Fog can have severe impacts on human life (e.g., accidents in transportation systems) because it causes low visibility. Areas prone to fog near the sea or a river require accurate fog forecasts. In this study, we analyzed the characteristics of fog occurrence around Gumi Reservoir, a part of the inland Nakdong River. We also simulated and predicted visibility using the Weather Research and Forecasting (WRF) model. Visibility was estimated using several point measurements and several different methods: Stoelinga and Warner (SW99), Rapid Update Cycle (RUC), the Forecast System Laboratory (FSL), Decision Tree (DT), revised visibility (RVIS) and Gultepe_06 methods. The DT method showed a pattern similar to the observed fog occurrences. In this study, the Gulteppe_06 method produced the best performance under the lowest visibility situation, although there is some discrepancy in microphysics' simulated results, followed by DT and RUC.

The detailed characteristics of fog over South Korea were analyzed using the three-years quality controlled (QC) 237 visibility meter data operated by Korea Meteorological Administration. The fog (dense fog: DFog) frequency varies greatly with season and geographic location. The fog frequency at inland is highest in autumn, but at the West Coast in spring and summer. Fog occurs frequently from spring to autumn in the mountainous regions. Unlike the fog, the DFog is mostly prevalent in summer at land, mountain, and coastal regions. The large coefficients of variation of fog and DFog at the three regions and four seasons indicate that the locality of fog over South Korea is very high. The formation and dissipation (FaD) of fog show strong diurnal variations irrespective of geographic location and season, strongest at inland and weakest at sea. Fog usually occurs from night to sunrise and dissipates from early morning to late morning. The maximum FaD time of fog show seasonal variation with the seasonal change in solar elevation angle. The frequency of fog is inversely proportional to the duration time, mostly less than 3 hours regardless of season and geographic location. Also, the duration of DFog is mostly within 1-3 hours.