Towards a better understanding of fog at Perth Airport

• A comprehensive climatology of fog events at Perth Airport was developed. • On average Perth Airport gets fog events per year, mainly in the cool season. • Fog often forms when the moist environmental westerlies and surface flow decouples. A comprehensive climatology of fog events at Perth Airport was developed with the aim of improving the forecasting of fog through an enhanced understanding of its nature. From 2002 to 2019 (inclusive) the airport experienced 8.9 ± 3.0 fog events per year with 96% of all events occurring in the cool season (from April to October inclusive). The fog events were ordered into different fog types, following a hierarchical classification method. Radiation fog were found to be the most prevalent in this region. A k-means clustering method was applied to 2300 UTC (0700 LST) observations at Perth Airport to divide the dataset into 5 different weather regimes. The majority of fog events occurred in a post-frontal/trough system with a high-pressure system to the west, bringing in moisture from the Indian Ocean. Perth Airport is located on a coastal plain with an escarpment 10 km to the east of the airport, roughly parallel to the coast and rising to about 300 m. In the 2002–2019 cool season period, 41% of fog events had northerly to easterly 10 m winds at 2300 UTC (0700 LST) while the 925 hPa winds were southerly to westerly. This compares with 15% of days with north to east winds at 10 m across all cool season days in the same period. These results support the current theory that fog often forms when nocturnal cooling on the coastal plain leads to increased stability in the near surface layer and a decoupling between the moist environmental westerlies and the surface flow at Perth airport. This allows the development of local circulations forced by the local topography.