Intrinsic Constraints on Asymmetric Turbulent Transport of Scalars Within the Constant Flux Layer of the Lower Atmosphere

Li, Dan; Katul, Gabriel G.; Li, Heping

doi:10.1002/2018gl077021

Cited by 22 publications

(28 citation statements)

References 45 publications

(71 reference statements)

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“…However, the analysis here suggests that a small

\overset{{w^{'}}^{2}}{true}

(suppressed by thermal stratification in the SBL) and the generation of flux transport terms associated with large eddies acting on the large mean gradients lead to ratios R LLu and R LLT exceeding unity (Figure S10). These large R LLu and R LLT are, of course, the main reason why flux‐gradient theory (or MOST) fails as discussed elsewhere (Li et al, ). The large magnitudes of in

\frac{\partial \overset{u}{true}}{\partial z}

and

\frac{\partial \overset{θ}{true}}{\partial z}

are, to large degree, anchored by boundary conditions at the surface (friction velocity and radiative cooling) as well as the SBL aloft.…”

Section: Resultsmentioning

confidence: 84%

“…The terms on the right‐hand side (RHS) of equations or include the production term associated with the mean profile of horizontal velocity (or potential temperature), the third‐order turbulent transport of momentum flux (or kinematic heat flux), the pressure decorrelation term due to the interaction between pressure and velocity (or temperature), and the buoyancy term associated with thermal stratification. Note that the molecular destruction term is ignored as its magnitude is much smaller than the other terms (Katul et al, , ; Li et al, ). The Rotta () model retaining the linear component is then employed to parameterize the pressure decorrelation terms

- \frac{1}{ρ} \frac{\partial \overset{u^{'} P^{'}}{true}}{\partial z} = - C \frac{\overset{true¯}{u^{'} w^{'}}}{τ_{u}},

- \frac{1}{ρ} \frac{\partial \overset{θ^{'} P^{'}}{true}}{\partial z} = - C \frac{\overset{true¯}{w^{'} θ^{'}}}{τ_{θ}},

where C is a proportionality coefficient, τ u and τ θ are the relaxation time scales delineating how fast a turbulent eddy loses its coherency (Li, ; Li et al, ).…”

Section: Resultsmentioning

confidence: 99%

“…where C is a proportionality coefficient, τ u and τ θ are the relaxation time scales delineating how fast a turbulent eddy loses its coherency (Li, 2019;Li et al, 2018). To simplify the calculation, C, τ u , and τ θ are considered as constants.…”

Section: Scaling Argumentsmentioning

confidence: 99%

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Large Eddies Regulate Turbulent Flux Gradients in Coupled Stable Boundary Layers

Lan

Katul

et al. 2019

Geophysical Research Letters

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Due to strong mean wind shear, the stable boundary layer (SBL) becomes vertically coupled. In a coupled SBL, large turbulent eddies enhance cross‐layer mixing and vertically mix turbulent kinetic energy. However, large gradients in momentum and heat fluxes are frequently observed, degrading the performance of Monin‐Obukhov similarity theory. It is shown that increased vertical gradients of mean variables (i.e., wind speed and potential temperature) can cause flux gradients. This process is operated upon by downward penetrating large eddies with altered phase difference, contributing unevenly to fluxes and thus causing flux gradients. In the coupled SBL with small gradients in mean variables, large eddies are vertically synchronized, contributing evenly to fluxes across layers and thus causing small flux gradients. As turbulent production becomes weak and the cross‐layer difference in turbulent flux transport increases, large eddies become less vertically synchronized, contributing unevenly to fluxes across layers and thus causing large flux gradients.

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“…However, the analysis here suggests that a small

\overset{{w^{'}}^{2}}{true}

\frac{\partial \overset{u}{true}}{\partial z}

and

\frac{\partial \overset{θ}{true}}{\partial z}

are, to large degree, anchored by boundary conditions at the surface (friction velocity and radiative cooling) as well as the SBL aloft.…”

Section: Resultsmentioning

confidence: 84%

- \frac{1}{ρ} \frac{\partial \overset{u^{'} P^{'}}{true}}{\partial z} = - C \frac{\overset{true¯}{u^{'} w^{'}}}{τ_{u}},

- \frac{1}{ρ} \frac{\partial \overset{θ^{'} P^{'}}{true}}{\partial z} = - C \frac{\overset{true¯}{w^{'} θ^{'}}}{τ_{θ}},

where C is a proportionality coefficient, τ u and τ θ are the relaxation time scales delineating how fast a turbulent eddy loses its coherency (Li, ; Li et al, ).…”

Section: Resultsmentioning

confidence: 99%

See 1 more Smart Citation

Large Eddies Regulate Turbulent Flux Gradients in Coupled Stable Boundary Layers

Lan

Katul

et al. 2019

Geophysical Research Letters

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“…Second moment turbulence closure models are useful for understanding the physics of turbulent mixing and have been extensively used to study the energetics of atmosphere and ocean flows (Kantha, ; Li et al, ; Valsala et al, ). Such models provide the diagnostics of microstructure properties in the boundary layer, which are helpful in interpreting the underlying mechanisms of turbulence and mixing.…”

Section: Resultsmentioning

confidence: 99%

On the Variability of Arabian Sea Mixing and its Energetics

et al. 2019

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The mechanism of interannual variability of Arabian Sea (AS) mixing is studied by examining its energetics from 60 years of model output generated using an ocean general circulation model. The model results are compared with ocean reanalysis data for the same period. Both model and reanalysis data show consistent patterns of interannual variability of AS mixing. It is observed that the dominant mode of interannual variability has a basin-wide structure with monotonic sign throughout the basin, albeit with a higher intensity in the central and southeast regions of the AS. The variability is primarily controlled by El Niño-Southern Oscillation with shallower (deeper) mixed layer during El Niño (La Niña). During a typical La Niña year, the wind stress forcing, heat fluxes, and evaporation minus precipitation together cause deepening of mixed layer depth (MLD) in AS. This MLD variability has the strongest signal in winter months (i.e., December to February [DJF]). Further, the resolved mixing energetics suggest that the dominant forcing of MLD variability changes with seasons and has a regional preference. Buoyancy production of turbulent kinetic energy (TKE) governs the mixing and associated MLD variability in central AS during September to November (SON) and in northeast AS during DJF. Both buoyancy and mechanical production of TKE govern MLD variability in southwest to central AS from June to August and southeast AS in SON. Thus, the study highlights basin-wide structure of interannual variability of AS mixing along with its controlling mechanisms with strong regional dependency owing to the contrasting nature of forcing prevailing over various parts of the AS.

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“…During daytime conditions, large eddies that are generated from heated surfaces ascend (i.e., ejections) and large eddies that originate in the well‐mixed layer (ML) aloft impinge on the ASL (sweeps), leading the ASL to interact and become somewhat coupled with the overlying ML (Garratt, 1992; D. Li et al., 2018). These two types of coherent large eddy motions contribute to fluxes throughout the atmospheric boundary layer including the ASL.…”

Section: Introductionmentioning

confidence: 99%

Non‐Closure of Surface Energy Balance Linked to Asymmetric Turbulent Transport of Scalars by Large Eddies

Gao

Katul

2021

JGR Atmospheres

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How large turbulent eddies influence non‐closure of the surface energy balance is an active research topic that cannot be uncovered by the mean continuity equation in isolation. It is demonstrated here that asymmetric turbulent flux transport of heat and water vapor by sweeps and ejections of large eddies under unstable atmospheric stability conditions reduce fluxes. Such asymmetry causes positive gradients in the third‐order moments in the turbulent flux budget equations, primarily attributed to substantially reduced flux contributions by sweeps and sustained large flux contributions by ejections. Small‐scale surface heterogeneity in heating generates ejecting eddies with larger air temperature variance than sweeping eddies, causing asymmetric flux transport in the atmospheric surface layer. Changes in asymmetry with increasing instability are congruent with observed increases in the surface energy balance non‐closure. To assess the contributions of asymmetric flux transport by large eddies to the non‐closure requires two eddy covariance systems on the tower to measure the gradients of the turbulent heat flux and other third‐order moments.

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Intrinsic Constraints on Asymmetric Turbulent Transport of Scalars Within the Constant Flux Layer of the Lower Atmosphere

Cited by 22 publications

References 45 publications

Large Eddies Regulate Turbulent Flux Gradients in Coupled Stable Boundary Layers

Large Eddies Regulate Turbulent Flux Gradients in Coupled Stable Boundary Layers

On the Variability of Arabian Sea Mixing and its Energetics

Non‐Closure of Surface Energy Balance Linked to Asymmetric Turbulent Transport of Scalars by Large Eddies

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