The diurnal boundary layer wind oscillation above sloping terrain

Abstract

The role of thermal forcing in the diurnal oscillation of the planetary boundary layer wind above sloping terrain is investigated. Because the gravitational force vector has a component parallel to a sloping boundary, the diurnal temperature oscillation in the boundary layer provides a source of potential energy which drives a diurnal oscillation in the boundary layer wind. Thus, the viscous (Ekman) boundary layer and the thermal boundary layer are coupled. A set of three second order differential equations which approximately governs the dynamics of the boundary layer is derived. Diurnally periodic solutions are obtained, and it is shown that the thermal forcing mechanism can account for the amplitude of the nocturnal low-level jet observed over the sloping Great Plains region of the United States. However, it appears that time and height variations in the eddy viscosity and eddy heat diffusion coefficients must be included to duplicate in detail the vertical structure and phase of the observed jet.