An observational study of the mean and turbulent structure of the strong stable nocturnal boundary layer
Cheng, Yung, writer on meteorology
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Results from a boundary layer observation from 8 m to 200 m as measured with a minisodar over a flat site in Northeastern Illinois are discussed. The very stable nocturnal boundary layer appears to be intermittent with evidence of significant turbulence intensity, turbulent momentum flux and turbulent kinetic energy in the surface layer and above the level of wind maximum. Shear instability is shown to be responsible for the generation of turbulence even at a layer above the level of the wind maximum. A significant wind increase at high levels which was produced by an upward turbulent momentum flux was observed. The spectra of velocity components are generalized. The wavelength and spectral density for the three- dimensional wind components decrease with height up to the maximum wind level, but increase with height above this maximum. This type of peak-shift is consistent with the two-layer turbulent structure. The relationships between the nondimensionalized turbulence intensity, lnw'2/u*2, turbulent momentum flux, lnx/u.2, and the height above the ground ln(l-z/ht), are statistically significant. Two quantitative formula to express the turbulence intensity and turbulent momentum flux in terms of surface quantities are obtained through regression analysis. The present study indicates that in moderate stable nocturnal boundary layer, the properties of the atmosphere tend to be evenly distributed due to the effect of mixing process. Thus two-layer structure, which is one of the major features in very stable nocturnal boundary layer, is not observed under moderate stable conditions. The magnitude of turbulence level in different layers is comparable to each other, resulting in constant distributions of wind speed, temperature, nondimensionlized vertical velocity variance and momentum flux with increasing height.