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Jeremy D. Bricker and Stephen G. Monismith

Abstract

A new method of wave–turbulence decomposition is introduced, for which the only instrument required is one high-frequency pointwise velocity sensor. This is a spectral method that assumes equilibrium turbulence and no wave–turbulence interaction. Nonetheless, laboratory and field experiments show that the new method produces results in good agreement with the results of established wave–turbulence decomposition methods. Therefore, this spectral method proves useful when neither a synchronized wave gauge, nor a second velocimeter, is available. Furthermore, this study indicates that uncertainty in velocimeter probe orientation is responsible for most of the wave bias occurring in turbulent velocity data, so that an accurate measurement of this orientation makes wave–turbulence decomposition unnecessary.

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Nicole L. Jones and Stephen G. Monismith

Abstract

The vertical distribution of the turbulent kinetic energy dissipation rate was measured using an array of four acoustic Doppler velocimeters in the shallow embayment of Grizzly Bay, San Francisco Bay, California. Owing to the combination of wind and tide forcing in this shallow system, the surface and bottom boundary layers overlapped. Whitecapping waves were generated for a significant spectral peak steepness greater than 0.05 or above a wind speed of 3 m s−1. Under conditions of whitecapping waves, the turbulent kinetic energy dissipation rate in the upper portion of the water column was greatly enhanced, relative to the predictions of wind stress wall-layer theory. Instead, the dissipation followed a modified deep-water breaking-wave scaling. Near the bed (bottom 10% of the water column), the dissipation measurements were either equal to or less than that predicted by wall-layer theory. Stratification due to concentration gradients in suspended sediment was identified as the likely cause for these periods of production–dissipation imbalance close to the bed. During 50% of the well-mixed conditions experienced in the month-long experiment, whitecapping waves provided the dominant source of turbulent kinetic energy over 90% or more of the water column.

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Kristen A. Davis and Stephen G. Monismith

Abstract

Results are presented from an observational study of stratified, turbulent flow in the bottom boundary layer on the outer southeast Florida shelf. Measurements of momentum and heat fluxes were made using an array of acoustic Doppler velocimeters and fast-response temperature sensors in the bottom 3 m over a rough reef slope. Direct estimates of flux Richardson number Rf confirm previous laboratory, numerical, and observational work, which find mixing efficiency not to be a constant but rather to vary with Frt, Reb, and Rig. These results depart from previous observations in that the highest levels of mixing efficiency occur for Frt < 1, suggesting that efficient mixing can also happen in regions of buoyancy-controlled turbulence. Generally, the authors find that turbulence in the reef bottom boundary layer is highly variable in time and modified by near-bed flow, shear, and stratification driven by shoaling internal waves.

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Derek A. Fong and Stephen G. Monismith

Abstract

The accuracy of an acoustic Doppler current profiler (ADCP) used with an internal bottom-tracking system is considered. The boat speed measured using bottom tracking is extremely accurate, comparable to the speeds measured by a high-resolution, real-time kinematic global positioning system (KGPS). The accuracy in the direction of boat motion reported by the bottom tracking is limited to the accuracy of the internal compass of the ADCP. Directional differences (after correcting for local magnetic declination) are about 3° between the ADCP bottom tracking and KGPS. An error of this magnitude is shown to result in a maximal measurement error in water velocity of less than 6%.

Nonetheless, an unexplained water velocity error is observed that is significantly larger than can be explained by a simple compass error. Repeated transects in opposing directions show a bias in measured water velocities in the direction of boat motion. The bias cannot be explained by an error in the compass or the bottom-tracked boat velocities. The difference in recorded velocity between two repeated transects with the boat moving in opposite directions exhibits an error of up to ±5 cm s−1 that has vertical variability.

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Jonah V. Steinbuck, Mark T. Stacey, and Stephen G. Monismith

Abstract

Techniques for the accurate estimation of the dissipation rate of temperature variance χT from temperature microstructure measurements are synthesized and evaluated. The techniques focus on the treatment of contamination in the integration of the vertical temperature gradient spectrum. The essential improvements come from estimating χT from the wavenumber domain in which the signal exceeds the noise and from reconstructing unresolved wavenumber contributions using the best-fit Batchelor spectrum. The improvement in the estimates of χT results in better Batchelor fits and more robust estimates of the dissipation rate of turbulent kinetic energy ɛ. High correlations of independent measurements of χT and ɛ from two collocated FP07 thermistors demonstrate the value of the techniques and highlight the improvements at the low end of the resolvable ranges of χT and ɛ.

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Mark T. Stacey, Stephen G. Monismith, and Jon R. Burau

Abstract

The authors present a field study of estuarine turbulence in which profiles of Reynolds stresses were directly measured using an ADCP throughout a 25-h tidal day. The dataset that is discussed quantifies turbulent mixing for a water column in northern San Francisco Bay that experiences a sequence of states that includes a weak ebb and flood that are stratified, followed by a strong, and eventually unstratified, ebb and flood. These measurements show that energetic turbulence is confined to a bottom mixed layer by the overlying stratification. Examination of individual Reynolds stress profiles along with profiles of Richardson number and turbulent Froude number shows that the water column can be divided into regions based on the relative importance of buoyancy effects.

Using the measured turbulence production rate P, the dissipation rate ϵ is estimated. The observed turbulence had values of ϵ/νN 2 > 20 all of the time and ϵ/νN 2 > 200 most of the time, suggesting that the observed motions were buoyancy affected turbulence rather than internal waves. However, at times, turbulent Froude numbers in much of the upper-water column were less than one, indicating important stratification effects. Taken as a whole, the data show that stratification affects the turbulent velocity variance q 2 most severely; that is, observed reductions in uw are largely associated with small values of q 2 rather than with a dramatic reduction in the efficiency with which turbulent motions produce momentum fluxes.

Finally, the dataset is compared to predictions made using the popular Mellor–Yamada level 2.5 closure. These comparisons show that the model tends to underestimate the turbulent kinetic energy in regions of strong stratification where the turbulence is strongly inhomogeneous and to overestimate the turbulent kinetic energy in weakly stratified regions. The length scale does not appear to compensate for these errors, and, as a result, similar errors are seen in the eddy viscosity predictions. It is hypothesized that the underestimation of q 2 is due to an inaccurate parameterization of turbulence self-transport from the near-bed region to the overlying stratification.

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Nicholas J. Nidzieko, James L. Hench, and Stephen G. Monismith

Abstract

A field experiment was conducted to examine stratified and unstratified curvature-generated lateral circulation and momentum balances in an estuarine tidal channel. Conductivity, temperature, depth, and current profiler data were collected vertically and laterally across the channel at a sharp bend over a fortnightly period to measure the terms of the lateral momentum budget. Well-mixed conditions allow the development of classic two-layer helical flow around a bend. Stratification strengthens curvature-induced lateral circulation, but the development of a lateral baroclinic pressure gradient opposes the resultant motions. The spatial and temporal response of this baroclinic pressure gradient is different than centrifugal acceleration, producing a three-layer profile. As the baroclinic term becomes stronger (or as centrifugal acceleration disappears as the flow exits the bend), two-layer flow with the opposite direction from curvature occurs. In both stratified and well-mixed conditions, downstream adjustment of lateral circulation (nonlinear advective acceleration) is of leading order in the lateral momentum budget; the depth-averaged term adjusts the streamline direction, while vertical deviations from the depth average account for changes in lateral circulation. The asymmetry of forcing mechanisms on flood and ebb, because of variations in stratification and strength of tidal flow, can strongly affect net lateral transport and generation of residual currents in regions of curvature.

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Hugo N. Ulloa, Kristen A. Davis, Stephen G. Monismith, and Geno Pawlak

Abstract

We examine temporal variability of thermally driven baroclinic cross-shore exchange in the context of a tropical fringing reef system focusing on the role of tidally driven alongshore flow. Ensemble diurnal phase averaging of cross-shore flow at the Kilo Nalu Observatory (KNO) in Oahu, Hawaii, shows a robust diurnal signal associated with an unsteady buoyancy/diffusive dynamic balance, although significant variability is observed at subdiurnal time scales. In particular, persistent fortnightly variability in the cross-shore diurnal flow pattern is consistent with modulation by the semidiurnal alongshore tidal flow. The alongshore flow plays a direct role in the cross-shore exchange momentum balance via Coriolis acceleration but also affects the cross-shore circulation indirectly via its influence on vertical turbulent diffusion. An idealized linear theoretical model for thermally driven cross-shore flow is formulated using the long-term time-averaged diurnal dynamic balance at KNO as a baseline. The model is driven at leading order by the surface heat flux, with contributions from the alongshore flow and cross-shore wind appearing as linear perturbations. Superposition of the idealized solutions for Coriolis and time-varying eddy viscosity perturbations are able to reproduce key aspects of the fortnightly variability. Modifying the model to consider a more realistic alongshore flow and considering effects of nightly convection lead to further improvements in comparisons with KNO observations. The ability of the theoretical approach to reproduce the fortnightly patterns indicates that semidiurnal variations in the alongshore flow are effective in modulating the cross-shore flow via Coriolis and vertical turbulent transport mechanisms.

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Ryan J. Lowe, James L. Falter, Stephen G. Monismith, and Marlin J. Atkinson

Abstract

The response of the circulation of a coral reef system in Kaneohe Bay, Hawaii, to incident wave forcing was investigated using field data collected during a 10-month experiment. Results from the study revealed that wave forcing was the dominant mechanism driving the circulation over much of Kaneohe Bay. As predicted theoretically, wave setup generated near the reef crest resulting from wave breaking established a pressure gradient that drove flow over the reef and out of the two reef channels. Maximum reef setup was found to be roughly proportional to the offshore wave energy flux above a threshold root-mean-square wave height of 0.7 m (at which height setup was negligible). On the reef flat, the wave-driven currents increased approximately linearly with incident wave height; however, the magnitude of these currents was relatively weak (typically <20 cm s−1) because of (i) the mild fore-reef slope of Kaneohe Bay that reduced setup resulting from a combination of frictional wave damping and its relatively wide surf zone compared to steep-faced reefs, and (ii) the presence of significant wave setup inside its coastally bounded lagoon, resulting from frictional resistance on the lagoon–channel return flows, which reduced cross-reef setup gradients by 60%–80%. In general, the dynamics of these wave-driven currents roughly matched predictions derived from quasi-one-dimensional mass and momentum balances that incorporated radiation stresses, setup gradients, bottom friction, and the morphological properties of the reef–lagoon system.

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Stephen G. Monismith, Liv M. M. Herdman, Soeren Ahmerkamp, and James L. Hench

Abstract

Observations of waves, setup, and wave-driven mean flows were made on a steep coral forereef and its associated lagoonal system on the north shore of Moorea, French Polynesia. Despite the steep and complex geometry of the forereef, and wave amplitudes that are nearly equal to the mean water depth, linear wave theory showed very good agreement with data. Measurements across the reef illustrate the importance of including both wave transport (owing to Stokes drift), as well as the Eulerian mean transport when computing the fluxes over the reef. Finally, the observed setup closely follows the theoretical relationship derived from classic radiation stress theory, although the two parameters that appear in the model—one reflecting wave breaking, the other the effective depth over the reef crest—must be chosen to match theory to data.

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