冷気流と冷気湖の動的相互作用に関する実測的研究

Abstract

Field investigations of the dynamic interaction between nocturnal drainage winds and a cold-air lake were conducted in a hilly area near Fukuoka City, Japan. It was found that nocturnal drainage winds can be classified into two types according to where they occur, that is, below or above the surface of the cold-air lake. The nocturnal drainage wind which contributes to the development of the cold-air lake over a basin and flows down into the surface layer of the lake is one type of nocturnal drainage wind (NDW I). This type of flow, in general, has a thickness of more than 1O m, and a speed which oscillates with periods ranging from 30 to 60 minutes, though the cause of the periodicity has yet to be determined. Within the cold-air lake, that is, on the hillside buried in the lake, another type of drainage wind (NDW II) can be found, the thickness of which is less than about 10 m. Winds of this type also oscillate, with periods of 20 to 30 minutes and of about 40 minutes. These periods agree fairly well with those of internal gravity waves induced in the cold-air lake, suggesting that the periodic behavior of the NDW II type is associated with the wave-like movement of cold air in the lake. Although cold air is produced over the entire ground surface, nocturnal drainage winds, which are the flow of cold air down the sloping surfaces, exhibit different characteristics according to whether the slope is located above or beneath the surface of the cold-air lake.