In situ observational research of the gap wind “Hijikawa-Arashi” in Japan

The Hijikawa-arashi, a gap wind occurring in Ozu City, Ehime Prefecture, Japan, was investigated through in situ observations of horizontal and vertical directions. Analysis of surface air temperature data revealed that the inland Ozu Basin was radiatively cooled on the days on which the Hijikawa-arashi events occurred. This induced a greater difference in air temperature between the basin and the estuary of the Hijikawa River in comparison to days that no basin cooling occurred. In addition, the wind speeds of the Hijikawa-arashi observed at the estuary of the Hijikawa River were strongly proportional to the sea-level pressure difference between the inland Ozu Basin and the estuary. Theoretical calculations indicated that this pressure gradient force was sufficient for driving the strong wind of the Hijikawa-arashi. Moreover, calculation of the Froude number using vertical meteorological data revealed that the Hijikawa-arashi developed as a supercritical flow. That is, the flow was intensified at the exit of the gap, in accordance with the hydraulic theory. The vertical observations detected the inversion layer over the Hijikawa-arashi and suggested an application of the shallow water theory to this gap wind. The Hijikawa-arashi is an interesting gap flow with a strong wind, despite its small-scale geography relative to other gap winds worldwide. There is an important trigger getting higher basin pressure upstream due to the radiative cooling of the atmosphere and formation of a cold pool at the basin.