## 5 Discussion  
The first Korean geostationary satellite COMS has been successfully operating since 2010. It has been operating stably, with relatively high accuracy of measured radiance to derive SST fields. Thus, the proper selection of SST retrieval equations and derivation of the SST coefficients are of importance for diverse atmospheric and oceanic applications. Concerning COMS/MI SST, this study raised the following questions: What kind of SST retrieval equations are the most appropriate for COMS data? Are there any problems associated with the use of SST formulations such as MCSST or NLSST over the COMS region? Are there any characteristic errors or regional biases in the estimation of SST? If there are significant regional biases, can be geostationary satellite SST algorithm stably used for regional seas, especially in the North Asian regions with low temperatures? To obtain potential explanations regarding such questions, we conducted the present study. Previously well-known SST algorithms showed relatively small RMSEs and bias errors [(18; 20; 49; 50; 51)]. However, some tendencies, such as overestimation of SSTs at low temperature  
Figure 13: Example of daily SST fields (\({}^{\circ}\)C) from (**a**) COMS/MI-derived SST and (**b**) OSTIA on March 26, 2015. (**c,d**) are SSTs in the seas around the Korean Peninsula, enlarged images from (**a,b**), respectively. ranges were detected from the SST equations regardless variations between day and night. Since most of the matchup points are concentrated in low-latitude regions, biases can be amplified, because the characteristics of low temperature are not properly considered in the retrieval procedure of SST coefficients. This problem can be an important issue in the derivation of regional SST fields in low-temperature areas in particular. Another issue concerns the errors of SST with respect to SZA. In order to remove the errors related to the SZA, additional terms for SZA have long been proposed in several algorithms [20; 51]. However, the geostationary SST provided by the previous algorithms showed that the overestimation phenomenon appeared even though the formulation contained the SZA correction term in the SST formulations. The error characteristics of the algorithms were related to the problem of COMS/MI-derived SST at low temperatures. Therefore, we focused on the utilization of full-disk data and proposed a SST retrieval formula without the single term of SZA correction to restrain peculiar biases at a range of low temperature. As can be seen from the results of this study, when calculating the SST using the radiance data observed from the COMS/MI, the appropriate algorithm should be decided according to the application purposes. If more accurate SST estimations from COMS/MI data are required at low temperatures, it is recommended that an equation be used that does not include the SZA correction term by removing the edges of the full-disk of COMS/MI. To understand the pattern of COMS SST for the whole area of the full-disk, one may use the calculation formula including the SZA correction term, but it is important to understand the characteristics of SST in the seas around the Korean Peninsula. Such characteristic errors of under- or over- estimation are likely to be caused by a lack of in situ measurements in the matchup database. Therefore, it is necessary to observe SSTs at high-latitude regions with low water temperatures in winter. The geostationary satellite-based SST data are of importance for utilization in the local area at relatively high-latitude regions in the marginal seas around the Korea, as well as for large ocean-scale studies. SST estimates from geostationary satellite measurements over full-disk region may have regional biases, especially near the edge of the full-disk region and at high-latitude regions with limited sampling of low temperatures. Therefore, this study pointed out that the COMS/MI SSTs should be regionally optimized for more reliable scientific research through the process of in-depth investigation of the SST errors.