The Sensitivities of Nocturnal Cooling and Drainage Flow over an Inclined Topography to Meteorological, Ground Surface, and Topographic Conditions

Abstract

On the basis of results of slope and valley drainage flow models Sato and Kondo, 1988), a simple method is presented for the simultaneous evaluation of nocturnal cooling and drainage flow over slopes and in valleys under weak ambient wind conditions.Evaluations are made of the sensitivities of nocturnal cooling, drainage flow characteristics (velocity and thickness), and sensible heat transport by the drainage flow to the following conditions: meteorological conditions, thermal constant of the ground, surface aerodynamic properties and topographic features.Due to the enhancement of sensible heat transport by the drainage flow, surface cooling of an inclined topography is smaller than that of a horizontal plain. The velocity of the drainage flow is sensitive to the conditions mentioned above, but its thickness is dependent only on the surface aerodynamic properties and the topographic features. The side-slope flow plays an essential role in the energy exchange between the atmosphere and a valley.Available observations of nocturnal cooling and drainage flow support the present method.

The Daytime PBL Heating Process over Complex Terrain in Central Japan under Fair and Calm Weather Conditions

“…Kondo and Okusa (1990) developed a simple model of nocturnal cooling in a basin, which can predict the temperature profile of the cold air lake and nighttime-averaged sensible heat flux with a few topographical parameters and given atmospheric conditions. Sato (1989) investigated the quantitative relationship between nocturnal cooling and the drainage flow over an inclined topography using a simple drainage flow model Sato and Kondo, 1988). These previous studies suggest that the surface energy exchange process over a complex terrain produces thermally induced meso-scale circulations such as mountainvalley winds, and that such circulations also control the surface energy exchange process and create a remarkable difference in the energy exchange between night and day.…”

Recent Studies of Local-scale Climate in Japan

“…Concerning the simultaneous evaluation of nocturnal cooling and drainage flow over slopes under weak wind conditions, Sato (1989) pre sented a slope and valley drainage model. Using this model, he showed that the rate of surface cooling on an inclined topography was smaller than that on a horizontal plain due to the enhancement of sensible heat transport by the drainage flow.…”