Transport in Stable Boundary Layers and Thin Drainage Flows

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The purpose of this project is to improve our understanding of the exchange of heat, moisture, momentum, and contaminants between the earth's surface and the lower atmosphere when the atmospheric layer near the ground is extremely stable. In these conditions, the variation of the wind with height and time and the vertical transport of heat and other material by the atmosphere do not conform to the theories appropriate for ordinary daytime conditions. During the day, surface heating causes the air next to the ground to become unstable, leading to convection, well-developed turbulence, predictable profiles of temperature, wind velocity, and humidity, and therefore predictable vertical fluxes of heat and water vapor. But at night, the surface is cold, the air stable, and the turbulence often intermittent and caused by vertical wind shear associated with terrain-induced drainage flows. The recent CASES-99 field program (Kansas, October 1999) showed that drainage winds often form over ground that is only gently sloping, and may be confined to only the lowest few meters of the atmosphere. The vertical flux of heat and other properties is determined by the shear and turbulence in the drainage flow, but these characteristics are not well known because measurements are ordinarily too widely separated in the vertical to give a good representation of the true structure. Two approaches are pursued in this project. One is to analyze existing data from past field programs (especially CASES-99). The analysis focuses on understanding the intermittent bursts of turbulence that account for much of the eddy transport of heat and momentum in stable conditions. The CASES data make it possible to distinguish between the turbulence generated locally by shear at the top of the drainage flow and that which is created elsewhere and advected over the observing site. The second approach is to conduct a small field program focussed on thin drainage flows employing new instruments to enable measurements with high resolution in space and time. These instruments include hot-film anemometers, sonic anemometers with short path lengths and miniaturized transducers, and laser-Doppler anemometry. The result of the research will be an improved formulation of transport processes in the very stable boundary layer for use in large-scale numerical models for weather forecasting and climate prediction.