Influence of the sampling time on the kinematics of turbulent diffusion from a continuous source

Eckman, Richard M.

doi:10.1017/s0022112094004301

Cited by 6 publications

(2 citation statements)

References 19 publications

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“…The cross-wind velocity variance, and therefore the plume spread, is sensitive to the choice of averaging time (e.g., [7]). Example time series of the cross-wind component for two different stability regimes are shown in Fig.…”

Section: Velocity Variancementioning

confidence: 99%

Observations of the cross-wind velocity variance in the stable boundary layer

Vickers

Mahrt

2007

Environ Fluid Mech

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Nine tower datasets over grassland, brush rangeland, snow covered plain, the ocean, three different pine forests, an aspen forest and an urban site, are used to document the scale-dependence of the cross-wind velocity variance in the stable boundary layer. The turbulence velocity variance scales with the surface momentum flux, as reported in previous studies. Such scaling removes the stability dependence of the variance at a given site, and also removes most of the differences between sites. The scaling is more effective with use of a record-dependent averaging time for defining the turbulent fluctuations. The variable averaging time is the timescale associated with the gap region in the heat flux multiresolution cospectra.On scales larger than turbulence and less than a few hours (mesoscale), variations in the cross-wind velocity variance at a given site are not related to local variables such as the friction velocity. Possible exceptions include suppression of turbulence and mesoscale motions in well-defined drainage flows and enhancement of turbulence and mesoscale motions in stronger winds downstream of a ridge. Larger mesoscale variance is associated with complex terrain and forested sites compared to the more homogeneous sites in flat terrain with short or no vegetation. These differences between sites are related to the absence of a gap region in the velocity spectra at the complex terrain and forested sites. The observed probability distribution functions of the total variance and the mesoscale variance are documented for different averaging times, stability classes and site characteristics.

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Section: Velocity Variancementioning

confidence: 99%

Observations of the cross-wind velocity variance in the stable boundary layer

Vickers

Mahrt

2007

Environ Fluid Mech

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“…can be sensitive to the range of scales included in its calculation. Correspondingly, the measurement of plume spread is sensitive to the choice of averaging time (Eckman 1994).…”

Section: Introductionmentioning

confidence: 99%

Dependence of Turbulent and Mesoscale Velocity Variances on Scale and Stability

Mahrt¹,

Moore²,

Vickers³

et al. 2001

J. Appl. Meteor.

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The scale dependence of velocity variances is studied using data collected from a grassland site, a heather site, and four forested sites. The dependence of velocity variances on averaging time, used to define the fluctuation quantities, is modeled. The crosswind velocity variance is emphasized, because it is more difficult to model than the other two components and is crucial input for applications such as dispersion modeling. The distinction between turbulence and mesoscale variances is examined in detail. Because mesoscale and turbulence motions are governed by different physics, meaningful study of the behavior of velocity variances requires adequate separation of turbulence and mesoscale motions from data. For stable conditions, the horizontal velocity variances near the surface exhibit a spectral gap, here corresponding to a very slow or nonexistent increase of variance with increasing averaging time. This ''gap region,'' when it occurs, allows separation of mesoscale and turbulence motions; however, the averaging times corresponding to this gap vary substantially with stability. A choice of typical averaging times for defining turbulent perturbations, such as 5 or 10 min, leads to the capture of significant mesoscale motions for very stable conditions and contributes to the disagreement with turbulence similarity theory. For unstable motions, the gap region for the horizontal velocity variances shrinks or becomes poorly defined, because large convective eddies tend to ''fill in'' the gap between turbulence and mesoscale motions. The formulation developed here allows turbulence and mesoscale motions to overlap into the same intermediate timescales. The mesoscale variances are less predictable, because a wide variety of physical processes contribute to mesoscale motions. Their magnitude and range of timescales vary substantially among sites. The variation of the behavior of turbulence variances among sites is significant but substantially less than that for the mesoscale motions.

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The expected mass fraction

Heagy¹,

Sullivan²

1996

Atmospheric Environment

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Influence of the sampling time on the kinematics of turbulent diffusion from a continuous source

Cited by 6 publications

References 19 publications

Observations of the cross-wind velocity variance in the stable boundary layer

Observations of the cross-wind velocity variance in the stable boundary layer

Dependence of Turbulent and Mesoscale Velocity Variances on Scale and Stability

The expected mass fraction

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