Albert J. Williams

Estimates of Kinetic Energy Dissipation under Breaking Waves

Abstract The dissipation of kinetic energy at the surface of natural water bodies has important consequences for many Physical and biochemical processes including wave dynamics, gas transfer, mixing of nutrients and pollutants, and photosynthetic efficiency of plankton. Measurements of dissipation close to the surface obtained in a large lake under conditions of strong wind forcing are presented that show a layer of enhanced dissipation exceeding wall layer values by one or two orders of magnitude. The authors propose a scaling for the rate of dissipation based on wind and wave parameters, and conclude that the dissipation rate under breaking waves depends on depth, to varying degrees, in three stages. Very near the surface, within one significant height, the dissipation rate is high (an order of magnitude greater than that predicted by wall layer theory) and roughly constant. Below this is an intermediate region where the dissipation decays as z−2. The thickness of this layer (relative to the significant...

Bottom stress estimates and their prediction on the Northern California continental shelf during CODE-1: the importance of wave-current interaction

Abstract High quality near-bottom boundary layer measurements obtained at a midshelf location (90 m water depth) in the CODE region off Northern California are described. Bottom tripod velocity measurements and supporting data obtained during typical spring and early summer conditions (June 1981 during CODE-1) are analyzed to obtain bath velocity profiles and mean bottom stress and bottom roughness estimates. During the time period described, the mean near-bottom ( 0.997) approximately 30 percent of the time. Effects induced by unsteadiness from internal waves result in some degradation of the profiles (0.96≤R≤0.997) the rest of the time. Mean stress profiles indicate the logarithmic layer is approximately a constant-stress layer. The near-bottom flow field is Composed of mean currents and oscillatory currents due to well. Typing mean u* values estimated from measurements greater than 30 cm above the bottom have magnitudes of 0.5–1.0 cm s−1. Mean stress valu...

A Composite Spectrum of Vertical Shear in the Upper Ocean

Abstract Results from three separate velocity profilers operated nearly simultaneously in the northwest Atlantic in 1975 are used to form a composite shear spectrum over vertical wavelengths from 100 m down to a few centimeters. This exercise constitutes an intercomparison of the three different measurement techniques and reveals a shear spectrum which is approximately fiat at a WKB-scaled level from k = 0.01 cpm through k0 ≈ 0.1 cpm, then falls as k−1 to a buoyancy wavenumber k0 = (N3/ϵ)1/2 determined by the local average Vaisala frequency N and the volume-averaged dissipation rate ϵ. Various consequences of the observed shear spectral shape are explored.

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...

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...

Sediment-transport events on the northern California continental shelf during the 1990–1991 STRESS experiment

Abstract Measurements of currents and light transmission were made at bottom tripods and moorings arrayed across the northern California continental shelf along the Coastal Ocean Dynamics Experiment (CODE) “C” transect as part of the 1990–1991 Sediment Transport Events on Shelves and Slopes (STRESS) experiment. In combination with meteorological and wave data from the National Data Buoy Center Buoy 46013, these measurements provide information about the physical forcing and resultant resuspension and transport of bottom material between 21 November and 8 March. Sixteen events were identified in the wave, wind and current-meter records for this period. Only two were local storms with southerly winds, but they caused about half of the seasonal net transport. Seven were swell events that combined long-period waves generated by distant storms with local currents. At the 90-m site, swells interacted with the mean northward flow to produce northward transport. During six northerly wind events, upwelling-favorable winds often were sufficient to slow or reverse the mean northward flow and thus caused southward transport. A single current event, which produced moderate southward transport, was observed at the 130-m site. Net transport during the winter experiment was offshore at all sites, northward at the inner- and mid-shelf sites, but southward at the outer-shelf site. The results suggest that local storms with southerly winds may dominate seasonal transport, as on the Washington shelf, but significant transport also can occur during fair weather and during periods of northerly winds.

Vertical Structure of Dissipation in the Nearshore

Abstract The vertical structure of the dissipation of turbulence kinetic energy was observed in the nearshore region (3.2-m mean water depth) with a tripod of three acoustic Doppler current meters off a sandy ocean beach. Surface and bottom boundary layer dissipation scaling concepts overlap in this region. No depth-limited wave breaking occurred at the tripod, but wind-induced whitecapping wave breaking did occur. Dissipation is maximum near the surface and minimum at middepth, with a secondary maximum near the bed. The observed dissipation does not follow a surfzone scaling, nor does it follow a “log layer” surface or bottom boundary layer scaling. At the upper two current meters, dissipation follows a modified deep-water breaking-wave scaling. Vertical shear in the mean currents is negligible and shear production magnitude is much less than dissipation, implying that the vertical diffusion of turbulence is important. The increased near-bed secondary dissipation maximum results from a decrease in the tu...

Salt fingers observed in the mediterranean outflow.

Salt fingers, convection cells which constitute a possible mixing mechanism of the ocean, have now been observed in the Mediterranean outflow. The fingers, 6 millimeters in diamneter and 24 centimeters long, were photographed below the salinity maximum of the outflow, in an interface 22 centimeters thick where temperature and salinity decreased O. 15�C and 0.03 per mil, respectively.

Images of ocean microstructure

Abstract Temperature and salinity structures at the molecular diffisuon scale, which create index of refraction anomalies in the ocean, have been observed using the shadowgraph method in an instrument named “SCIMP”. Images of salt fingers — the convective cells driven by the differential diffusivities of heat and salt — were obtained in the Mediterranean Outflow, in the Caribbean, and in the deep Tyrrhenian Sea. The square-packed domains, which the counterflowing fingers form, converge the light in a pattern of bright vertical bands behind diagonal rows of high index fingers. Random alignment of the domains in a field of fingers makes it probable that some domain is aligned with its diagonal rows parallel to the optic axis. Folded interfaces of high gradient in temperature and salinity appeared in images recorded in the upper 300 m in the Caribbean and near the surface in the other regions. The shadowgraph image of a single interface is a line graded dark on the low index side and bright on the high index side. Curved interface surfaces produce an outline of the portion which lies parallel to the optic axis. Several are produced by deeply convoluted interfaces. Images of salt fingers or folded interfaces are an indication of mixing.

Budgets of turbulent kinetic energy and scalar variance in the continental shelf bottom boundary layer

A local turbulent kinetic energy (TKE) budget, in which dissipation balances production minus buoyancy flux, and a nonlocal turbulent scalar variance (TSV) budget, in which dissipation balances production minus TSV flux divergence, are tested observationally. The calculations are based on 6 week long records of velocity and sound speed measured between 0.7 and 5.4 m above bottom on the New England shelf. Estimates of fluxes and gradients are direct, while estimates of dissipation rates are indirectly obtained from inertial subrange spectra. Within 2 m of the bottom, buoyancy flux is not an important component of the TKE budget (flux Richardson number Rf 0.8 and regression coefficient = 0.89 ×/÷ 1.06 and 1.08 ×/÷ 1.05 at 1.65 and 0.74 m above the bottom, respectively). The local TKE budget at 4.35 m above the bed is not closed, however. TSV production and dissipation estimates are strongly correlated at all heights (r2>0.64), but dissipation is systematically greater than production, particularly at 0.74 m above the bed (regression coefficient = 2.42 ×/÷ 1.28). Including TSV flux divergence in the balance reduces, statistically significantly, the observed discrepancy. The implications of the results for turbulence closure models and microstructure diffusivity estimates are discussed.