The Atmospheric Boundary Layer

Catalog description

The atmospheric boundary layer is the layer of the atmosphere that is in frequent contact with the surface of the earth. It is the layer where life exists, and which mediates exchanges of energy, momentum, and chemicals between the earth's surface and the atmosphere. The scales of motion in the atmospheric boundary layer, because of the presence of the earth's surface, are small compared to the rest of the atmosphere. The dynamics, therefore, differ from those found in the 'free' atmosphere. This course describes the physical properties of the layer of the earth's atmosphere that is in frequent contact with the earth's surface, the atmospheric boundary layer. The course includes a descriptive overview of this layer using observations, then presents the governing equations and common simplifications used to describe the boundary layer. Conservation of mass, energy, and momentum, are covered. A core principle is the decomposition of the governing equations into a mean state and turbulent components, and the challenges introduced by this decomposition. The concepts of eddy diffusivity and closure methods are motivated by this challenge. These principles and governing equations are used to understand the typical evolution of the atmospheric boundary layer as a function of time of day. Convective and stable boundary layer conditions are contrasted. The contrasting conditions are linked to changes in the exchange of energy, momentum and water vapor at the earth's surface. The fundamentals of plume dispersion are described and tested. A simple numerical model of the atmospheric boundary layer is discussed and applied to atmospheric data. Stability conditions in the atmosphere are further explored using the equation for turbulent kinetic energy. Parameters describing the turbulence state of the surface layer and boundary layer, including the Obukhov length, friction velocity, convective velocity scale, and Richardson number, are discussed and applied to typical boundary layer conditions and observations. Similarity theory is discussed as a means of describing turbulent properties of the atmospheric boundary layer as a function of stability conditions. Monin-Obukhov similarity theory for the surface layer is applied to atmospheric observations. Additional common atmospheric boundary layer states are described, including cloud topped boundary layers, marine boundary layers, and boundary layers in heterogeneous terrain. Observational, measurement and numerical methods are presented and used in class assignments.