Coefficients of Resistance to Cold-Air-Drainage Winds of a Grass-Covered Slope

Abstract

Cold–air–drainage (CAD) winds occur during calm and clear nights on slopes and this phenomenon is closely related to the thermal environment of people, animals and plants. The driving force of CAD winds is the surplus of density or the deficit of potential temperature produced by radiative cooling in the surface air layer, therefore the CAD wind speed is affected by the vertical profile of potential temperature. On the other hand, there are two kinds of resistance forces that are exerted on the cold–air drainage layer (CADL); one is the force exerted by the ground surface and the other is that by the surrounding atmosphere. They were assumed to be proportional to the drainage speed squared, the coefficient of proportionality being the coefficient of resistance. These coefficients of resistance of the ground surface and the surrounding atmosphere are not constant but change with CAD wind speed. This paper describes the results of analyzing the observations taken on a grass–covered slope of Mt. Kuju and estimating the coefficient.