Daytime Boundary Layer Evolution in a Deep Valley. Part I: Observations in the Ina Valley

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  • a Tohoku National Agricultural Experiment Station, Morioka, Japan
  • | b Geophysical Institute, Tohoku University, Sendai, Japan
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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.

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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