## 2 Materials
### In Situ Measurements  
Solar radiation from space passing through the atmosphere and incident to the land surface can be classified into three categories: direct solar radiation, diffuse solar radiation, and global solar radiation. Direct solar radiation represents the solar radiation that is not scattered and reflected by atmospheric molecules or particulates but is directly incident to the land surface; diffuse solar radiation denotes the solar radiation that arrives at the land surface after scattering or reflection by atmospheric molecules or particulates; global solar radiation is defined as the total solar radiation incident to the land surface as the  
Figure 1: (**a**) Map of East Asia including Korea, where the blue rectangle indicates the study area, and (**b**) MODIS land cover from the Annual International Geosphere–Biosphere Program around the Korean Peninsula in 2019.  
aggregation of the direct component and the diffuse component. For planning photovoltaic power generation, the global solar radiation must be monitored. Therefore, we calculated the global solar radiation (hereafter referred to as SSI) from the GK2A/AMI using an ML technique. Because the Korean Peninsula has complicated geographical and meteorological properties, each region has different characteristics affected by SSI. The KMA has established 81 Automated Surface Observing System (ASOS) stations for monitoring meteorological conditions in real time. Among these AOS stations, only 48 AOS stations observe SSI in real time using pyranometers every minute (Figure 2). The KMA conducted the quality control of these AOS pyroanometers based on the criteria and guidance provided by the World Meteorological Organization (WMO) to maintain in situ SSI monitoring with high accuracy [26]. Because SSI fluctuates rapidly depending on the weather conditions, quality control is difficult. Thus, the KMA distributes the in situ SSI measurements taken every minute after preprocessing by aggregating the SSI over an hour as the operational data. The hourly SSI ground measurements from AOS stations were provided as the operational in situ SSI measurements of the KMA and were used to provide reference data for training and validating the ML model ([[https://data.kma.go.kr/cmmn/main.do](https://data.kma.go.kr/cmmn/main.do)]([https://data.kma.go.kr/cmmn/main.do](https://data.kma.go.kr/cmmn/main.do)), accessed on 17 December 2021). In order to test the model on other ground-based measurements, we used in situ measurements from the Ieodo Ocean Research Station (IORS) operated by the Korea Hydrographic and Oceanographic Agency (KHOA) and flux towers operated by the National Institute of Forest Science (NIFoS) (Figure 2). IORS was located on 125\({}^{\circ}\)10\({}^{\prime}\)56\({}^{\prime\prime}\) E and 32\({}^{\circ}\)07\({}^{\prime\prime}\)22\({}^{\prime\prime}\) N, 149 km southwest of Jeju Island, in 2003. KHOA operates IORS in real time for monitoring both marine and atmospheric environments every minute ([[http://www.khoa.go.kr/oceangrid/koods/kov/oldobservation/obs_past_search.do](http://www.khoa.go.kr/oceangrid/koods/kov/oldobservation/obs_past_search.do)]([http://www.khoa.go.kr/oceangrid/koods/kov/oldobservation/obs_past_search.do](http://www.khoa.go.kr/oceangrid/koods/kov/oldobservation/obs_past_search.do)), accessed on 28 January 2022). NIFoS operates six flux towers over Korea; the flux towers observe the environmental conditions twice every hour ([[http://know.nifos.go.kr/know/service/flux/fluxIntro.do](http://know.nifos.go.kr/know/service/flux/fluxIntro.do)]([http://know.nifos.go.kr/know/service/flux/fluxIntro.do](http://know.nifos.go.kr/know/service/flux/fluxIntro.do)), accessed on 28 January 2022). The in situ SSI measurements from IORS and the NIFoS flux towers are also quality-controlled according to the criteria of WMO. To use in situ SSI data from IORS and the flux towers for validation, we converted them to hourly SSI by aggregating the SSI over an hour.