Eugene A. Terray

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...

Oceanic Turbulence Dissipation Measurements in SWADE

Abstract Recent experiments measuring turbulence dissipation rates in the upper ocean can be divided into two types: those supporting an analogy between the upper ocean and lower atmosphere, with dissipation rates following wall layer behavior, and those finding oceanic dissipation rates to be much higher than wall layer predictions. In an attempt to reconcile these two diverse acts of observations, Terray et al. proposed a wave-dependent scaling of the dissipation rate based on the significant wave height and the rate of energy input from the wind to the waves. Their parameterization was derived from observations of strongly forced, fetch-limited waves, although they conjectured that it would apply in typical oceanic conditions as well. This paper reports new measurements of turbulent kinetic energy dissipation made in the North Atlantic Ocean from a SWATH ship during the recent Surface Waves Dynamics Experiments (SWADE).These data support the scaling of Terray et al., verifying its validity when applied...

Wave-Turbulence Interactions in the Upper Ocean. Part II. Statistical Characteristics of Wave and Turbulent Components of the Random Velocity Field in the Marine Surface Layer

Abstract We present the results of an analysis of field data collected by Donelan who used a miniature drag sphere to measure velocities beneath wind waves on Lake Ontario. Linear statistical techniques are used to separate the velocity into wave and turbulent parts. While we mostly aim at demonstrating the effects of surface wind waves on the statistical characteristics of the turbulent field in the upper mixed layer, we also interpret several features of the data on the hags of recent theoretical results. One of the most intriguing features of the turbulent velocity spectra so obtained is a large peak near the dominant wave frequency. We review various possible explanation for this behavior although we prefer a model in which the turbulence is assumed frozen on the timescale of the Waves. This model requires no new dynamics and gives explicit formulae relating the dissipation rate to the magnitude of the spectral densities for high and low frequencies. On this basis we have determined a dissipation leng...

Variation in Surface Turbulence and the Gas Transfer Velocity over a Tidal Cycle in a Macro-tidal Estuary

The gradient flux technique, which measures the gas transfer velocity (k), and new observational techniques that probe turbulence in the aqueous surface boundary layers were conducted over a tidal cycle in the Plum Island Sound, Massachusetts. Efforts were aimed at testing new methods in an estuarine system and to determine if turbulence created by tidal velocity can be responsible for the short-term variability ink. Measurements were made during a low wind day, at a site with tidal excursions of 2.7 m and a range in tidal velocity of nearly 1 m s−1. Estimates ofk using the gradient flux technique were made simultaneously with the Controlled Flux Technique (CFT), infrared imagery, and high-resolution turbulence measurements, which measure the surface renewal rate, turbulent scales, and the turbulent dissipation rate, respectively. All measurements were conducted from a small mobile catamaran that minimizes air- and water-side flow distortions. Infrared imagery showed considerable variability in the turbulent scales that affect air-water gas exchange. These measurements were consistent with variation in the surface renewal rate (range 0.02 to 2 s−1), the turbulent dissipation rate (range 10−7 to 10−5 W kg−1), andk (range 2.2 to 12.0 cm hr−1). During this low wind day, all variables were shown to correlate with tidal speed. Taken collectively our results indicate the promise of these methods for determining short-term variability in gas transfer and near surface turbulence in estuaries and demonstrate that turbulent transport associated with tidal velocity is a potentially important factor with respect to gas exchange in coastal systems.

ASIS—A New Air–Sea Interaction Spar Buoy: Design and Performance at Sea

Abstract This paper describes a new, compact buoy, the Air–Sea Interaction Spar (ASIS), capable of reliably and accurately measuring directional wave spectra, atmospheric surface fluxes, and radiation in the the open ocean. The ASIS buoy is a stable platform and has low flow disturbance characteristics in both atmospheric and oceanic surface boundary layers. The buoy has been deployed for sea trials in the waters off Miami, Florida; in the northeastern region of the Gulf of Mexico; and in the northwestern Mediterranean. The acquired measurements of directional wave spectra, momentum and heat fluxes, and profile data—as well as general meteorological and oceanographic parameters—obtained from the buoy are well suited for enhancing research on air–water interfacial processes, wave dynamics, remote sensing, and gas transfer. In this paper the design is described and the performance of the buoy using field data is characterized.

Performance of a broad-band acoustic Doppler current profiler

A new broadband acoustic Doppler current profiler (ADCP) is described, with a useful range comparable to that of a commercially available narrowband (incoherent) system of the same acoustic frequency, but having enhanced performance. The extra performance may be traded off among (1) reduced velocity variance, (2) reduced averaging time, and (3) finer depth resolution. This improvement permits the observation of phenomena with smaller time and space scales than is now possible with available ADCPs. An expression predicting r.m.s. velocity error in terms of system parameters and the measured acoustic data is given and is shown to be consistent with the independently measured velocity error among redundant beams. Two major sources of bias error in incoherent ADCPs are shown to be much reduced for the broadband system. Field data demonstrating the improved performance over the existing incoherent ADCP are shown for cases of both strong and weak shear. >

Observations of Turbulence in the Ocean Surface Boundary Layer: Energetics and Transport

Observations of turbulent kinetic energy (TKE) dynamics in the ocean surface boundary layer are presented here and compared with results from previous observational, numerical, and analytic studies. As in previous studies, the dissipation rate of TKE is found to be higher in the wavy ocean surface boundary layer than it would be in a flow past a rigid boundary with similar stress and buoyancy forcing. Estimates of the terms in the turbulent kinetic energy equation indicate that, unlike in a flow past a rigid boundary, the dissipation rates cannot be balanced by local production terms, suggesting that the transport of TKE is important in the ocean surface boundary layer. A simple analytic model containing parameterizations of production, dissipation, and transport reproduces key features of the vertical profile of TKE, including enhancement near the surface. The effective turbulent diffusion coefficient for heat is larger than would be expected in a rigid-boundary boundary layer. This diffusion coefficient is predicted reasonably well by a model that contains the effects of shear production, buoyancy forcing, and transport of TKE (thought to be related to wave breaking). Neglect of buoyancy forcing or wave breaking in the parameterization results in poor predictions of turbulent diffusivity. Langmuir turbulence was detected concurrently with a fraction of the turbulence quantities reported here, but these times did not stand out as having significant differences from observations when Langmuir turbulence was not detected.

Turbulent transport in the outer region of rough-wall open-channel flows: the contribution of large coherent shear stress structures (LC3S)

Author Posting.

The dependence of microwave backscatter from the sea on illuminated area: Correlation times and lengths

During the Synthetic Aperture Radar (SAR) and X band Ocean Nonlinearities-Forschungsplattform Nordsee experiment, we mounted two continuous wave microwave systems on an elevator on the German Research Platform Nordsee for the purpose of investigating the dependence of microwave backscatter from the sea surface on illuminated area. The two systems operated at X and Ka bands (10 and 35 GHz) and collected HH and VV polarized backscattered signals simultaneously. The elevator system allowed us to vary the altitude of the two microwave systems above the sea surface from 7.5 to 27 m, always in the far field of the antennas. Most data were collected at a 45° incidence angle, which implied that the Ka band system illuminated areas from 0.4 to 6.0 m2 while the X band system viewed spots between 2.9 and 41.3 m2. We examined the dependence of the normalized radar cross section (σ0), its variance, and the bandwidth of the Doppler spectrum on illuminated areas. We were unable to detect any dependence of σ0 on area but found a definite decrease in its variance as area increased. At X band the variance divided by the square of σ0, the normalized variance, decreased from values near 12 for small areas to values near 2 for large areas. At Ka band, corresponding values were 40 and 2. The normalized variance was always slightly larger for HH polarization. By fitting the area dependence of the normalized variance to available theory, we deduce that correlation lengths are on the order of 10 times the microwave wavelength at both X and Ka band. Values for the normalized variance of an elementary scattering facet were also inferred and are presented in this paper. From the Doppler bandwidths we obtained radial velocity spreads over the illuminated areas and found that they agreed well at X and Ka band. These velocity spreads, which are inversely proportional to the correlation time of the backscatter, increased rapidly with illuminated area for small areas but tended to level off to values of about 0.5 m s−1 at large areas. This implies a decorrelation time for large illuminated areas of about 10 ms at X band and 3 ms at Ka band but somewhat larger values for small areas. The dependence of the velocity spread was found to be well explained by theory if an intrinsic velocity spread of 0.07 m s−1 was used to represent scatterer lifetime effects.

Measuring waves and currents with an upward-looking ADCP

Routine monitoring of waves and currents in the nearshore region is of great interest both scientifically and to the general public because of their role in coastline erosion and their impact on recreational activities. Historically, the technology for measuring these quantities has been distinct, requiring separate instrumentation for each. In this contribution the authors show that it is possible in shallow water to estimate both wave height and direction from a conventional bottom-mounted, upward-looking acoustic Doppler current profiler. Height and direction spectra compare well with a co-located array of pressure gages.