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  • Author or Editor: Tsuneo Kuwagata x
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Fujio Kimura
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Tsuneo Kuwagata

Abstract

A new concept of a thermally induced local circulation is presented by numerical and observational studies. This wind system transports a low-level air mass from a plain to a basin, passing over a mountain ridge. The characteristics of the wind system are investigated using two- and three-dimensional numerical models.

Upslope winds develop over the mountain slopes surrounding the basin until late afternoon. These winds are composed of separate individual circulations both inside and outside the basin. The atmospheric temperature in the boundary layer within the basin becomes higher than that outside, so that the surface pressure becomes lower at the bottom of the basin than that outside.

At dusk, the thermal forcing due to the surface heat flux decreases, weakening the upslope winds, and then a plain-to-basin wind develops over the mountain ridges due to the pressure difference formed in the daytime. The plain-to-basin circulation is generated when the altitude of the mountain range is almost equal to or less than the maximum mixing height developed over the plain. Higher mountain ranges act as potential barriers of the circulation.

The plain-to-basin winds are most remarkable when the horizontal scale of the basin is less than approximately 100 km and the height of the mountain range is approximately equal to the maximum mixing height. For larger horizontal scales, the velocity of the plain-to-basin wind is weaker.

Two observational examples of the plain-to-basin wind are presented. The first example is known as a part of the system that carries pollutants from the Tokyo area to the Saku Basin and develops over a mountain pass in the evening. The other wind system develops during the afternoon over a valley that connects a basin to a plain. This wind system is observed for the first time in the present work.

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Tsuneo Kuwagata
and
Fujio Kimura

Abstract

The thermally induced circulation in a deep valley during fair weather and weak synoptic wind conditions is simulated by a two-dimensional numerical model, in order to investigate the daytime planetary boundary layer evolution observed in the Ina Valley, a deep, two-dimensional valley in Japan. The numerical model can simulate the observed structure of the PBL fairly well, along with the daytime variations of the observed valley surface air temperature and surface pressure.

The numerical simulations suggest that the thermally induced cross-valley circulation creates a two-layer PBL structure. That is, a turbulent mixed layer develops due to sensible heating from the surface, reaching to heights of about 500–1000 m above the valley floor, while a quasi–mixed layer is formed above the turbulent mixed layer by the heat transport of the cross-valley circulation. The quasi–mixed layer is a new feature of the PBL. The upper limit of the quasi–mixed layer corresponds to the top of the cross-valley circulation, being somewhat higher than both sides of the mountains. The quasi–mixed layer can be clearly distinguished during the daytime in a deep valley having a depth of greater than about 1500 m. Since the quasi–mixed layer has a slightly stable stratification, the magnitude of the coefficient of vertical turbulence in this layer is much less than that in the turbulent mixed layer.

The results of the simulations reveal that the thermally induced cross-valley circulation transports heat from the mountainous regions to the central part of the valley, while water vapor is transported in the opposite manner. The potential temperature becomes horizontally uniform during the afternoon, except in the shallow layer of the upslope flow along the side slopes. On the other hand, the daytime distribution of specific humidity in the valley is rather complex, being affected not only by the cross-valley circulation, but also by the ambient wind along the direction of the cross valley. Water vapor tends to be accumulated over the mountainous regions during the daytime, resulting in the formation of cumulus clouds. Visible images observed by the NOAA satellite confirm the development of cumulus clouds over the mountainous regions in the Ina Valley during the afternoon.

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Tsuneo Kuwagata
and
Fujio Kimura

Abstract

The development process of the daytime boundary layer under fair weather and weak synoptic wind conditions was observed in the Ina Valley, a deep two-dimensional valley in Japan. The daytime boundary layer over the bottom of the valley consisted of two sublayers. The lower sublayer is a turbulent mixed layer that reached to heights of 500–1000 m above the surface. The upper sublayer is formed by the local subsidence, which is part of the thermally induced cross-valley circulation, remaining a slightly stable stratification during the daytime. The specific humidity did not become vertically uniform in the upper sublayer due to the weakness of the turbulent mixing.

The heating rate of the boundary layer was larger over the valley bottom while smaller over the mountainous areas. The observed results suggest that the thermally induced cross-valley circulation (e.g., upslope flow along the side slopes) plays a role in the heat transport from the mountainous regions to the central part of the valley. The structures of the boundary layer obtained during these observations were also consistent with previous results observed in other basins and valleys.

The cross-valley circulation prevailed until the early afternoon. However, an up-valley wind along the valley developed during the late afternoon and continued to flow until midnight. The intensity of the up-valley wind increased with increasing the thermal contrast between the coastal and inland regions in central Japan.

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