Terray
Eugene A.
Terray
Eugene A.
No Thumbnail Available
Search Results
Now showing
1 - 7 of 7
-
ArticleMeasurements of momentum and heat transfer across the air–sea interface(American Meteorological Society, 2008-05) Gerbi, Gregory P. ; Trowbridge, John H. ; Edson, James B. ; Plueddemann, Albert J. ; Terray, Eugene A. ; Fredericks, Janet J.This study makes direct measurements of turbulent fluxes in the mixed layer in order to close heat and momentum budgets across the air–sea interface and to assess the ability of rigid-boundary turbulence models to predict mean vertical gradients beneath the ocean’s wavy surface. Observations were made at 20 Hz at nominal depths of 2.2 and 1.7 m in 16 m of water. A new method is developed to estimate the fluxes and the length scales of dominant flux-carrying eddies from cospectra at frequencies below the wave band. The results are compared to independent estimates of those quantities, with good agreement between the two sets of estimates. The observed temperature gradients were smaller than predicted by standard rigid-boundary closure models, consistent with the suggestion that wave breaking and Langmuir circulation increase turbulent diffusivity in the upper ocean. Similarly, the Monin–Obukhov stability function ϕh was smaller in the authors’ measurements than the stability functions used in rigid-boundary applications of the Monin–Obukhov similarity theory. The dominant horizontal length scales of flux-carrying turbulent eddies were found to be consistent with observations in the bottom boundary layer of the atmosphere and from laboratory experiments in three ways: 1) in statically unstable conditions, the eddy sizes scaled linearly with distance to the boundary; 2) in statically stable conditions, length scales decreased with increasing downward buoyancy flux; and 3) downwind length scales were larger than crosswind length scales.
-
ArticleAir-sea CO2 exchange in the equatorial Pacific(American Geophysical Union, 2004-08-28) McGillis, Wade R. ; Edson, James B. ; Zappa, Christopher J. ; Ware, Jonathan D. ; McKenna, Sean P. ; Terray, Eugene A. ; Hare, Jeffrey E. ; Fairall, Christopher W. ; Drennan, William M. ; Donelan, Mark A. ; DeGrandpre, Michael D. ; Wanninkhof, Rik ; Feely, Richard A.GasEx-2001, a 15-day air-sea carbon dioxide (CO2) exchange study conducted in the equatorial Pacific, used a combination of ships, buoys, and drifters equipped with ocean and atmospheric sensors to assess variability and surface mechanisms controlling air-sea CO2 fluxes. Direct covariance and profile method air-sea CO2 fluxes were measured together with the surface ocean and marine boundary layer processes. The study took place in February 2001 near 125°W, 3°S in a region of high CO2. The diurnal variation in the air-sea CO2 difference was 2.5%, driven predominantly by temperature effects on surface solubility. The wind speed was 6.0 ± 1.3 m s−1, and the atmospheric boundary layer was unstable with conditions over the range −1 < z/L < 0. Diurnal heat fluxes generated daytime surface ocean stratification and subsequent large nighttime buoyancy fluxes. The average CO2 flux from the ocean to the atmosphere was determined to be 3.9 mol m−2 yr−1, with nighttime CO2 fluxes increasing by 40% over daytime values because of a strong nighttime increase in (vertical) convective velocities. The 15 days of air-sea flux measurements taken during GasEx-2001 demonstrate some of the systematic environmental trends of the eastern equatorial Pacific Ocean. The fact that other physical processes, in addition to wind, were observed to control the rate of CO2 transfer from the ocean to the atmosphere indicates that these processes need to be taken into account in local and global biogeochemical models. These local processes can vary on regional and global scales. The GasEx-2001 results show a weak wind dependence but a strong variability in processes governed by the diurnal heating cycle. This implies that any changes in the incident radiation, including atmospheric cloud dynamics, phytoplankton biomass, and surface ocean stratification may have significant feedbacks on the amount and variability of air-sea gas exchange. This is in sharp contrast with previous field studies of air-sea gas exchange, which showed that wind was the dominating forcing function. The results suggest that gas transfer parameterizations that rely solely on wind will be insufficient for regions with low to intermediate winds and strong insolation.
-
ArticleTurbulent transport in the outer region of rough-wall open-channel flows : the contribution of large coherent shear stress structures (LC3S)(Cambridge University Press, 2007-02-15) Hurther, D. ; Lemmin, U. ; Terray, Eugene A.Acoustic Doppler velocity profiler (ADVP) measurements of instantaneous three-dimensional velocity profiles over the entire turbulent boundary layer height, δ, of rough-bed open-channel flows at moderate Reynolds numbers show the presence of large scale coherent shear stress structures (called LC3S herein) in the zones of uniformly retarded streamwise momentum. LC3S events over streamwise distances of several boundary layer thicknesses dominate the mean shear dynamics. Polymodal histograms of short streamwise velocity samples confirm the subdivision of uniform streamwise momentum into three zones also observed by Adrian et al. (J. Fluid Mech., vol. 422, 2000, p. 1). The mean streamwise dimension of the zones varies between 1δ and 2.5δ. In the intermediate region (0.2
-
ArticleThree-dimensional mapping of fluorescent dye using a scanning, depth-resolving airborne lidar(American Meteorological Society, 2007-06) Sundermeyer, Miles A. ; Terray, Eugene A. ; Ledwell, James R. ; Cunningham, A. G. ; LaRocque, P. E. ; Banic, J. ; Lillycrop, W. J.Results are presented from a pilot study using a fluorescent dye tracer imaged by airborne lidar in the ocean surface layer on spatial scales of meters to kilometers and temporal scales of minutes to hours. The lidar used here employs a scanning, frequency-doubled Nd:YAG laser to emit an infrared (1064 nm) and green (532 nm) pulse 6 ns in duration at a rate of 1 kHz. The received signal is split to infrared, green, and fluorescent (nominally 580–600 nm) channels, the latter two of which are used to compute absolute dye concentration as a function of depth and horizontal position. Comparison of dye concentrations inferred from the lidar with in situ fluorometry measurements made by ship shows good agreement both qualitatively and quantitatively for absolute dye concentrations ranging from 1 to >10 ppb. Uncertainties associated with horizontal variations in the natural seawater attenuation are approximately 1 ppb. The results demonstrate the ability of airborne lidar to capture high-resolution three-dimensional “snapshots” of the distribution of the tracer as it evolves over very short time and space scales. Such measurements offer a powerful observational tool for studies of transport and mixing on these scales.
-
ArticleDrift and mixing under the ocean surface : a coherent one-dimensional description with application to unstratified conditions(American Geophysical Union, 2006-03-24) Rascle, Nicolas ; Ardhuin, Fabrice ; Terray, Eugene A.Waves have many effects on near-surface dynamics: Breaking waves enhance mixing, waves are associated with a Lagrangian mean drift (the Stokes drift), waves act on the mean flow by creating Langmuir circulations and a return flow opposite to the Stokes drift, and, last but not least, waves modify the atmospheric surface roughness. A realistic ocean model is proposed to embrace all these aspects, focusing on near-surface mixing and surface drift associated with the wind and generated waves. The model is based on the generalized Lagrangian mean that separates the momentum into a wave pseudomomentum and a quasi-Eulerian momentum. A wave spectrum with a reasonably high frequency range is used to compute the Stokes drift. A turbulent closure scheme based on a single evolution equation for the turbulent kinetic energy includes the mixing due to breaking wave effects and wave-turbulence interactions. The roughness length of the closure scheme is adjusted using observations of turbulent kinetic energy near the surface. The model is applied to unstratified and horizontally uniform conditions, showing good agreement with observations of strongly mixed quasi-Eulerian currents near the surface when waves are developed. Model results suggest that a strong surface shear persists in the drift current because of the Stokes drift contribution. In the present model the surface drift only reaches 1.5% of the wind speed. It is argued that stratification and the properties of drifting objects may lead to a supplementary drift as large as 1% of the wind speed.
-
ArticleObservations of turbulence in the ocean surface boundary layer : energetics and transport(American Meteorological Society, 2009-05) Gerbi, Gregory P. ; Trowbridge, John H. ; Terray, Eugene A. ; Plueddemann, Albert J. ; Kukulka, TobiasObservations 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.
-
Technical ReportSpatial variability of bottom turbulence over a linear sand ridge mooring deployment and AUTOSUB AUV survey cruise report R/V RRS Challenger, cruise number 146 Broken Bank, North Sea, U.K., 17 – 28 August 1999 cruise report(Woods Hole Oceanographic Institution, 2001-08) Voulgaris, George ; Trowbridge, John H. ; Terray, Eugene A.Two successful AUTOSUB deployments were carried out during August 1999 as part of the AUTOSUB Thematic Program project titled “Spatial Variability of Bottom Turbulence over a Linear Sand Ridge,” funded by the Natural Environment Research Council (NERC), U.K. The AUTOSUB Autonomous Underwater Vehicle (AUV) was deployed and used to survey flow patterns at a location near the Broken Bank, southern North Sea, U.K. The AUV was equipped with acoustic flow and turbulence sensors and its surveys aimed at mapping the spatial variation of flow and turbulence near the bed and over topographic features. Three instrumented bottom mounted frames were also deployed, around the AUV survey area, for a period of approximately 5 days. The purpose of this array was to gather information on the temporal variability of the flow and turbulence near the seabed and to identify the important terms that drive circulation around the bank. Additional data were gathered including CTD casts, seabed samples and acoustic images of the seabed (side-scan sonar). The purpose of this data collection was to help identify the flow patterns around ridges and to understand the mechanisms controlling the maintenance and evolution of such features. This report describes the operations carried out by researchers from the University of South Carolina, Woods Hole Oceanographic Institution, Southampton Oceanography Centre and the AUTOSUB Team on the R.V. RRS Challenger during the period 17th –28th August 1999.