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Williams, GJ, Sandin SA, Zgliczynski BJ, Fox MD, Gove JM, Rogers JS, Furby KA, Hartmann AC, Caldwell ZR, Price NN, Smith JE.  2018.  Biophysical drivers of coral trophic depth zonation. Marine Biology. 165   10.1007/s00227-018-3314-2   AbstractWebsite

Depth is used often as a proxy for gradients in energetic resources on coral reefs and for predicting patterns of community energy use. With increasing depth, loss of light can lead to a reduced reliance on autotrophy and an increased reliance on heterotrophy by mixotrophic corals. However, the generality of such trophic zonation varies across contexts. By combining high-resolution oceanographic measurements with isotopic analyses (delta C-13, delta N-15) of multiple producer and consumer levels across depths (10-30 m) at a central Pacific oceanic atoll, we show trophic zonation in mixotrophic corals can be both present and absent within the same reef system. Deep-water internal waves that deliver cool particulate-rich waters to shallow reefs occurred across all sites (2.5-5.6 events week(-1) at 30 m) but the majority of events remained depth-restricted (4.3-9.7% recorded at 30 m propagated to 10 m). In the absence of other particulate delivery, mixotrophs increased their relative degree of heterotrophy with increasing depth. However, where relatively long-lasting downwelling events (1.4-3.3 times the duration of any other site) occurred simultaneously, mixotrophs displayed elevated and consistent degrees of heterotrophy regardless of depth. Importantly, these long-lasting surface pulses were of a lagoonal origin, an area of rich heterotrophic resource supply. Under such circumstances, we hypothesize heterotrophic resource abundance loses its direct linkage with depth and, with resources readily available at all depths, trophic zonation is no longer present. Our results show that fine-scale intra-island hydrographic regimes and hydrodynamic connectivity between reef habitats contribute to explaining the context specific nature of coral trophic depth zonation in shallow reef ecosystems.

Smith, JA.  2015.  Revisiting oceanic acoustic gravity surface waves. Journal of Physical Oceanography. 45:2953-2958.: American Meteorological Society   10.1175/JPO-D-14-0256.1   Abstract

AbstractThe reintroduction of compressibility into the equations for surface gravity waves can permit mixed acoustic?gravity modes that are periodic in the vertical as well as horizontal directions. These modes interact with the bottom even in deep water, so bottom motion can excite them. Because they propagate rapidly, it has been suggested they may be useful as precursors of tsunamis. Here the equations are revisited, and, using some robust approximations, some physical understanding and interpretation of the phenomena are presented. It is posed that these new modes can alternatively be thought of as acoustic modes slightly modified by a gravity wave boundary condition at the surface, rather than as surface waves dramatically modified by compressibility. Their potential use is not diminished; indeed, this alternative perspective should help make implementation more practical.

Smith, JA, Pinkel R, Goldin MA.  2015.  A vertical slice of acoustic intensity and velocity [2-D images of waves and turbulence]. Current, Waves and Turbulence Measurement (CWTM), 2015 IEEE/OES Eleventh. :1-5.   10.1109/CWTM.2015.7098151   Abstract

Phased Array Doppler Sonars (PADS) have been developed at Scripps over the past 20 years. These provide sector-scan images of radial velocity and acoustic intensity over ranges of 0.1 m (medical ultra sound) to 1.5 km (ocean surface wave measurement). Most recently we've deployed a 200 kHz PADS from the R/V Revelle during EquatorMix (October 2012, at 140W on the Equator). The pie-shaped measurement area was oriented to probe a vertical slice under the ship parallel to the direction of travel, so that moving averages along the plane can be formed. At the surface, our speed over ground was near 1 m/s, while near 120 m depth, the core of the Equatorial Undercurrent exceeded our speed by about 0.5 m/s; i.e., the undercurrent was moving at 1.5 m/s relative to the surface, which was nearly stationary in an Earth-referenced frame. The objective was to image descending shear-layers that often occur in the early morning before sunrise, along with any associated turbulence or internal waves. Averages can be formed moving with the flow as a function of depth, yielding focused images of acoustic backscatter strength, Doppler bandwidth, and both sine- and cosine- weighted velocity estimates. Where conditions are favorable, the last pair can be recombined to yield horizontal and vertical velocities in some detail.

Smith, JA.  2014.  A bistatic phased-array Doppler sonar for wave-current research. Journal of Atmospheric and Oceanic Technology. 31:1628-1641.   10.1175/jtech-d-13-00187.1   AbstractWebsite

Wave breaking and wave-forced flows are important to air-sea interactions and to the transport and dispersal of materials at sea. But recent measurements have shown a discrepancy in the Eulerian response to wave groups compared to scientists' current theoretical understanding of wave-current interactions. Flow structures on scales of centimeters to meters occur underneath breaking waves, and larger-scale flows are driven by wave-current interactions (e.g., Langmuir circulation, alongshore flows). Such details of the vertically resolved flow are just beginning to be modeled, and observational guidance is needed. Here a new instrument is described that is intended to measure waves and currents over a 2D vertical plane underwater, resolving two components of velocity on this plane. Initial observations were made near the Scripps Pier (La Jolla, California), where steep waves and strong currents can be reliably found, yet logistics are not too burdensome. To get the spatial resolution desired using 200-kHz sound, ping-to-ping "coherent processing" would have be used for Doppler estimation; however, near shore the reverberations remain too strong for far too long to get any coherence, unlike the previous experience in deep water. In view of this, using much higher frequencies (>1 MHz) with "incoherent processing" is suggested; the increased attenuation at higher frequencies then would subdue the reverberation problem, but with comparable space-time resolution.

Smith, JA, Pinkel R, Goldin M, Sun O, Nguyen S, Hughen T, Bui M, Aja A.  2012.  Wirewalker dynamics. Journal of Atmospheric and Oceanic Technology. 29:103-115.   10.1175/jtech-d-11-00049.1   AbstractWebsite

A wirewalker exploits the difference in vertical motion between a wire attached to a surface buoy and the water at the depth of a profiling body to provide the power to execute deep profiles: when the wire's relative motion is upward, the profiler lets go; when it is downward, the profiler clamps on, and the weight attached at depth pulls the wire down, dragging the profiler downward against its buoyancy. The difference between the upward wire and profiler motion has to exceed the buoyancy-driven upward acceleration of the profiler body for this to work. Because the relative motion of the wire and water decreases as the surface is approached, the profiler might get stuck near the surface, especially when it is calm. However, two things mitigate this: 1) the system has a damped resonant response (similar to 1.3 Hz), which induces relative motion between the buoy and water even at the surface; and 2) for waves too gentle to directly exceed the required acceleration, drag on the profiler can pull the clamped-together system down sufficiently that the buoy and wire without the profiler attached can suddenly release and bob upward faster than the profiler. For system parameters as estimated here, the latter requires submersion of less than 0.005 m below its equilibrium depth. Several such "bounces" can occur over a portion of the wave phase. These two effects explain why, in practice, the profiler does not stay long near the surface (although it does proceed downward a bit more slowly there).

Pinkel, R, Goldin MA, Smith JA, Sun OM, Aja AA, Bui MN, Hughen T.  2011.  The Wirewalker: a vertically profiling instrument carrier powered by ocean waves. Journal of Atmospheric and Oceanic Technology. 28:426-435.   10.1175/2010jtecho805.1   AbstractWebsite

Ocean wave energy is used to drive a buoyant instrument platform down a wire suspended from a surface float. At the lower terminus of the profiling range, the cam that rectifies wave vertical motion is released and the package, termed the Wirewalker, free ascends. No electronic components are used in the profiler, and only a few moving parts are involved. The Wirewalker is tolerant of a broad range of payloads: the ballast is adjusted by adding discrete foam blocks. The Wirewalker profiles 1000-3000 km month(-1), vertically, with typical missions lasting from days to months. A description of the profiler is presented along with a discussion of basic profiling dynamics.

Smith, JA, Brulefert C.  2010.  Evolution of persistent wave groups. Journal of Physical Oceanography. 40:67-84.   10.1175/2009jpo4149.1   AbstractWebsite

During the near-field leg of the Hawaiian Ocean-Mixing Experiment (HOME-NF), short, steep surface wave groups were observed that elicited strong group-forced responses in the wave-filtered surface current field, as reported by Smith. Some of these wave groups persisted for 17 wave periods, yet were only about 1 wavelength long in the along-wind direction. Here, the authors consider the evolution of wave groups of the form observed and find that this persistence is consistent with linear dispersion in spite of the very compact form. The key aspects enhancing persistence are 1) that the wave crests within the group are oriented at an angle with respect to the group envelope and 2) they are much wider in the crosswind direction than along-wind (in the example examined in detail, about 5 times). According to a simplified model, groups with the observed 5-to-1 aspect ratio and this "slant-wave'' structure can persist for up to 20 wave periods, consistent with the observations (cf. 8 periods for a collinear wave group). The maximum persistence increases in proportion to the across-wind length of the group.

Tejada-Martinez, AE, Grosch CE, Gargett AE, Polton JA, Smith JA, MacKinnon JA.  2009.  A hybrid spectral/finite-difference large-eddy simulator of turbulent processes in the upper ocean. Ocean Modelling. 30:115-142.   10.1016/j.ocemod.2009.06.008   AbstractWebsite

A three-dimensional numerical model for large-eddy simulation (LES) of oceanic turbulent processes is described. The numerical formulation comprises a spectral discretization in the horizontal directions and a high-order compact finite-difference discretization in the vertical direction. Time-stepping is accomplished via a second-order accurate fractional-step scheme. LES subgrid-scale (SGS) closure is given by a traditional Smagorinsky eddy-viscosity parametrization for which the model coefficient is derived following similarity theory in the near-surface region. Alternatively, LES closure is given by the dynamic Smagorinsky parametrization for which the model coefficient is computed dynamically as a function of the flow. Validation studies are presented demonstrating the temporal and spatial accuracy of the formulation for laminar flows with analytical solutions. Further validation studies are described involving direct numerical simulation (DNS) and LES of turbulent channel flow and LES of decaying isotropic turbulence. Sample flow problems include surface Ekman layers and wind-driven shallow water flows both with and without Langmuir circulation (LC) generated by wave effects parameterized via the well-known Craik-Leibovich (C-L) vortex force. In the case of the surface Ekman layers, the inner layer (where viscous effects are important) is not resolved and instead is parameterized with the Smagorinsky models previously described. The validity of the dynamic Smagorinsky model (DSM) for parameterizing the surface inner layer is assessed and a modification to the surface stress boundary condition based on log-layer behavior is introduced improving the performance of the DSM. Furthermore, in Ekman layers with wave effects, the implicit LES grid filter leads to LC subgrid-scales requiring ad hoc modeling via an explicit spatial filtering of the C-L force in place of a suitable SGS parameterization. (C) 2009 Elsevier Ltd. All rights reserved.

Polton, JA, Smith JA, MacKinnon JA, Tejada-Martinez AE.  2008.  Rapid generation of high-frequency internal waves beneath a wind and wave forced oceanic surface mixed layer. Geophysical Research Letters. 35   10.1029/2008gl033856   AbstractWebsite

High-frequency internal waves generated by Langmuir motions over stratified water may be an important source of turbulent mixing below the surface mixed layer. Large eddy simulations of a developing mixed layer and inertial current are employed to investigate this phenomena. Uniform surface wind stress and parallel Stokes drift wave forcing rapidly establishes a turbulent mixed-layer flow, which ( as the inertial motion veers off the wind) generates high-frequency internal waves in the stratified fluid below. The internal waves evolve such that their vector phase velocity matches the depth-averaged mixed-layer velocity that rotates as an inertial oscillation. The internal waves drain energy and momentum from the mixed layer on decay time-scales that are comparable to those of near-inertial oscillations. The high-frequency waves, which are likely to be trapped in the transition layer, may significantly contribute to mixing there and thus provide a potentially important energy sink for mixed-layer inertial motions.

Smith, JA.  2008.  Vorticity and divergence of surface velocities near shore. Journal of Physical Oceanography. 38:1450-1468.   10.1175/2007jpo3865.1   AbstractWebsite

The nearshore environment is complex, with many competing dynamical elements. Surface waves and edge waves (a form of surface wave trapped to the shore) can generally be separated from other forms of motion because of their fast propagation speeds. However, other motions such as internal waves, shear waves, density flows, and isolated vortex pairs can move at comparable speeds. A tool to help separate these dynamical elements is decomposition of the surface 2D flow into two parts, one nondivergent and the other irrotational (solenoidal and potential flows, respectively). Here, an efficient algorithm for this separation is developed and applied, and two examples are examined from data taken at Duck, North Carolina, in 1997 as part of the SandyDuck experiment. The first example is a fresher-water density flow propagating downcoast (probably from the Chesapeake Bay). It is seen that 1) the wave-driven alongshore flow leads the flow, generating a "surge" of offshore surface flow in its wake; 2) the isolation of the irrotational (2D divergent) part of the flow permits estimates of some dynamical characteristics of the flow; and 3) the nondivergent part of the flow indicates a meander in the alongshore flow that moves downcoast with the surge. The second example is a hypothesized form of isolated vortical structure, such as might be generated by a pulsed rip current that detaches from the shore and bottom and coasts offshore some distance before dissipating. A kinematically self-consistent structure is formulated that would have both divergence and vorticity fields associated with it. However, the observations inspiring the hypothesis are inconclusive, so the existence of such a structure has not been verified.

Apotsos, A, Raubenheimer B, Elgar S, Guza RT, Smith JA.  2007.  Effects of wave rollers and bottom stress on wave setup. Journal of Geophysical Research-Oceans. 112   10.1029/2006jc003549   AbstractWebsite

[1] Setup, the increase in the mean water level associated with breaking waves, observed between the shoreline and about 6-m water depth on an ocean beach is predicted well by a model that includes the effects of wave rollers and the bottom stress owing to the mean flow. Over the 90-day observational period, the measured and modeled setups are correlated ( squared correlation above 0.59) and agree within about 30%. Although rollers may affect setup significantly on beaches with large-amplitude ( several meters high) sandbars and may be important in predicting the details of the cross-shore profile of setup, for the data discussed here, rollers have only a small effect on the amount of setup. Conversely, bottom stress ( calculated using eddy viscosity and undertow formulations based on the surface dissipation, and assuming that the eddy viscosity is uniform throughout the water column) significantly affects setup predictions. Neglecting bottom stress results in underprediction of the observed setup in all water depths, with maximum underprediction near the shoreline where the observed setup is largest.

Smith, JA.  2006.  Observed variability of ocean wave Stokes drift, and the Eulerian response to passing groups. Journal of Physical Oceanography. 36:1381-1402.   10.1175/jpo2910.1   AbstractWebsite

Waves and currents interact via exchanges of mass and momentum. The mass and momentum fluxes associated with surface waves are closely linked to their Stokes drift. Both the variability of the Stokes drift and the corresponding response of the underlying flow are important in a wide range of contexts. Three methods are developed and implemented to evaluate Stokes drift from a recently gathered oceanic dataset, involving surface velocities measured continually over an area 1.5 km in radius by 45 degrees. The estimated Stokes drift varies significantly, in line with the occurrence of compact wave groups, resulting in highly intermittent maxima. One method also provides currents at a fixed level (Eulerian velocities). It is found that Eulerian counterflows occur that completely cancel the Stokes drift variations at the surface. Thus, the estimated Lagrangian surface flow has no discernable mean response to wave group passage. This response is larger than anticipated and is hard to reconcile with current theory.

Smith, JA.  2006.  Wave-current interactions in finite depth. Journal of Physical Oceanography. 36:1403-1419.   10.1175/jpo2911.1   AbstractWebsite

The energy, momentum, and mass-flux exchanges between surface waves and underlying Eulerian mean flows are considered, and terms in addition to the classical wave "radiation stress" are identified. The formulation is made in terms of the vertically integrated flow. The various terms are identified with other analyses and interpreted in terms of physical mechanisms, permitting reasonable estimates of the associated depth dependencies. One term is identified with the integrated "CL vortex force" implemented, for example, in simulations of Langmuir circulation. However, as illustrated with a simple example of steady refraction across a shear zone, other terms of the same order can significantly alter the results. The classic example of long waves forced by short-wave groups is also revisited. In this case, an apparent singularity arising in shallow water is countered by finite-amplitude dispersion corrections, these being formally of the same order as the forced long-wave response, and becoming significant or dominant as shallow water is approached.

D'Spain, GL, Terrill E, Chadwell CD, Smith JA, Lynch SD.  2006.  Active control of passive acoustic fields: Passive synthetic aperture/Doppler beamforming with data from an autonomous vehicle. Journal of the Acoustical Society of America. 120:3635-3654.   10.1121/1.2346177   AbstractWebsite

The maneuverability of autonomous underwater vehicles (AUVs) equipped with hull-mounted arrays provides the opportunity to actively modify received acoustic fields to optimize extraction of information. This paper uses ocean acoustic data collected by an AUV-mounted two-dimensional hydrophone array, with overall dimension one-tenth wavelength at 200-500 Hz, to demonstrate aspects of this control through vehicle motion. Source localization is performed using Doppler shifts measured at a set of receiver velocities by both single elements and a physical array. Results show that a source in the presence of a 10-dB higher-level interferer having exactly the same frequency content (as measured by a stationary receiver) is properly localized and that white-noise-constrained adaptive beamforming applied to the physical aperture data in combination with Doppler bearnforming provides greater spatial resolution than physical-aperture-alone bearnforming and significantly lower sidelobes than single element Doppler beamforming. A new broadband beamformer that adjusts for variations in vehicle velocity on a sample by sample basis is demonstrated with data collected during a high-acceleration maneuver. The importance of including the cost of energy expenditure in determining optimal vehicle motion is demonstrated through simulation, further illustrating how the vehicle characteristics are an integral part of the signal/array processing structure. (c) 2006 Acoustical Society of America.

Smith, JA.  2002.  Continuous time-space sampling of near-surface velocities using sound. Journal of Atmospheric and Oceanic Technology. 19:1860-1872.   10.1175/1520-0426(2002)019<1860:ctsson>;2   AbstractWebsite

A phased- array Doppler sonar (PADS) system is described that uses sound at frequencies near 200 kHz to probe an area several hundred meters on a side with 7- 20- m spatial resolution. The area can be sampled every second or less with under 2 cm s(-1) rms velocity error per sample. Radial velocity estimates from two or more systems can be combined to produce time series of horizontal velocity vector maps over the irregularly shaped overlapping region. Such extensive and continuous sampling in time and space permits analysis via direct 3D Fourier transformation, for example, producing complete wavenumber- frequency spectra. Free waves, bound harmonics, finite- amplitude effects, Doppler shifting by currents, etc., can be studied. Extended temporal sampling permits investigations into lower- frequency vortical and internal wave modes as well as surface waves, and of the modulation of these by tides. A pair of PADS was deployed as part of SandyDuck, a large collaborative field experiment held in 1997 near Duck, North Carolina. An example drawn from SandyDuck data illustrates use of the technique, demonstrating that both mean flow and oscillatory (wave) motions can be detected.

Smith, JA.  2002.  The use of phased-array Doppler sonars near shore. Journal of Atmospheric and Oceanic Technology. 19:725-737.   10.1175/1520-0426(2002)019<0725:tuopad>;2   AbstractWebsite

Phased-array Doppler sonars (PADS) have been used to probe an area several hundred meters on a side with 8-m spatial resolution, sampling every second or less with under 2 cm s(-1) rms velocity error per sample. Estimates from two systems were combined to produce horizontal velocity vectors. Here, concerns specific to use of PADS in shallow water are addressed. In particular, the shallower the water is, the larger the fraction of bottom backscatter, so the stronger the bias is toward zero Doppler shift in the estimates. First, direct comparisons are made with other current measurements made during the multi-investigator field experiment "SandyDuck,'' sponsored by the Office of Naval Research, which took place in the autumn of 1997 off the coast of Duck, North Carolina. The coherences between PADS and in situ current measurements are high, but the amplitude of the sonar response is generally low. To explore this further, a simplified model of wave shoaling is developed, permitting estimates of wave-frequency velocity variances from point measurements to be extrapolated over the whole field of view of PADS for comparison. The resulting time-space movies of sonar response are consistent with quasi-steady acoustic backscatter intensity from the bottom competing with a variable backscatter level from the water volume. The latter may arise, for example, from intermittent injection of bubbles by breaking waves, producing patches of high or low acoustic response that advect with the mean flow. Once this competition is calibrated via the surface wave variance comparison, instantaneous measured total backscatter intensities can be compared with an estimated bottom backscatter level (which is updated on a longer timescale, appropriate to evolution of the water depth or bottom roughness) to provide corrected sonar estimates over the region.

Smith, JA.  2001.  Observations and theories of Langmuir circulation: a story of mixing. Fluid mechanics and the environment : dynamical approaches : a collection of research papers written in commemoration of the 60th birthday of Sidney Leibovich. ( Leibovich S, Lumley JL, Eds.).:295-314., Berlin ; New York: Springer Abstract
Smith, JA.  2001.  Phased-array Doppler sonar measurements of near-surface motion: Langmuir circulation, surface waves, and breaking. Acoustical Oceanography, Proceedings of the Institute of Acoustics. ( Leighton TG, Heald GJ, Griffiths H, Eds.).:163-170. Abstract
Smith, JA.  1999.  Doppler sonar observations of Langmuir circulation. Air-sea exchange : physics, chemistry, and dynamics. ( Geernaert GL, Ed.).:539-555., Dordrecht ; Boston: Kluwer Academic Publishers Abstract
Smith, JA.  1999.  Observations of wind, waves, and the mixed layer: the scaling of surface motion. The Wind-Driven Air-Sea Interface. ( Banner ML, Ed.).:231-238., Sydney, Australia: University of New South Wales Abstract
Smith, JA.  1998.  Evolution of Langmuir circulation during a storm. Journal of Geophysical Research-Oceans. 103:12649-12668.   10.1029/97jc03611   AbstractWebsite

Wind stress, waves, stratification, velocity profiles, and surface fields of radial velocity and acoustic backscatter intensity were measured along a drift track 50 to 15O km off Point Arguello, California. On March 8, 1995, the wind increased from calm to 12 m/s from the SE, opposing swell from the NW. It increased to 15 m/s at noon UTC on March 9, remained steady over the next 12 hours, briefly dropped and veered by 60 degrees, then returned. A mixed layer deepened quickly to 25 m, then held roughly steady through the next 2 days, in spite of gusty winds continuing at 15-25 m/s. A phased-array Doppler sonar system took measurements covering 250 m by 150 m of the surface, with 5 m by 10 m spatial resolution. Averages over 6 min removed surface waves, permitting continuous assessment of strength, orientation, spacing, and degree of organization of features associated with surface motion (e.g., Langmuir circulation), even when conditions were top rough for vi:sual assessment. Several results stand out: (1) As found previously, most wind mixing arises from inertial shear across the thermocline. (2) Consistent with wind/wave forcing of Langmuir circulation, Plueddemann et al. [1996] suggest that surface velocity variance < V-2 > scales like (u*U-s), where u* is friction velocity and U-s is the surface Stokes' drift; however, the measurements here scale with (U-s)2 alone, once Langmuir circulation is established. (3) The velocity variance is weaker here than expected, given the magnitudes of wind and waves, leading to a smaller estimated mixing effect. (4) Large vacillations in LC strength are seen just before the brief veering of the wind; it is suggested that bubble buoyancy could play a dynamic role. (5) Mean orientation and spacing can differ for intensity versus radial velocity features.

Rieder, KF, Smith JA.  1998.  Removing wave effects from the wind stress vector. Journal of Geophysical Research-Oceans. 103:1363-1374.   10.1029/97jc02571   AbstractWebsite

The presence of ocean surface waves hats been observed to affect both the magnitude and direction of the wind stress. Here concurrent wind and wave data are employed to study their relationship. To help isolate the influence of the waves, the wind stress is broken into three frequency bands: "low" (frequencies below 0.06 Hz), corresponding to large-scale motions in the boundary layer at frequencies below any significant wave energy; "middle" (frequencies between 0.06 and 0.16 Hz), corresponding to the frequencies of the dominant swell and wind waves; and "high" (frequencies above 0.16 Hz), corresponding to waves too short to influence coherently the wind fluctuations at the anemometer site 8 m above the surface. Most often, the low band holds the most stress. The magnitude of the wind stress within the low band increases roughly with the square of the mean wind speed, the high band appears to increase with the wind speed to the fourth power, and the middle band exhibits varied dependence. The direction of the wind stress in the low band is closely tied to the mean wind direction. In contrast, the directions in the middle and high bands are influenced by the waves and can be significantly off the mean wind direction. The middle band is biased toward the direction of long-period swell, while the high band is biased toward the direction of short-period seas, which is closer to the wind direction. Thus it is mainly within the middle band that large deviations in stress versus wind magnitude and direction are found. To further isolate the influence of waves, a wave-correlated fraction of the wind stress is estimated using direct correlations between the surface elevation and wind fluctuations. Removing this wave-correlated stress from the total results in a residual stress that is better behaved: the magnitude of the residual stress in the middle band is modeled by a simple wind speed dependent drag coefficient, and the direction is very nearly aligned with the wind in both the middle and high bands. These results indicate that waves are indeed closely associated with the observed deviations from "bulk formula" stress estimates. They also suggest a new method by which to estimate the wind stress; namely, partitioning the stress into three separately modeled parts: a low-frequency stress, a high-frequency wave-correlated stress, and a high-frequency residual stress.

Smith, JA, Rieder KF.  1997.  Wave induced motion of FLIP. Ocean Engineering. 24:95-110.   10.1016/0029-8018(96)00008-x   AbstractWebsite

The value of data gathered from R/P FLIP is enhanced if the motion of FLIP is described. At surface wave frequencies, FLIP's motion can be estimated from measurements of apparent acceleration and heading. This minimal set of measurements is often available in past data sets (e.g., from the Surface Wave Process Program, or SWAPP). In addition, the accelerometer data is often of superior quality, with less noise and greater dynamic range than other available data. The challenge is to partition the motion between true horizontal acceleration and tilt. For this, a quasi-linear dynamic model of FLIP's response to forcing by surface waves is developed, including added mass and drag terms. The model is refined on the basis of comparisons with more extensive motion data, gathered over a variety of wind and wave conditions: estimates of FLIP's tilt from measurements of the earth's magnetic flux vector, combined with gyro-compass heading, and horizontal velocity at 35 m estimated from surface-scanning Doppler sonars. The overall agreement between model estimates derived from accelerometer data and the others is good. Copyright (C) 1996 Elsevier Science LtdSee correction: vol 24(5), p. 497

Rieder, KF, Smith JA, Weller RA.  1996.  Some evidence of colinear wind stress and wave breaking. Journal of Physical Oceanography. 26:2519-2524.   10.1175/1520-0485(1996)026<2519:seocws>;2   AbstractWebsite

Data collected during the Surface Waves and Processes Program are employed to investigate a possible interrelation between wind stress and surface wave breaking. From comparison of data from 15 half-hour long time segments, the directions of the wind stress and the whitecap motion are observed to be generally colinear, with both lying between the mean wind and the swell. As well, a nondimensionalized whitecap speed is found to correlate with the drag coefficient. These results suggest that the magnitude and direction of the wind stress might be estimated from wave breaking information.See correction: vol 27(1), p. 213

Plueddemann, AJ, Smith JA, Farmer DM, Weller RA, Crawford WR, Pinkel R, Vagle S, Gnanadesikan A.  1996.  Structure and variability of Langmuir circulation during the Surface Waves Processes Program. Journal of Geophysical Research-Oceans. 101:3525-3543.   10.1029/95jc03282   AbstractWebsite

A cooperative, multiplatform field experiment was conducted in the eastern North Pacific during February and March of 1990 as part of the Surface Waves Processes Program (SWAPP). One of the experimental objectives was to investigate Langmuir circulation so that its role in the evolution of the oceanic surface boundary layer could be better understood. The concurrent use of different observational techniques, ranging from simple surface drifters to complex Doppler sonar systems, resulted in new information about Langmuir circulation structure and variability. Estimates of Langmuir cell spacing indicated that a broad range of scales, from about 2 to 200 m, was excited during periods of strong surface forcing and that the energy containing scales evolved with time. Estimates of cell spacing based on Doppler velocities from a surface-scanning sonar directed crosswind showed this scale evolution, but estimates based on backscattered intensity did not. This was attributed to the fact that the intensity-based estimates were only indirectly related to circulation strength. The near-surface convergent velocities from the sonar were used to form an objective, quantitative measure of the temporal variations in Langmuir circulation strength. As expected, the circulation strength increased dramatically during strong wind events. However, circulation strength and wind stress did not decrease simultaneously, and Langmuir circulation was detectable for up to a day after abrupt reductions in wind stress. Energy from the surface wave field, which decayed more slowly than the wind, was apparently responsible for maintaining the circulation. The variation of circulation strength was found to be better related to (u*U-s)(1/2) than to u*, where u* = (tau/rho)(1/2) is the friction velocity, tau is the wind stress, and U-s is the surface wave Stokes drift. This scaling is consistent with wave-current interaction theories of Langmuir cell generation.