## 3 Results
### MERRA-2 GDB (SSR Trends) and Its Evaluation
#### 3.2.2 The MERRA-2 GDB on Global, Hemispherical and Regional Scales
& \(-\)6.16 \(\pm\) 0.38 & \(-\)5.58 \(\pm\) 0.47 & \(-\)0.33 \(\pm\) 0.46 & \(-\)0.08 \(\pm\) 0.39 \\ \hline Land-Globe & \(-\)2.58 \(\pm\) 0.25 & 0.53 \(\pm\) 0.35 & \(-\)2.45 \(\pm\) 0.38 & 0.25 \(\pm\) 0.39 \\ \hline Land-N.H. & \(-\)2.07 \(\pm\) 0.30 & 0.60 \(\pm\) 0.43 & \(-\)2.44 \(\pm\) 0.48 & \(-\)0.59 \(\pm\) 0.41 \\ \hline Land-S.H. & \(-\)2.98 \(Given that GDB is a regional phenomenon (Figure 11), and that the 40-year GDB results in Figures 11 and 12 are based on linear trending analysis, an additional investigation was performed on a regional scale, in order to reveal decadal scale GDB features. To this aim, three specific regions of the Globe were selected, which have a satisfactory coverage by stations and exhibit homogeneous GDB patterns (Figure 11). These regions, shown in rectangles in Figure 11, are the land areas of Europe, East Asia and India. Three time series of deseasonalized SSR anomalies (for the period January 1980-December 2019) were produced for the selected world regions, one for the GEBA stations lying in each region (125 stations in Europe, 36 in E. Asia and 8 in India) that satisfy the 2 criteria of slope estimation as mentioned in Section 2.2, a second one of the respective MERRA-2 pixels (including the stations) of the region, and a third one of all MERRA-2 pixels of the region and are shown in Figure 13. Along with the time series, the corresponding applied linear regression and computed trends are also given. This analysis allows testing the ability of MERRA-2 to reproduce the inter-annual variability and trend of SSR at the locations of GEBA stations within each region, as well as examining whether the station (or corresponding pixel areas) based trends of SSR are representative for the entire areas of Europe, India and East Asia. In general, it is found that MERRA-2 satisfactorily reproduces the month-to-month and year-to-year variability of SSR (correlation coefficients equal to 0.88, 0.77 and 0.66, for Europe, East Asia and India, respectively), although GEBA measurements exhibit a slightly larger variability, especially for Europe and East Asia. For India, it is found that GEBA measurements indicate distinct negative SSR anomalies during the decade 2010-2019, against quite smaller, still negative, anomalies for MERRA-2. On the other hand, large positive GEBA SSR anomalies are found for East Asia during the decade 2010-2019, against smaller positive anomalies for MERRA-2. As a result of these discrepancies, a stronger decreasing trend of SSR, i.e., a stronger dimming, is found for India according to GEBA (\(-\)33.81 W/m\({}^{2}\)) than corresponding MERRA-2 pixels (\(-\)17.97 W/m\({}^{2}\)), as well as a larger increasing GEBA-based trend of SSR, i.e., stronger brightening, for East Asia (+5.31 W/m\({}^{2}\)) than corresponding MERRA-2 pixels (+0.37 W/m\({}^{2}\)). On the other hand, the trends for Europe are about equal (+6.86 and +4.03 W/m\({}^{2}\) according to GEBA and MERRA-2 pixels, respectively), indicating brightening in both cases. With respect to the representativeness of the station (or corresponding MERRA-2 pixels) based SSR trends for the entire world regions, it is found that, indeed, they are, since there is a similarity between the sign of their trends, both showing the same phases of GDB. In addition, a great similarity is also found in terms of magnitude of GDB's for East Asia and India, with slight differences for Europe, where there is a slightly smaller trend for the entire region (+1.78 W/m\({}^{2}\)) than for the MERRA-2 pixels (+4.03 W/m\({}^{2}\)) and GEBA stations (+6.86 W/m\({}^{2}\)). Our results prove the ability of MERRA-2 to qualitatively reproduce the GEBA stations-based GDB (dimming or brightening) over the continental areas of Europe, East Asia and India. Moreover, they also indicate that the GDB based on measurements of GEBA stations in East Asia and India can be considered as representative for the entire areas to a high degree, but this is not entirely valid for Europe.