Impacts of urban expansion on fog types in Shanghai, China: Numerical experiments by WRF model

Abstract Fog is a hazard to transportation activities in Shanghai, China, but it is not known how this fog is influenced by urban expansion. Here we use a numerical model to run, for the first time for Shanghai, sensitivity experiments of the fog response to urban expansion, including the changes of land use and anthropogenic heat. Instead of using ‘fog days’ as a measure, we use the 29-year (1989–2017) meteorological observations of fog events at Hongqiao International Airport in central Shanghai, and classify the fog into radiation, advection, advection–radiation, and precipitation types. The results show that (1) Fog events generally decrease over these 29 years, with the decline in winter accounting for 50.2% of the total reduction. (2) Radiation fog decreases the most, but remains the most common type throughout the period. (3) Numerical sensitivity experiments show that the urban expansion in the past 29 years caused both an increase in surface air temperature and a decrease in water-vapor mixing ratio, resulting in a decrease in relative humidity and an increase in visibility for radiation fog. (4) For advection fog, the increased surface air temperature allowed an increase in water-vapor mixing ratio, but a decrease in liquid water. (5) Due to warmer near-surface air, the inversion layer weakened. Hence, urban expansion in Shanghai has reduced the amount of not only radiation fog, but also advection fog.