Velocity fluctuations near an interface between a turbulent region and a stably stratified layer

Carruthers, David; Hunt, J. C. R.

doi:10.1017/s002211208600318x

Cited by 89 publications

(81 citation statements)

References 29 publications

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“…The interaction between the stable layer in the upper canopy and turbulent eddies impinging on it from above is similar in some ways to that identified by Carruthers and Hunt (1986) to explain gravity wave generation in the capping inversion above the convective boundary layer (CBL). Their hypothesis is that buoyant eddies impinging on the stable layer from below can supply energy to trapped wave modes within that layer.…”

Section: Toward a Model Of The Resonant Wave Eventsmentioning

confidence: 55%

Daytime turbulent exchange between the Amazon forest and the atmosphere

Fitzjarrald¹,

Moore²,

Cabral³

et al. 1990

J. Geophys. Res.

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Detailed observations of turbulence just above and below the crown of the Amazon rain forest during the wet season are presented. The forest canopy is shown to remove high‐frequency turbulent fluctuations while passing lower frequencies. Filter characteristics of turbulent transfer into the Amazon rain forest canopy are quantified. In spite of the ubiquitous presence of clouds and frequent rain during this season, the average horizontal wind speed spectrum and the relationship between the horizontal wind speed and its standard deviation are well described by dry convective boundary layer similarity hypotheses originally found to apply in flat terrain. Diurnal changes in the sign of the vertical velocity skewness observed above and inside the canopy are shown to be plausibly explained by considering the skewness budget. Simple empirical formulas that relate observed turbulent heat fluxes to horizontal wind speed and variance are presented. Changes in the amount of turbulent coupling between the forest and the boundary layer associated with deep convective clouds are presented in three case studies. Even small raining clouds are capable of evacuating the canopy of substances normally trapped by persistent static stability near the forest floor. Recovery from these events can take more than an hour, even during midday.

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Section: Toward a Model Of The Resonant Wave Eventsmentioning

confidence: 55%

Daytime turbulent exchange between the Amazon forest and the atmosphere

Fitzjarrald¹,

Moore²,

Cabral³

et al. 1990

J. Geophys. Res.

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“…In conclusion, the recent work of Carruthers & Hunt (1986) is noted. That work describes an analysis of the mixing a t the surface of a turbulent layer bounded above by a linearly stratified fluid, a configuration they describe as a class (i) flow and with little similarity to the problem studied here.…”

Section: W~/ [ C T~] mentioning

confidence: 58%

Turbulent mixing at a shear-free density interface

Hannoun¹,

List

1988

J. Fluid Mech.

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The interaction of a sharp density interface with oscillating-grid-induced shear-free turbulence was experimentally investigated. A linear photodiode array was used in conjunction with laser-induced fluorescence to measure the concentration of dye that was initially only in the less dense layer. A laser-Doppler velocimeter was used to measure the vertical velocity in and above the density interface at a point where the dye concentration was also measured. Potential refractive-index-fluctuation problems were avoided using solutes that provided a homogeneous optical environment across the density interface. Internal wave spectra, amplitudes and velocities, as well as the vertical mass flux were measured. The results indicate that mixing occurs in intermittent bursts and that the gradient (local) Richardson number remains constant for a certain range of the overall Richardson number R3, defined in terms of an integral lengthscale, buoyancy jump and turbulence intensity. The spectra of the internal waves decay as f -' a t frequencies below the maximum Brunt-Vaisala frequency. These findings give support to a model for oceanic mixing proposed by Phillips (1977) in which the internal waves are limited in their spectral density by sporadic local instabilities and breakdown to turbulence. The results also indicate that, for a certain R, range, the thickness of the interfacial layer (normalized by the integral lengthscale of the turbulence) is a decreasing function of R3. At sufficiently high R, the interfacial thickness becomes limited by diffusive effects. Finally, we discuss a simple model for entrainment a t a density interface in the presence of shearfree turbulence.

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“…It is therefore possible that wind shear influenced the results in some cases. As shown by Carruthers and Hunt (1986), the values of the length scales depend on the atmospheric stability of the upper layer. Mason (1989) also observed that the results by Lenschow and Stankov (1986) near the top of the CBL could have been influenced by the presence of large-scale motion in the stable layer above the inversion.…”

Section: Fig 3 Eulerian Autocorrelation Function Rmentioning

confidence: 99%

Relating Eulerian and Lagrangian Statistics for the Turbulent Dispersion in the Atmospheric Convective Boundary Layer

Dosio

Arellano

Holtslag

et al. 2005

Journal of the Atmospheric Sciences

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Eulerian and Lagrangian statistics in the atmospheric convective boundary layer (CBL) are studied by means of large eddy simulation (LES). Spectra analysis is performed in both the Eulerian and Lagrangian frameworks, autocorrelations are calculated, and the integral length and time scales are derived. Eulerian statistics are calculated by means of spatial and temporal analysis in order to derive characteristic length and time scales. Taylor's hypothesis of frozen turbulence is investigated, and it is found to be satisfied in the simulated flow.Lagrangian statistics are derived by tracking the trajectories of numerous particles released at different heights in the turbulent flow. The relationship between Lagrangian properties (autocorrelation functions) and dispersion characteristics (particles' displacement) is studied through Taylor's diffusion relationship, with special emphasis on the difference between horizontal and vertical motion. Results show that for the horizontal motion, Taylor's relationship is satisfied. The vertical motion, however, is influenced by the inhomogeneity of the flow and limited by the ground and the capping inversion at the top of the CBL. The Lagrangian autocorrelation function, therefore, does not have an exponential shape, and consequently, the integral time scale is zero. If distinction is made between free and bounded motion, a better agreement between Taylor's relationship and the particles' vertical displacement is found.Relationships between Eulerian and Lagrangian frameworks are analyzed by calculating the ratio ␤ between Lagrangian and Eulerian time scales. Results show that the integral time scales are mainly constant with height for z/z i Ͻ 0.7. In the upper part of the CBL, the capping inversion transforms vertical motion into horizontal motion. As a result, the horizontal time scale increases with height, whereas the vertical one is reduced. Current parameterizations for the ratio between the Eulerian and Lagrangian time scales have been tested against the LES results showing satisfactory agreement at heights z/z i Ͻ 0.7.

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Velocity fluctuations near an interface between a turbulent region and a stably stratified layer

Cited by 89 publications

References 29 publications

Daytime turbulent exchange between the Amazon forest and the atmosphere

Daytime turbulent exchange between the Amazon forest and the atmosphere

Turbulent mixing at a shear-free density interface

Relating Eulerian and Lagrangian Statistics for the Turbulent Dispersion in the Atmospheric Convective Boundary Layer

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