John H. Trowbridge

Evaluation of the Acoustic Doppler Velocimeter (ADV) for Turbulence Measurements

Abstract Accuracy of the acoustic Doppler velocimeter (ADV) is evaluated in this paper. Simultaneous measurements of open-channel flow were undertaken in a 17-m flume using an ADV and a laser Doppler velocimeter. Flow velocity records obtained by both instruments are used for estimating the true (“ground truth”) flow characteristics and the noise variances encountered during the experimental runs. The measured values are compared with estimates of the true flow characteristics and values of variance (〈u′2〉, 〈w′2〉) and covariance (〈u′w′〉) predicted by semiempirical models for open-channel flow. The analysis showed that the ADV sensor can measure mean velocity and Reynolds stress within 1% of the estimated true value. Mean velocities can be obtained at distances less than 1 cm from the boundary, whereas Reynolds stress values obtained at elevations greater than 3 cm above the bottom exhibit a variation that is in agreement with the predictions of the semiempirical models. Closer to the boundary, the measure...

Fluid-mud processes on the Amazon continental shelf

Abstract A sediment transport study conducted on the Amazon continental shelf as part of AmasSeds (A Multi-disciplinary Amazon Shelf SEDiment Study) revealed extensive regions of dense nearbed suspensions of sediment, known as fluid mud (suspended-sediment concentration > 10 g1−1 ). Fluid mud was found near the river mouth on the inner- and middle-shelf, in the region of the bottom salinity front, and was most extensive during periods of rising and high river discharge. Fluid mud, up to 7.25 m thick, but generally 1–2 m thick, appears to form by processes similar to those occurring at an estuarine turbidity maximum, i.e. enhanced settling and lateral convergence of near-bottom flows. A modeling study showed that vertical mixing was controlled by the suppression of turbulence, due to the stratification induced by suspended sediment, and established an upper bound for the total amount of suspended sediment that may be carried in suspension. Sediment leaving the Amazon River appears to go through cycles of trapping and resuspension at the river mouth, before being partially advected seaward and alongshelf, where it is largely incorporated into fluid mud along the bottom salinity front. The fluid muds have far-reaching effects on the Amazon shelf system by reducing boundary shear stresses, affecting water-column seabed exchange, and serving as the agent of outward growth of the subaqueous delta through episodic offshore transport.

ON A TECHNIQUE FOR MEASUREMENT OF TURBULENT SHEAR STRESS IN THE PRESENCE OF SURFACE WAVES

Abstract Surface waves can produce large biases in estimates of turbulent shear stress obtained from single-sensor measurements of velocity if there is even a small uncertainty in the orientation of either the velocity sensor or the principal axes of the wave-induced velocity field. The wave-induced bias can be diminished substantially by differencing measurements obtained from two velocity sensors separated by a distance larger than the correlation scale of the turbulence but small in comparison to the inverse wavenumber of the surface waves. If the scale separation is sufficiently large, then minus the density times half of the covariance between horizontal and vertical velocity differences is a nearly wave-free estimate of the average of the turbulent shear stresses at the two sensors. A theoretical analysis determines the bias associated with this technique under simplified conditions, in which waves and turbulence are uncorrelated and the waves are weakly nonlinear and narrow-banded in both frequency...

The Bottom Boundary Layer Over the Northern California Shelf

Abstract Moored temperature and shipboard CTD observations from a northern California coastal upwelling region reveal variable bottom mixed-layer heights that are typically 5–15 m, but occasionally exceed 50 m. Observations from Oregon, northern California, and Peru, indicate that in coastal upwelling regions, maximum bottom mixed-layer heights tend to increase with water depth over the shelf, but rarely exceed half the water depth. Over the northern California shelf the bottom mixed-layer height is shown to depend on the stratification, the current magnitude, and the current direction. The dependence on current direction tends to dominate the response, with thicker bottom mixed layers during poleward flows and thinner bottom mixed layers during equatorward flows. This asymmetric response to poleward and equatorward currents is consistent with model results which indicate that the asymmetric response is due to the up- or downslope Ekman transport of buoyancy along the bottom.

Asymmetric behavior of an oceanic boundary layer above a sloping bottom

Abstract The effects of stratification, planetary rotation and a sloping bottom combine to produce an asymmetric response in which the characteristics of an oceanic bottom boundary layer depend on the direction, in addition to the magnitude, of the along-isobath velocity in the inviscid interior. The asymmetric response is examined theoretically under idealized conditions in which the motion begins from rest, the flow is uniform in the along-isobath and cross-isobath directions, and the water column is initially uniformly stratified. The analysis is based on an integrated model, in which the bottom stress is determined from a quadratic drag law, and the height of the boundary layer is determined from a Pollard–Rhines–Thompson mixing criterion. The model indicates rapid mixing at the onset of forcing to a height limited by planetary rotation and interior stratification, followed by evolution in which the height of the boundary layer may either increase or remain fixed near its initial value, depending on t...

The Coupled Boundary Layers and Air–Sea Transfer Experiment in Low Winds

The Office of Naval Researchs Coupled Boundary Layers and Air–Sea Transfer (CBLAST) program is being conducted to investigate the processes that couple the marine boundary layers and govern the exchange of heat, mass, and momentum across the air–sea interface. CBLAST-LOW was designed to investigate these processes at the low-wind extreme where the processes are often driven or strongly modulated by buoyant forcing. The focus was on conditions ranging from negligible wind stress, where buoyant forcing dominates, up to wind speeds where wave breaking and Langmuir circulations play a significant role in the exchange processes. The field program provided observations from a suite of platforms deployed in the coastal ocean south of Marthas Vineyard. Highlights from the measurement campaigns include direct measurement of the momentum and heat fluxes on both sides of the air–sea interface using a specially constructed Air–Sea Interaction Tower (ASIT), and quantification of regional oceanic variability over sca...

Structure and dynamics of fluid muds on the Amazon Continental Shelf

Thick layers of fluid mud occur in strong tidal flows over the inner portion of the Amazon continental shelf in regions of strong salinity fronts associated with the plume discharged from the Amazon River. Detailed shipboard profile measurements obtained in this region during A Multidisciplinary Amazon Shelf Sediment Study (AMASSEDS) provide an unprecedented opportunity to examine the structure and dynamics of fluid muds under natural conditions. The analysis focuses on flows in which the motion is fully turbulent and suspended sediment dominates the stratification. Under these conditions a comparison of measurements and one-dimensional model calculations indicates that vertical transport is controlled by suppression of turbulent mixing at gradient Richardson numbers near 1/4. This constraint produces a distinctive vertical structure and leads to an upper bound on the total amount of suspended sediment that may be carried in a turbulent suspension by a tidal flow.

Near-bottom turbulence measurements in a partially mixed estuary : Turbulent energy balance, velocity structure, and along-channel momentum balance

Abstract A set of moored, bottom-mounted and shipboard measurements, obtained in a straight section of the lower Hudson estuary during late summer and early fall of 1995, determine velocity, density, and along-channel pressure gradient throughout the 15-m water column, as well as providing direct eddy-correlation estimates of Reynolds stress and indirect inertial-range estimates of dissipation within 3 m of the bottom. The analysis focuses on testing 1) a simplified turbulent kinetic energy equation, in which production balances dissipation; 2) the Prandtl–Karman law of the wall, which is a relationship between bottom stress and near-bottom velocity gradient; and 3) a simplified depth-integrated along-channel momentum balance involving local acceleration, pressure gradient, and bottom stress. Estimates of production and dissipation agree well throughout the entire record. The relationship between bottom stress and velocity gradient is consistent with the law of the wall within approximately 1 m of the sea...

The Direct Estimation of Near-Bottom Turbulent Fluxes in the Presence of Energetic Wave Motions

Abstract Velocities produced by energetic waves can contaminate direct covariance estimates of near-bottom turbulent shear stress and turbulent heat flux. A new adaptive filtering technique is introduced to minimize the contribution of wave-induced motions to measured covariances. The technique requires the use of two sensors separated in space and assumes that the spatial coherence scale of the waves is much longer than the spatial coherence scale of the turbulence. The proposed technique is applied to an extensive set of data collected in the bottom boundary layer of the New England shelf. Results from the oceanic test demonstrate that the technique succeeds at removing surface-wave contamination from shear stress and heat flux estimates using pairs of sensors separated in the vertical dimension by a distance of approximately 5 times the height of the lower sensor, even during the close passage of hurricanes. However, the technique fails at removing contamination caused by internal motions that occur oc...

Turbulence Measurements in the Surf Zone

Abstract Velocity measurements within 1 m of the bottom in approximately 4.5-m water depth on a sand beach provide estimates of turbulent Reynolds shear stress, using a dual-sensor technique that removes contamination by surface waves, and inertial-range estimates of dissipation. When combined with wave measurements along a cross-shore transect and nearby wind measurements, the dataset provides direct estimates of the terms in simplified equations for alongshore momentum and turbulence energetics and permits examination of semiempirical relationships between bottom stress and near-bottom velocity. The records are dominated by three events when the measurement site was in the outer part of the surf zone. Near-bottom turbulent shear stress is well correlated with (squared correlation coefficient r2 = 0.63), but smaller than (regression coefficient b = 0.51 ± 0.03 at 95% confidence), wind stress minus cross-shore gradient of wave-induced radiation stress, indicating that estimates of one or more of these ter...