Numerical simulations of upslope fog observed at Beppu Bay in Oita Prefecture, Japan

Abstract

The present study focussed on the formation of upslope fog. This fog causes chronic expressway closures near Beppu Bay (BB) in Oita Prefecture, Japan. A dense fog event, occurring on 3 and 4 May 2020, resulting from the passage of an extratropical cyclone through the southern Japanese islands, was simulated using the Weather Research and Forecasting (WRF) numerical model. The simulation results indicated that warm and humid air with a lifting condensation level less than 20 m flowed into a plain from BB. Simulated fog and poor visibility were more severe on mountain slope adjacent to BB than on the surroundings. Here, the westward horizontal flux of water vapour by the wind flow was prolonged at lower heights. Analyses of the simulated elements verified that the upslope fog was induced by moist adiabatic cooling of the air lifted by the upslope winds. At the mature stage of upslope fog, a simulated local vertical circulation generated in conjunction with the upper synoptic flow extended the dense fog into the upper atmosphere over BB. During the decaying stage of upslope fog on mountain, a fog covered on the entire BB. This was associated with weakening winds at lower heights. Sensitivity experiments for model boundary conditions supported the importance of mountain steepness and the rectangular shape of BB for the generation of upslope fog. Finally, an application possibility of dense fog forecasts was discussed by using a simulated difference in the equivalent potential temperature between the base of the mountain and Beppuwan expressway interchange located midslope.

Editorial: Atmospheric processes and applications in urban, coastal and mountainous terrain

“…Finally, a great challenge for meteorological models is represented by the forecasting of fog, fundamental for applications related to road management and security. Ohashi and Suido (2021) present an application of the WRF model to reproduce upslope fog in a coastal region in the Oita Prefecture (Japan), proposing a practical technique for fog prediction.…”

Development of statistical and machine learning models to predict the occurrence of radiation fog in Japan

“…In Japan, traffic accidents often occur due to visibility problems caused by fog (e.g., Yamamoto, 2000; Ohashi & Suido, 2021). Notably, basins in Japan have been found to have more fog days than plains; that is, basins are more prone to radiation fog than plains (Akimoto & Kusaka, 2010).…”