MERRA-2 Reanalysis and GEBA (**i-a**), and BSRN (**ii-a**) SSR fluxes and between MERRA-2 and GEBA (**i-b**) and BSRN (**ii-b**) deseasonalized SSR anomalies.