Sweeping Effects Modify Taylor’s Frozen Turbulence Hypothesis for Scalars in the Roughness Sublayer

Everard, K.; Katul, Gabriel G.; Lawrence, Gregory A.; Christen, Andreas; Parlange, Marc B.

doi:10.1029/2021gl093746

Cited by 8 publications

(6 citation statements)

References 52 publications

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“…For example, Everard et al . (2021) showed that in canopy flows (in their study

z/h

varied between 0.2 and 2)

{U}_{\mathrm{adv}}

was a constant fraction of local mean wind speed at all observation heights, indicative that

{U}_{\mathrm{adv}}

increased with height (since wind speed increases with height). Their findings are in qualitative agreement with other

{U}_{\mathrm{adv}}

profiles for sparse canopies (Huang et al, 2009).…”

Section: Resultsmentioning

confidence: 95%

“…In such conditions,

\theta

isolines fluctuate vertically over the whole subcanopy domain, whereas

{L}_w

may be affected by canopy elements breaking turbulent eddies and thus decreasing

{L}_w

. Third, there are large uncertainties in utilizing Taylor's frozen turbulence hypothesis in canopy flows (Huang et al, 2009; Brunet, 2020; Everard et al, 2021). Hence, estimating the integral length scale of

w

from time series using a mean advection velocity become problematic.…”

Section: Resultsmentioning

confidence: 99%

“…There is a discrepancy between the two forests that might be due to a slightly incorrect value for eddy advection velocity U adv . For example, Everard et al (2021) showed that in canopy flows (in their study z∕h varied between 0.2 and 2) U adv was a constant fraction of local mean wind speed at all observation heights, indicative that U adv increased with height (since wind speed increases with height). Their findings are in qualitative agreement with other U adv profiles for sparse canopies (Huang et al, 2009).…”

Section: Dependence Of Length-scale Profiles On Stabilitymentioning

confidence: 95%

See 2 more Smart Citations

Probing eddy size and its effective mixing length in stably stratified roughness sublayer flows

Peltola

Aurela

Launiainen

et al. 2022

Quart J Royal Meteoro Soc

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Stably stratified roughness sublayer flows are ubiquitous yet remain difficult to represent in models and to interpret using field experiments. Here, continuous high‐frequency potential temperature profiles from the forest floor up to 6.5 times the canopy height observed with distributed temperature sensing (DTS) are used to link eddy topology to roughness sublayer stability correction functions and coupling between air layers within and above the canopy. The experiments are conducted at two forest stands classified as hydrodynamically sparse and dense. Near‐continuous profiles of eddy sizes (length scales) and effective mixing lengths for heat are derived from the observed profiles using a novel conditional sampling approach. The approach utilizes potential temperature isoline fluctuations from a statically stable background state. The transport of potential temperature by an observed eddy is assumed to be conserved (adiabatic movement) and we assume that irreversible heat exchange between the eddy and the surrounding background occurs along the (vertical) periphery of the eddy. This assumption is analogous to Prandtl's mixing‐length concept, where momentum is transported rapidly vertically and then equilibrated with the local mean velocity gradient. A distinct dependence of the derived length scales on background stratification, height above ground, and canopy characteristics emerges from the observed profiles. Implications of these findings for (1) the failure of Monin–Obukhov similarity in the roughness sublayer and (2) above‐canopy flow coupling to the forest floor are examined. The findings have practical applications in terms of analysing similar DTS data sets with the proposed approach, modelling roughness sublayer flows, and interpreting nocturnal eddy covariance measurements above tall forested canopies.

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“…For example, Everard et al . (2021) showed that in canopy flows (in their study

z/h

varied between 0.2 and 2)

{U}_{\mathrm{adv}}

was a constant fraction of local mean wind speed at all observation heights, indicative that

{U}_{\mathrm{adv}}

increased with height (since wind speed increases with height). Their findings are in qualitative agreement with other

{U}_{\mathrm{adv}}

profiles for sparse canopies (Huang et al, 2009).…”

Section: Resultsmentioning

confidence: 95%

“…In such conditions,

\theta

isolines fluctuate vertically over the whole subcanopy domain, whereas

{L}_w

may be affected by canopy elements breaking turbulent eddies and thus decreasing

{L}_w

w

from time series using a mean advection velocity become problematic.…”

Section: Resultsmentioning

confidence: 99%

Section: Dependence Of Length-scale Profiles On Stabilitymentioning

confidence: 95%

See 1 more Smart Citation

Probing eddy size and its effective mixing length in stably stratified roughness sublayer flows

Peltola

Aurela

Launiainen

et al. 2022

Quart J Royal Meteoro Soc

View full text Add to dashboard Cite

show abstract

“…2009; Everard et al. 2021). It could be possible that the discrepancy between the two scalings for the counter-gradient quadrants is a result of the failure of Taylor's hypothesis.…”

Section: Resultsmentioning

confidence: 99%

“…Contrarily, for ejections and sweeps, this difference is insignificant. While converting Γ w to a length scale, Taylor's hypothesis is needed, which often is questionable in canopy turbulence (Finnigan et al 2009;Everard et al 2021). It could be possible that the discrepancy between the two scalings for the counter-gradient quadrants is a result of the failure of Taylor's hypothesis.…”

Section: An Alternative Scaling With Integral Scalesmentioning

confidence: 99%

A scale-wise analysis of intermittent momentum transport in dense canopy flows

2022

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We investigate the intermittent dynamics of momentum transport and its underlying time scales in the near-wall region of the neutrally stratified atmospheric boundary layer in the presence of a vegetation canopy. This is achieved through an empirical analysis of the persistence time scales (periods between successive zero-crossings) of momentum flux events, and their connection to the ejection–sweep cycle. Using high-frequency measurements from the GoAmazon campaign, spanning multiple heights within and above a dense canopy, the analysis suggests that, when the persistence time scales ( $t_p$ ) of momentum flux events from four different quadrants are separately normalized by $\varGamma _{w}$ (integral time scale of the vertical velocity), their distributions $P(t_p/\varGamma _{w})$ remain height-invariant. This result points to a persistent memory imposed by canopy-induced coherent structures, and to their role as an efficient momentum-transporting mechanism between the canopy airspace and the region immediately above. Moreover, $P(t_p/\varGamma _{w})$ exhibits a power-law scaling at times $t_{p}<\varGamma _{w}$ , with an exponential tail appearing for $t_{p} \geq \varGamma _{w}$ . By separating the flux events based on $t_p$ , we discover that around 80 % of the momentum is transported through the long-lived events ( $t_{p} \geq \varGamma _{w}$ ) at heights immediately above the canopy, while the short-lived ones ( $t_{p} < \varGamma _{w}$ ) only contribute marginally ( $\approx 20\,\%$ ). To explain the role of instantaneous flux amplitudes in momentum transport, we compare the measurements with newly developed surrogate data and establish that the range of time scales involved with amplitude variations in the fluxes tends to increase as one transitions from within to above the canopy.

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The vertical turbulent structure within the surface boundary layer above a Vineyard in California’s Central Valley during GRAPEX

Alfieri¹,

Kustas²,

Prueger

et al. 2022

Irrig Sci

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Sweeping Effects Modify Taylor’s Frozen Turbulence Hypothesis for Scalars in the Roughness Sublayer

Cited by 8 publications

References 52 publications

Probing eddy size and its effective mixing length in stably stratified roughness sublayer flows

Probing eddy size and its effective mixing length in stably stratified roughness sublayer flows

A scale-wise analysis of intermittent momentum transport in dense canopy flows

The vertical turbulent structure within the surface boundary layer above a Vineyard in California’s Central Valley during GRAPEX

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