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1. Estimation of Turbulence Parameters in the Lower Troposphere from ShUREX (2016–2017) UAV Data

   https://doi.org/10.3390/atmos10070384  

   Luce, Hubert ; Kantha, Lakshmi ; Hashiguchi, Hiroyuki ; Lawrence, Dale ( July 2019 , Atmosphere)

   null (Ed.)

   Turbulence parameters in the lower troposphere (up to ~4.5 km) are estimated from measurements of high-resolution and fast-response cold-wire temperature and Pitot tube velocity from sensors onboard DataHawk Unmanned Aerial Vehicles (UAVs) operated at the Shigaraki Middle and Upper atmosphere (MU) Observatory during two ShUREX (Shigaraki UAV Radar Experiment) campaigns in 2016 and 2017. The practical processing methods used for estimating turbulence kinetic energy dissipation rate ε and temperature structure function parameter C T 2 from one-dimensional wind and temperature frequency spectra are first described in detail. Both are based on the identification of inertial (−5/3) subranges in respective spectra. Using a formulation relating ε and C T 2 valid for Kolmogorov turbulence in steady state, the flux Richardson number R f and the mixing efficiency χ m are then estimated. The statistical analysis confirms the variability of R f and χ m around ~ 0.13 − 0.14 and ~ 0.16 − 0.17 , respectively, values close to the canonical values found from some earlier experimental and theoretical studies of both the atmosphere and the oceans. The relevance of the interpretation of the inertial subranges in terms of Kolmogorov turbulence is confirmed by assessing the consistency of additional parameters, the Ozmidov length scale L O , the buoyancy Reynolds number R e b , and the gradient Richardson number Ri. Finally, a case study is presented showing altitude differences between the peaks of N 2 , C T 2 and ε , suggesting turbulent stirring at the margin of a stable temperature gradient sheet. The possible contribution of this sheet and layer structure on clear air radar backscattering mechanisms is examined. 

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2. The Role of Sand in Wave Boundary Layers Over Primarily Muddy Seabeds: Implications for Wave‐Supported Gravity Flows

   https://doi.org/10.1029/2020JC016621  

   Han, Zhuochen ; Horner‐Devine, Alexander R. ; Ogston, Andrea S. ; Hsu, Tian‐Jian ( April 2021 , Journal of Geophysical Research: Oceans)

   We performed laboratory experiments to investigate the influence of sand content on the dynamics of wave‐supported gravity flows in mud‐dominant environments. The experiments were carried out in an oscillatory water tunnel with a sediment bed of either 1% or 13% sand. Low and high energy regimes are differentiated based on a Stokes Reynolds numberRe_Δ ≈ 500. In the low energy regime, the sand fraction influences flow dynamics primarily through ripple formation; no ripples form in the 1% sand experiments, whereas ripples form in the 13% experiments that increase turbulence and the wave boundary layer thickness,δ_m. In the high energy regime, small ripples form in both the 1% and 13% sand experiments and we observe high near‐bed suspended sediment concentrations. The influence of stratification on the boundary layer flow is characterized in terms of the gradient Richardson numberRi_g. The flow is weakly stratified inside the boundary layer for all runs and critically stratified at or above the top of the boundary layer. In the lower energy regime, the sand content reduces the relative influence of stratification in the boundary layer, shifting the elevation of critical stratification,L_B, from approximately 1.3δ_mto 2.5δ_min the 1% and 13% experiments, respectively. In both sets of experimentsL_B ≈ δ_mat the strongest wave energy, indicating a transition to strongly stratified dynamics.

    

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3. Scaling of Shear‐Generated Turbulence: The Equatorial Thermocline, a Case Study

   https://doi.org/10.1029/2020JC016978  

   Richards, K. J. ; Natarov, A. ; Carter, G. S. ( May 2021 , Journal of Geophysical Research: Oceans)

   We formulate an expression for the turbulent kinetic energy dissipation rate,ϵ, associated with shear‐generated turbulence in terms of quantities in the ocean or atmosphere that, depending on the situation, may be measurable or resolved in models. The expression depends on the turbulent vertical length scale,ℓ_v, the inverse time scaleN, and the Richardson numberRi = N²/S², whereSis the vertical shear, withℓ_vscaled in a way consistent with theories and observations of stratified turbulence. Unlike previous studies, the focus is not so much on the functional form ofRi, but the vertical variation of the length scaleℓ_v. Using data from two ∼7‐day time series in the western equatorial Pacific, the scaling is compared with the observedϵ. The scaling works well with the estimatedϵcapturing the differences in amplitude and vertical distribution of the observedϵbetween the two times series. Much of those differences are attributable to changes in the vertical distribution of the length scaleℓ_v, and in particular the associated turbulent velocity scale,u_t. We relateu_tto a measure of the fine‐scale variations in velocity,. Our study highlights the need to consider the length scale and its estimation in environmental flows. The implications for the vertical variation of the associated turbulent diffusivity are discussed.

    

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4. Scalewise Return to Isotropy in Stratified Boundary Layer Flows

   https://doi.org/10.1029/2020JD032732  

   Ayet, A. ; Katul, G. G. ; Bragg, A. D. ; Redelsperger, J. L. ( August 2020 , Journal of Geophysical Research: Atmospheres)

   Anisotropic turbulence is ubiquitous in atmospheric and oceanic boundary layers due to differences in energy injection mechanisms. Unlike mechanical production that injects energy in the streamwise velocity component, buoyancy affects only the vertical velocity component. This anisotropy in energy sources, quantified by the flux Richardson numberRi_f, is compensated by a “return to isotropy” (RTI) tendency of turbulent flows. Describing RTI in Reynolds‐averaged models and across scales continues to be a challenge in stratified turbulent flows. Using phenomenological models for spectral energy transfers, the necessary conditions for which the widely‐used Rotta model captures RTI across variousRi_fand eddy sizes are discussed for the first time. This work unravels adjustments to the Rotta constant, withRi_fand scale, necessary to obtain consistency between RTI models and the measured properties of the atmospheric surface layer for planar‐homogeneous and stationary flows in the absence of subsidence. A range ofRi_fand eddy sizes where the usage of a conventional Rotta model is prohibited is also found. Those adjustments lay the groundwork for new closure schemes.

    

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5. A comparative study of turbulent stratified shear layers: effect of density gradient distribution

   https://doi.org/10.1007/s10652-022-09873-2  

   Pham, Hieu T. ; Sarkar, Sutanu ( May 2022 , Environmental Fluid Mechanics)

   Abstract Direct numerical simulations are performed to compare the evolution of turbulent stratified shear layers with different density gradient profiles at a high Reynolds number. The density profiles include uniform stratification, two-layer hyperbolic tangent profile and a composite of these two profiles. All profiles have the same initial bulk Richardson number ( $$Ri_{b,0}$$ R i b , 0 ); however, the minimum gradient Richardson number and the distribution of density gradient across the shear layer are varied among the cases. The objective of the study is to provide a comparative analysis of the evolution of the shear layers in term of shear layer growth, turbulent kinetic energy as well as the mixing efficiency and its parameterization. The evolution of the shear layers in all cases shows the development of Kelvin–Helmholtz billows, the transition to turbulence by secondary instabilities followed by the decay of turbulence. Comparison among the cases reveals that the amount of turbulent mixing varies with the density gradient distribution inside the shear layer. The minimum gradient Richardson number and the initial bulk Richardson number do not correlate well with the integrated TKE production, dissipation and buoyancy flux. The bulk mixing efficiency for fixed $$Ri_{b,0}$$ R i b , 0 is found to be highest in the case with two-layer density profile and lowest in the case with uniform stratification. However, the parameterizations of the flux coefficient based on buoyancy Reynolds number and the ratio of Ozmidov and Ellison scales show similar scaling in all cases. 

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