## 5 Discussions
### Spatial Distribution of SSI around Korea  
#### 5.2.1. \(\mathrm{GK2A/AMI}\) SSI  
SSI is a key factor in climatological, agricultural, and renewable energy applications. To apply SSI data in these studies and fields, it is essential to understand the spatial and temporal distribution of SSI. Thus, we produced \(\mathrm{GK2A/AMI}\)-derived CNN-based daily SSI measurements by accumulating hourly SSI for an SZA of less than 80 degrees from 1 January 2020 to 31 December 2021. Based on the daily SSI, the monthly mean daily SSI was calculated for each administrative district over Korea (Figure 12). Among the monthly mean daily SSI values over Korea, the largest value (20.451 M) m\({}^{-2}\)) and the smallest value (8.400 M) m\({}^{-2}\)) were observed in April and January, respectively (Table 3). The period of April to June showed higher mean daily SSI values compared with other periods. Under a clear sky, the SSI generally increased as the amount of ESR increased, and the ESR increased and decreased in summer and winter, respectively. However, because the Korean Peninsula has a monsoon climate, the coverage and intensity of clouds increases as summer approaches, and the incident solar radiation is reduced by clouds [50; 51; 52]. In contrast, from late spring to early summer, before the summer monsoon starts, a high SZA and clear skies are usually observed. Hence, around Korea, from July to September, the SSI was affected by intense clouds, and a low mean daily SSI was observed compared with the period of April to June. The north-south gradient of SSI over Korea reversed in July and August. The \(\mathrm{GK2A/AMI}\)-derived SSI was higher in the northern region on July; however, the SSI was higher in the southern region in August. In summer, the monsoon front around Korea is affected by air masses such as the North Pacific High over a low latitude and the Okhotsk High over a high latitude. In early summer, i.e., late June and early July, the Okhotsk High is generally stronger than the North Pacific High; thus, the monsoon front is located over the southern region of Korea [53]. However, in late summer, i.e., late July and early August, when the North Pacific High is strong, the monsoon front moves northward and is located  
Figure 11: Feature permutation of the trained CNN model, with feature permutation ranked by the increase in the RMSE of \(\mathrm{GK2A/AMI}\)-derived SSI. over the northern region of Korea [54]. Therefore, in July, the southern regions are generally affected by clouds derived from the monsoon front and show lower SSI than the northern regions; in August, the northern regions are generally affected by clouds derived from the monsoon front and show lower SSI than the southern regions. The annual mean daily SSI for 2020 and 2021 was calculated at SZA values of less than 80 degrees (Figure 13). The annual mean daily SSI over Korea in 2020 and 2021 was 14.351 M] m\({}^{-2}\) and 14.536 M] m\({}^{-2}\), respectively. Except for some provinces, most administrative districts showed a higher SSI in 2021 than in 2020. Because the Korean Peninsula has a monsoon climate, the Korean summer rainfall system known as Changma occurs and is accompanied by intense clouds and consecutive days of heavy precipitation from mid-June to early September [55]. More specifically, Korea was affected by 15 consecutive \begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline
**Month** & **Jan.** & **Feb.** & **Mar.** & **Apr.** & **May** & **Jun.** & **Jul.** & **Aug.** & **Sep.** & **Oct.** & **Nov.** & **Dec.** \\ \hline Mean (MJ m\({}^{-2}\)) & 8.400 & 12.068 & 16.051 & 20.451 & 19.212 & 20.069 & 16.872 & 14.931 & 13.542 & 13.075 & 9.665 & 8.721 \\ Min. (MJ m\({}^{-2}\)) & 6.975 & 10.944 & 15.190 & 19.359 & 18.422 & 17.771 & 14.963 & 12.784 & 12.115 & 12.097 & 8.856 & 7.592 \\ Max. (MJ m\({}^{-2}\)) & 9.115 & 12.840 & 16.514 & 21.040 & 19.800 & 20.706 & 18.355 & 16.613 & 14.941 & 14.017 & 10.549 & 10.110 \\ \hline \hline \end{tabular}
\end{table}