Development of the Fog Stability Index at Incheon International Airport

Incheon International Airport (IIA) is located on a reclaimed island off the west coast of the Korean Peninsula. A significant number of flight cancelation and returning is resulted from reduced visibility due to fog. Although numerical weather prediction models have been improved, fog remains hard to be forecasted because of fog's local nature and limited vertical and horizontal resolutions of the models. As an alternative, the Fog Stability Index (FSI), an empirical method originally developed in the late 1970s, can be used. It showed more reasonable fog prediction than numerical models, according to Holtslag et al. (2010). The FSI needs four variables that represent humidity, stability and wind speed. It is supposed to be appropriate for radiation fogs. The FSI is formulated as : FSI = 2(T-T_d) + 2(T-T₈₅₀) + W₈₅₀, where T and T_d are the temperature and dew point at the surface (°C), T₈₅₀ and W₈₅₀ are the temperature (°C) and wind speed (kt) at 850 hPa, respectively. The FSI value is an indicator of fog probability. According to Holtslag et al. (2010), FSI < 31 states a high probability, 31 < FSI < 51 signifies moderate probability, FSI > 55 implies low probability. We evaluated the FSI at IIA during the period, 2011.06.01-2011.12.31 and compared with observed fog occurrence. We also did the same analysis for another place, Osan (~100 km from IIA) for comparison. The mean FSI during the fog periods was 34.6 at IIA and it was 30.1 at Osan. The average FSI for the whole period including both fog and non-fog periods was higher than 31 at IIA, indicating that the threshold of 31 suggested by Holtslag et al. (2010) was inappropriate at IIA. Because the fogs at IIA were mostly sea fogs advected from the Yellow Sea. Therefore we plan to develop a new FSI formulation and the threshold value that may incorporate the nature of fog at IIA with the analysis of more data and the result will be presented at the conference.