# Development of Fog Detection Algorithm Using GK2A/AMI and Ground Data
## 4 Discussion
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Previous Study** & **Satellite Data** & **Data Set** & **Data for Validation** & **Average Results** \\ \hline \hline \multirow{2}{*}{[PERSON] [37]} & \multirow{2}{*}{GOES-13} & \multirow{2}{*}{For 2012} & \multirow{2}{*}{71 ASOS \({}^{1}\)/AWOS \({}^{2}\)} & POD = 0.41 \\  & & & & FAR = 0.75 \\ \hline \multirow{2}{*}{[PERSON] et al. [2]} & \multirow{2}{*}{COMS} & \multirow{2}{*}{5 fog cases in 2015} & \multirow{2}{*}{235 visibility meters} & POD = 0.83 \\  & & & & FAR = 0.54 \\ \hline \multirow{2}{*}{[PERSON] et al. [38]} & \multirow{2}{*}{SEVIRI \({}^{3}\)} & \multirow{2}{*}{\begin{tabular}{c} 51 scenes for training \\ and 4439 pixels for \\ validation (2016.10–2017.04) \\ \end{tabular} } & \multirow{2}{*}{18 METAR \({}^{4}\)} & POD = 0.69 \\  & & & & in Italy \\ \hline \multirow{2}{*}{[PERSON] et al. [18]} & \multirow{2}{*}{SEVIRI} & 342,328 scenes (2006–2015) & \multirow{2}{*}{273 METAR and 11 SYNOP \({}^{5}\)} & POD = 0.61 \\  & & & & FAR = 0.41 \\ \hline \multirow{2}{*}{[PERSON] et al. [39]} & \multirow{2}{*}{AHI, ABI, and IASI \({}^{6}\)} & \multirow{2}{*}{400 scenes for validation (2010–2015)} & \multirow{2}{*}{SYNOP} & POD = 0.71 \\  & & & & FAR = 0.34 \\ \hline \multirow{2}{*}{[PERSON] et al. [3]} & \multirow{2}{*}{Himawari-8} & \multirow{2}{*}{\
\begin{tabular}{c} 54 scenes for both training \\ and validation in 2015 \\ \end{tabular} } & \multirow{2}{*}{Visibility meter} & POD = 0.75 \\  & & & FAR = 0.44 \\ \hline \multirow{2}{*}{[PERSON] et al. [20]} & \multirow{2}{*}{Himawari-8} & \multirow{2}{*}{
\begin{tabular}{c} 8 fog cases for both training \\ and validation \\ \end{tabular} } & \multirow{2}{*}{Visibility meter} & POD = 0.64 \\  & & & FAR = 0.56 \\ \hline \hline \end{tabular}  
* AOS: Automated Surface Observing System. \({}^{2}\) AWOS: Automated Weather Observing System. \({}^{3}\) SEVIRI: Spinning Enhanced Visible and Infrared Imager. \({}^{4}\) METAR: Meteorological Terminal Aviation Routine Weather Report. \({}^{5}\) SYNOP: surface synoptic observations. \({}^{6}\) IASI: Infrared Atmospheric Sounding Interferometer.  
\end{table} 
Table 7: Summary of the validation results using ground observation data of previous studies during the past five years (2014–2019). Figure 13: _Cont._  
The detection skills of GK2A_FDA over the sea were not quantitatively evaluated in this study because of the limited validation data. In many studies, Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) data have been used for fog detection validation over sea [15; 16; 17; 20; 40]. Figure 14 shows a validation result for the sea fog detection of GK2A_FDA using Vertical Feature Mask (VFM) data produced from CALIPSO. The VFM data has an issue of incorrectly defining the sea fog layer as the surface (e.g., 37\({}^{\circ}\)N-40\({}^{\circ}\)N). As suggested by [PERSON] et al. [40], if the backscattering of the layer defined as the surface is greater than 0.03 km\({}^{-1}\)sr\({}^{-1}\), it is redefined as the sea fog layer. As the duration of sea fog is long, unlike land fog, the GK2A data were validated \(\pm\) 20 min based on the passing time of CALIPSO. The VFM data showed that GK2A_FDA effectively detected the sea fog (POD: 0.80, FAR: 0.20, KSS: 0.60, and Bias: 1.00). When considering advection fogs that frequently occur in the western and southern seas to the Korean Peninsula, quantitative validation of the fog detection algorithm in the sea should be performed using the CALIPSO data and others [15; 32; 41]. In addition, for stable fog detection in the long term, it is necessary to improve the level of fog detection on daytime land and for weak or local fog. In particular, the quality of numerical models and background data should be improved for the accurate separation of fog from low to mid clouds. Figure 14: _Cont._  
Figure 13: Sample images of spatio-temporal discontinuities obtained from (**a**) 06:10 KST to (**b**) 06:20 KST on 14 July 2019 and at (**c**) 07:10 KST on 30 September 2019). Bright sky blue lines indicate the SZA.