Thermally Induced Wind Passing from Plain to Basin over a Mountain Range

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.