Hydraulic jump in the atmosphere visualized by fog

Abstract

This study deals with the mechanism of a local wind called "Hijikawa Arashi" which is a down slope wind observed along the Hijikawa river near Nagahama in Ehime Prefecture. This river is connected to an inland basin (Ohsu basin) where a cold air mass is accu nulated during nighttime by radiation cooling and overflows to the sea (Iyo-nada of Seto Inland Sea) through a valley along the Hijikawa river. Hijikawa Arashi gives a considerable influence to daily life of the people in the area, because the wind speed sometimes exceeds 10m/s and fog associated with the wind reduces the visibility significantly. The mechanism of the strong wind is not known yet, and remains as an interesting problem to meteorologists.
In this study, Hijikawa Arashi is studied by means of a shallow water theory, laboratory experiment and field observation. It is shown that the strong wind is caused by a hydraulic jump of the down slope wind which is produced by the effect of topography of the valley. The theory and the laboratory experiments predict that a steady hydraulic jump is produced in the downstream side of the narrowest portion of the valley. The result was confirmed by a videotape record of the fog which visualized the hydraulic jump of the Hijikawa Arashi.