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1. Benthic Flow and Mixing in a Shallow Shoal Grass (Halodule wrightii) Fringe

   https://doi.org/10.3390/geosciences11030115  

   Cannon, David; Kibler, Kelly; Kitsikoudis, Vasileios (March 2021, Geosciences)

   null (Ed.)

   Mean flow and turbulence measurements collected in a shallow Halodule wrightii shoal grass fringe highlighted significant heterogeneity in hydrodynamic effects over relatively small spatial scales. Experiments were conducted within the vegetation canopy (~4 cm above bottom) for relatively sparse (40% cover) and dense (70% cover) vegetation, with reference measurements collected near the bed above bare sediment. Significant benthic velocity shear was observed at all sample locations, with canopy shear layers that penetrated nearly to the bed at both vegetated sites. Turbulent shear production (P) was balanced by turbulent kinetic energy dissipation (ϵ) at all sample locations (P/ϵ≈1), suggesting that stem-generated turbulence played a minor role in the overall turbulence budget. While the more sparsely vegetated sample site was associated with enhanced channel-to-shore velocity attenuation (71.4 ± 1.0%) relative to flows above bare sediment (51.7 ± 2.2%), unexpectedly strong cross-shore currents were observed nearshore in the dense canopy (VNS), with magnitudes that were nearly twice as large as those measured in the main channel (VCH; VNS/VCH¯ = 1.81 ± 0.08). These results highlight the importance of flow steering and acceleration for within- and across-canopy transport, especially at the scale of individual vegetation patches, with important implications for nutrient and sediment fluxes. Importantly, this work represents one of the first hydrodynamic studies of shoal grass fringes in shallow coastal estuaries, as well as one of the only reports of turbulent mixing within H. wrightii canopies. 

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2. Energetics and mixing in buoyancy-driven near-bottom stratified flow

   https://doi.org/10.1017/jfm.2019.184  

   Puthan, Pranav; Jalali, Masoud; Chalamalla, Vamsi K.; Sarkar, Sutanu (June 2019, Journal of Fluid Mechanics)

   Turbulence and mixing in a near-bottom convectively driven flow are examined by numerical simulations of a model problem: a statically unstable disturbance at a slope with inclination $$\unicode[STIX]{x1D6FD}$$ in a stable background with buoyancy frequency $$N$$ . The influence of slope angle and initial disturbance amplitude are quantified in a parametric study. The flow evolution involves energy exchange between four energy reservoirs, namely the mean and turbulent components of kinetic energy (KE) and available potential energy (APE). In contrast to the zero-slope case where the mean flow is negligible, the presence of a slope leads to a current that oscillates with $$\unicode[STIX]{x1D714}=N\sin \unicode[STIX]{x1D6FD}$$ and qualitatively changes the subsequent evolution of the initial density disturbance. The frequency, $$N\sin \unicode[STIX]{x1D6FD}$$ , and the initial speed of the current are predicted using linear theory. The energy transfer in the sloping cases is dominated by an oscillatory exchange between mean APE and mean KE with a transfer to turbulence at specific phases. In all simulated cases, the positive buoyancy flux during episodes of convective instability at the zero-velocity phase is the dominant contributor to turbulent kinetic energy (TKE) although the shear production becomes increasingly important with increasing  $$\unicode[STIX]{x1D6FD}$$ . Energy that initially resides wholly in mean available potential energy is lost through conversion to turbulence and the subsequent dissipation of TKE and turbulent available potential energy. A key result is that, in contrast to the explosive loss of energy during the initial convective instability in the non-sloping case, the sloping cases exhibit a more gradual energy loss that is sustained over a long time interval. The slope-parallel oscillation introduces a new flow time scale $$T=2\unicode[STIX]{x03C0}/(N\sin \unicode[STIX]{x1D6FD})$$ and, consequently, the fraction of initial APE that is converted to turbulence during convective instability progressively decreases with increasing $$\unicode[STIX]{x1D6FD}$$ . For moderate slopes with $$\unicode[STIX]{x1D6FD}<10^{\circ }$$ , most of the net energy loss takes place during an initial, short ( $$Nt\approx 20$$ ) interval with periodic convective overturns. For steeper slopes, most of the energy loss takes place during a later, long ( $Nt>100$ ) interval when both shear and convective instability occur, and the energy loss rate is approximately constant. The mixing efficiency during the initial period dominated by convectively driven turbulence is found to be substantially higher (exceeds 0.5) than the widely used value of 0.2. The mixing efficiency at long time in the present problem of a convective overturn at a boundary varies between 0.24 and 0.3. 

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3. Shear Instability and Turbulent Mixing in the Stratified Shear Flow Behind a Topographic Ridge at High Reynolds Number

   https://doi.org/10.3389/fmars.2022.829579  

   Chen, Jia-Lin; Yu, Xiao; Chang, Ming-Huei; Jan, Sen; Yang, Yiing Jang; Lien, Ren-Chieh (May 2022, Frontiers in Marine Science)

   Observations on the lee of a topographic ridge show that the turbulence kinetic energy (TKE) dissipation rate due to shear instabilities is three orders of magnitude higher than the typical value in the open ocean. Laboratory-scale studies at low Reynolds number suggest that high turbulent dissipation occurs primarily within the core region of shear instabilities. However, field-scale studies indicate that high turbulence is mainly populated along the braids of shear instabilities. In this study, a high-resolution, resolving the Ozmidov-scale, non-hydrostatic model with Large Eddy Simulation (LES) turbulent closure is applied to investigate dominant mechanisms that control the spatial and temporal scales of shear instabilities and resulting mixing in stratified shear flow at high Reynolds number. The simulated density variance dissipation rate is elevated in the cusp-like bands of shear instabilities with a specific period, consistent with the acoustic backscatter taken by shipboard echo sounder. The vertical length scale of each cusp-like band is nearly half of the vertical length scale of the internal lee wave. However, it is consistent with instabilities originating from a shear layer based on linear stability theory. The model results indicate that the length scale and/or the period of shear instabilities are the key parameters to the mixing enhancement that increases with lateral Froude number Fr L , i.e. stronger shear and/or steeper ridge. 

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4. Persistent Turbulence in the Samoan Passage

   https://doi.org/10.1175/JPO-D-19-0116.1  

   Cusack, Jesse M.; Voet, Gunnar; Alford, Matthew H.; Girton, James B.; Carter, Glenn S.; Pratt, Larry J.; Pearson-Potts, Kelly A.; Tan, Shuwen (December 2019, Journal of Physical Oceanography)

   Abyssal waters forming the lower limb of the global overturning circulation flow through the Samoan Passage and are modified by intense mixing. Thorpe-scale-based estimates of dissipation from moored profilers deployed on top of two sills for 17 months reveal that turbulence is continuously generated in the passage. Overturns were observed in a density band in which the Richardson number was often smaller than ¼, consistent with shear instability occurring at the upper interface of the fast-flowing bottom water layer. The magnitude of dissipation was found to be stable on long time scales from weeks to months. A second array of 12 moored profilers deployed for a shorter duration but profiling at higher frequency was able to resolve variability in dissipation on time scales from days to hours. At some mooring locations, near-inertial and tidal modulation of the dissipation rate was observed. However, the modulation was not spatially coherent across the passage. The magnitude and vertical structure of dissipation from observations at one of the major sills is compared with an idealized 2D numerical simulation that includes a barotropic tidal forcing. Depth-integrated dissipation rates agree between model and observations to within a factor of 3. The tide has a negligible effect on the mean dissipation. These observations reinforce the notion that the Samoan Passage is an important mixing hot spot in the global ocean where waters are being transformed continuously. 

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5. Kelvin–Helmholtz Instability “Tube” and “Knot” Dynamics. Part II: KHI T&K Dynamics in a Multiscale Gravity Wave Direct Numerical Simulation

   https://doi.org/10.1175/JAS-D-22-0193.1  

   Fritts, David C.; Wang, Ling (October 2023, Journal of the Atmospheric Sciences)

   Abstract A companion paper by Fritts et al. reviews evidence for Kelvin–Helmholtz instability (KHI) “tube” and “knot” (T&K) dynamics that appear to be widespread throughout the atmosphere. Here we describe the results of an idealized direct numerical simulation of multiscale gravity wave dynamics that reveals multiple larger- and smaller-scale KHI T&K events. The results enable assessments of the environments in which these dynamics arise and their competition with concurrent gravity wave breaking in driving turbulence and energy dissipation. A larger-scale event is diagnosed in detail and reveals diverse and intense T&K dynamics driving more intense turbulence than occurs due to gravity wave breaking in the same environment. Smaller-scale events reveal that KHI T&K dynamics readily extend to weaker, smaller-scale, and increasingly viscous shear flows. Our results suggest that KHI T&K dynamics should be widespread, perhaps ubiquitous, wherever superposed gravity waves induce intensifying shear layers, because such layers are virtually always present. A second companion paper demonstrates that KHI T&K dynamics exhibit elevated turbulence generation and energy dissipation rates extending to smaller Reynolds numbers for relevant KHI scales wherever they arise. These dynamics are suggested to be significant sources of turbulence and mixing throughout the atmosphere that are currently ignored or underrepresented in turbulence parameterizations in regional and global models. Significance StatementAtmospheric observations reveal that Kelvin–Helmholtz instabilities (KHI) often exhibit complex interactions described as “tube” and “knot” (T&K) dynamics in the presence of larger-scale gravity waves (GWs). These dynamics may prove to make significant contributions to energy dissipation and mixing that are not presently accounted for in large-scale modeling and weather prediction. We explore here the occurrence of KHI T&K dynamics in an idealized model that describes their behavior and character arising at larger and smaller scales due to superposed, large-amplitude GWs. The results reveal that KHI T&K dynamics arise at larger and smaller scales, and that their turbulence intensities can be comparable to those of the GWs. 

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