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J. J. Finnigan

Abstract

We describe a reliable method for distinguishing the mean, wave and turbulence fields when internal waves with changing amplitude perturb the turbulent boundary layer. By integrating the component wave and turbulence kinetic energy budgets through the turbulent layer, we show that only mechanism trasnferring energy between wave and turbulent fields is the work done by the periodic part of the turbulent stress against the wave rate of strain. When these components are π/2 out of phase. the net energy transfer is zero. Eight wave-turbulence interaction events of differing stability are analyzed and interpreted using rapid distortion theory. When density stratification is approximately steady, the phase relationship does not change from π/2 However, periodicity in the stratification changes the phase angle and leads to strong energy transfer from wave to turbulence.

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F. Einaudi
and
J. J. Finnigan

Abstract

New data obtained at the Boulder Atmospheric Observatory (BAO) has been compared with a linear stability analysis of the background atmospheric state as measured by rawinsonde ascents. Good agreement was obtained between measured wave parameters such as wavelength, period, and vector phase velocity, and the eigenvalues of the linear solution, but linear eigenvectors scaled by measured pressure at the base of the BAO tower agreed less well with measurement.

An investigation of the wave kinetic energy budget revealed that buoyant production of wave energy was a significant gain despite the strong stability (Ri ≳ 5). Further analysis of the budgets of wave heat flux and temperature variance revealed the essential role of wave-turbulence interaction in maintaining a large amplitude temperature wave and countergradient wave heat flux. A consideration of the turbulent kinetic energy budget showed many of the same features as the wave budget.

A comparison with earlier near-neutral and stable cases analyzed in comparable detail suggests that countergradient wave heat fluxes maintained by nonlinear wave-turbulence interaction and an essential transfer of kinetic energy from wave to turbulence may be generic features of such situations. A mechanism for maintenance of turbulence by waves in strongly stratified boundary layers is described, which emphasizes that the time-mean Richardson number is an irrelevant parameter at such times. In the analysis of the data, general methods are described for extracting wave signals from nonstationary turbulence records and for assessing the statistical significance of the waveform so derived.

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J. J. Finnigan
,
F. Einaudi
, and
D. Fua

Abstract

Observations have been made of a stably-stratified nighttime boundary layer perturbed by Kelvin-Helmholtz internal waves with critical levels around 600 m. Significant turbulence intensities were measured although the time-mean gradient Richardson numbers were large and positive. It is shown by constructing energy budgets of wave and turbulent components separately that there is an essential flow of kinetic energy from wave to turbulence and that the mechanics of this exchange process depend upon the nonlinear character of the wave field.

Turbulent energy budgets were followed through a wave cycle and revealed that turbulence production occurred during only one quarter of a wave period, the rest of the time being taken up by redistribution of turbulent kinetic energy (tke) among the three orthogonal components, relaxation under the effects of density stratification and dissipation. The principal path of energy dissipation is through conversion of vertical component tke to density fluctuations, which are in turn dissipated by molecular conductivity. Direct viscous dissipation of tke is negligible in comparison. This behavior is consistent with the quasi-two-dimensional character imposed on the turbulence by the strong stability and is clearly apparent in the behavior of the velocity and temperature spectra.

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F. Einaudi
,
J. J. Finnigan
, and
D. Fuà

Abstract

The analysis of an earlier work on the interaction between an internal gravity wave and the wave-induced turbulence is extended here to the case where the wave is generated by vertical wind shear. The initial system is described by a background wind and a temperature distribution such that the Richardson number is less than ¼. A gravity wave is generated by such a dynamically unstable system and grows exponentially in time. The wave modifies the Richardson number and lowers it, particularly in the neighborhood of the critical level. When the generalized Richardson number falls below an assumed critical level, turbulence is assumed to develop and is described by a “1½th order” scheme. A diffusion coefficient can then be calculated which has a mean and a fluctuating part. It is the latter which turns out to be responsible for the positive feedback between the wave and the wave-induced turbulence resulting in the wave growing at a faster rate than the one predicted by linear theory.

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F. Einaudi
,
A. J. Bedard Jr.
, and
J. J. Finnigan

Abstract

We present a climatological study of gravity waves and other coherent disturbances at the Boulder Atmospheric Observatory, during the period mid-March-mid-April 1984. The data were collected by a network of microbarographs, and by sensors on the 300 m tower. The total observational period was divided into 522 time segments of 5120 s each. Coherent and incoherent motions were identified on the basis of a cross-correlation coefficient, calculated from the microbarograph network for each time segment and frequency band analyzed, using the assumption that the atmospheric state can be described by an equivalent plane wave. Five passbands were considered in the period range 1–20 min.

The analysis indicates that the atmospheric state at these passbands displays highly coherent structure, most of the time. During the interval from 0800 to 1800 LST, coherent motions with cross-correlation coefficient larger than 0.5 are present about 25% of the time for periods between 1 and 5 min and more than 80% of the time for periods between 10 and 20 min. In the remaining hours of the day, the percentages rise to more than 40% and 95% of the time, respectively.

A relationship is illustrated between the turbulent kinetic energy measured on the tower and the amplitude of the rms pressure field at the ground for disturbances having up to 5 min periods. For longer periods, such a relationship appears to be absent, indicating that the longer the scales, the deeper the atmospheric zone important to the dynamics of the pressure fluctuations.

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M. T. Decker
,
F. Einaudi
, and
J. J. Finnigan

Abstract

During the 1978 PHOENIX experiment at the Boulder Atmospheric Observatory in Colorado, the presence of atmospheric gravity waves was detected by various independent remote sensing instruments. Fluctuations in the zenith atmospheric radiation were measured at 22.235 and 55.45 GHz in the water vapor and oxygen absorption bands and compared with corresponding fluctuations of surface pressure and the height of FM-CW radar echo returns. These fluctuations are explained, qualitatively and quantitatively, in terms of an internal gravity wave generated by wind shear above the boundary layer. The analysis shows that the oscillations at 22.235 GHz are essentially due to fluctuations of water vapor in the antenna beam while those at 55.45 GHz are due to temperature variations.

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Edward G. Patton
,
Peter P. Sullivan
,
Roger H. Shaw
,
John J. Finnigan
, and
Jeffrey C. Weil

Abstract

Large-eddy simulation of atmospheric boundary layers interacting with a coupled and resolved plant canopy reveals the influence of atmospheric stability variations from neutral to free convection on canopy turbulence. The design and implementation of a new multilevel canopy model is presented. Instantaneous fields from the simulations show that organized motions on the scale of the atmospheric boundary layer (ABL) depth bring high momentum down to canopy top, locally modulating the vertical shear of the horizontal wind. The evolution of these ABL-scale structures with increasing instability and their impact on vertical profiles of turbulence moments and integral length scales within and above the canopy are discussed. Linkages between atmospheric turbulence and biological control impact horizontal scalar source distributions. Decreasing spatial correlation between momentum and scalar fluxes with increasing instability results from ABL-scale structures spatially segregating momentum and scalar exchange at canopy top. In combination, these results suggest the need for roughness sublayer parameterizations to incorporate an additional length or time scale reflecting the influence of ABL-scale organized motions.

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Thomas Foken
,
Marc Aubinet
,
John J. Finnigan
,
Monique Y. Leclerc
,
Mattthias Mauder
, and
Kyaw Tha Paw U

No Abstract available.

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