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Atmospheric Boundary Layer FlowsTheir Structure and Measurement$
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J. C. Kaimal and J. J. Finnigan

Print publication date: 1994

Print ISBN-13: 9780195062397

Published to Oxford Scholarship Online: November 2020

DOI: 10.1093/oso/9780195062397.001.0001

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Flow Over Flat Uniform Terrain

Flow Over Flat Uniform Terrain

1 (p.3) Flow Over Flat Uniform Terrain
Atmospheric Boundary Layer Flows

J. C. Kaimal

J. J. Finnigan

Oxford University Press

We start with the simplest of boundary layers, that over an infinite flat surface. Here we can assume the flow to be horizontally homogeneous. Its statistical properties are independent of horizontal position; they vary only with height and time. This assumption of horizontal homogeneity is essential in a first approach to understanding a process already complicated by such factors as the earth's rotation, diurnal and spatial variations in surface heating, changing weather conditions, and the coexistence of convective and shear-generated turbulence. It allows us to ignore partial derivatives of mean quantities along the horizontal axes (the advection terms) in the governing equations. Only ocean surfaces come close to the idealized infinite surface. Over land we settle for surfaces that are locally homogeneous, flat plains with short uniform vegetation, where the advection terms are small enough to be negligible. If, in addition to horizontal homogeneity, we can assume stationarity, that the statistical properties of the flow do not change with time, the time derivatives in the governing equations vanish as well. This condition cannot be realized in its strict sense because of the long-term variabilities in the atmosphere. But for most applications we can treat the process as a sequence of steady states. The major simplification it permits is the introduction of time averages that represent the properties of the process and not those of the averaging time. These two conditions clear the way for us to apply fluid dynamical theories and empirical laws developed from wind tunnel studies to the atmosphere's boundary layer. We can see why micrometeorologists in the 1950s and 1960s scoured the countryside for flat uniform sites. The experiments over the plains of Nebraska, Kansas, and Minnesota (USA), Kerang and Hay (Australia), and Tsimliansk (USSR) gave us the first inklings of universal behavior in boundary layer turbulence. Our concept of the atmospheric boundary layer (ABL) and its vertical extent has changed significantly over the last few decades.

Keywords:   Absolute temperature, Buoyancy parameter, Capping inversion, Diffusivity, Entrainment, Friction velocity, Gravity waves, Heat flux budget, Imbalance term, Lapse rate

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