On Breaking Waves and Turbulence at the Air-sea Interface

On Breaking Waves and Turbulence at the Air-sea Interface
Author: Peter Jesse Sutherland
Publisher:
Total Pages: 117
Release: 2013
Genre:
ISBN: 9781303566660

Wave fields in the open ocean evolve according to the radiative transfer equation of wave energy or action, which has three source terms, wind input, non-linear wave-wave interactions, and dissipation. Of these, dissipation is thought to be the least well understood, but is expected to be dominated by wave breaking. This dissertation is an investigation of the physical processes associated with the wave breaking and dissipation. Data were taken during three field experiments on R/P FLIP. These experiments took place in September 2009 south of Hawaii (Radiance in a Dynamic Ocean experiment), off the coast of Northern California in June of 2010 (High Resolution Air-Sea Interaction experiment, HIRES), and in the Southern California bight in December 2010 (an extension of HIRES). Between the three campaigns, winds of 0 to 18 m/s and significant wave heights of 0.5 to 5 m were experienced. Stereo Long Wave Infra Red (LWIR) video cameras mounted on one of FLIP's booms were used to reconstruct the 3D structure of an approximately 3x4 m patch of sea surface. Using surface temperature structure as a passive tracer, pattern imaging velocimetry (PIV) was applied to consecutive video frames to extract the velocity field of this patch. An important statistic of breakers is [Lambda](c), the distribution of crest length per unit area of sea surface as a function of breaker velocity c. A new technique, based on image texture, was developed to track breaking waves on the stereo IR reconstructed surface. These waves ranged from large air-entraining breakers to micro breakers that would be undetectable in visible imagery. This allowed measurements of [Lambda](c) that also cover the high-wavenumber gravity wave spectrum. Stress (or wave momentum flux) and dissipation can be related to the fourth and fifth moments of [Lambda](c), and comparisons of these moments with wind stress and wave field dissipation showed that micro-breaking without air entrainment is dynamically significant. A new technique was developed, whereby irrotational surface waves can be separated from rotational turbulence using a Helmholtz decomposition. Turbulent kinetic energy (TKE) dissipation at the sea surface was then estimated using this rotational velocity field. Synchronized subsurface velocity measurements from an array of profiling pulse-coherent acoustic Doppler profilers allowed the calculation of the dissipation profile to depths O(10) significant wave heights. Tying surface and subsurface measurements together allows estimation of total TKE dissipation in the surface wave zone of the marine boundary layer. Turbulence measurements were supported by wind and wave data, allowing us to measure the wave coherence of TKE dissipation and relate it to wind and wave conditions, especially wave breaking.

The Air-Sea Interface

The Air-Sea Interface
Author: M. A. Donelan
Publisher: University of Miami Iberian Studies Institute
Total Pages: 832
Release: 1996
Genre: Science
ISBN:

Ocean-Atmosphere Interactions of Gases and Particles

Ocean-Atmosphere Interactions of Gases and Particles
Author: Peter S. Liss
Publisher: Springer
Total Pages: 366
Release: 2013-12-18
Genre: Science
ISBN: 3642256430

The oceans and atmosphere interact through various processes, including the transfer of momentum, heat, gases and particles. In this book leading international experts come together to provide a state-of-the-art account of these exchanges and their role in the Earth-system, with particular focus on gases and particles. Chapters in the book cover: i) the ocean-atmosphere exchange of short-lived trace gases; ii) mechanisms and models of interfacial exchange (including transfer velocity parameterisations); iii) ocean-atmosphere exchange of the greenhouse gases carbon dioxide, methane and nitrous oxide; iv) ocean atmosphere exchange of particles and v) current and future data collection and synthesis efforts. The scope of the book extends to the biogeochemical responses to emitted / deposited material and interactions and feedbacks in the wider Earth-system context. This work constitutes a highly detailed synthesis and reference; of interest to higher-level university students (Masters, PhD) and researchers in ocean-atmosphere interactions and related fields (Earth-system science, marine / atmospheric biogeochemistry / climate). Production of this book was supported and funded by the EU COST Action 735 and coordinated by the International SOLAS (Surface Ocean- Lower Atmosphere Study) project office.

The Effects of Ice and Currents on Wave-breaking Turbulence at the Ocean Surface

The Effects of Ice and Currents on Wave-breaking Turbulence at the Ocean Surface
Author: Seth Zippel
Publisher:
Total Pages: 131
Release: 2017
Genre:
ISBN:

Breaking waves are critical to the exchange of momentum, gasses, and heat between the atmosphere and ocean. In open water, these exchanges control the growth and decay of waves, and have implications for global heat and gas budgets. However, local geophysical properties can significantly alter these exchanges. At river inlets, strong currents influence swell that has grown over a long ocean fetch, dramatically increasing surface fluxes. The reverse effect is seen in the presence of ice, where ocean wave properties are decoupled from atmospheric forcing, decreasing exchanges across the air/sea interface. Here, measurements of ocean waves and near surface turbulence are presented to show the modification of surface boundary processes from ice and currents. Measurements from free drifting buoys at the Mouth of the Columbia River are used to evaluate wave breaking parameterizations, where breaking occurs in intermediate depths and in the presence of vertically sheared currents. Breaking waves were identified using images collected with cameras onboard the buoys, and the breaking activity is well-correlated with wave steepness. Vertical shear in the currents produces a frequency-dependent effective current that modifies the linear dispersion relation. Accounting for these sheared currents in the wavenumber spectrum is essential in calculating the correct wave steepness; without this, wave steepness can be over (under) estimated on opposing (following) currents by up to 20%. The observed bulk wave steepness values suggest a limiting value of 0.4. The observed fraction of breaking waves is in good agreement with several existing models, each based on wave steepness. Further, a semi-spectral model designed for all depth regimes also compares favorably with measured breaking fractions. In this model, the majority of wave breaking is predicted to occur in the higher frequency bands (i.e., short waves). There is a residual dependance on directional spreading, in which wave breaking decreases with increasing directional spread. Observations at the Columbia River Mouth are also used to investigate the source and vertical structure of turbulence in the surface boundary layer. Turbulent velocity data collected onboard SWIFT buoys were corrected for platform motions to estimate turbulent kinetic energy (TKE) and TKE dissipation rates. Both of these quantities are correlated with wave steepness, which has been previously shown to determine wave breaking within the same dataset. Estimates of the turbulent length scale increase linearly with distance from the free surface, and roughness lengths estimated from velocity statistics scale with significant wave height. The vertical decay of turbulence is consistent with a balance between vertical diffusion and dissipation. Below a critical depth, a power law scaling commonly applied in the literature works well to fit the data. Above this depth, an exponential scaling fits the data well. These results, which are in a surface-following reference frame, are reconciled with results from the literature in a fixed reference frame. A mapping between free-surface referenced and mean-surface reference coordinates suggests 30% of the TKE is dissipated above the mean sea surface. Lastly, wind, wave, turbulence, and ice measurements from the Arctic Marginal Ice Zone are used to evaluate the response of the ocean surface to a given wind stress, with a focus on the local wind input to waves and subsequent ocean surface turbulence. Observations are from the Beaufort Sea in the summer and early fall of 2014, with fractional ice cover of up to 50%. Observations showed strong damping and directional modification of short waves, which, in turn, decreased the wind energy input to waves. Near-surface turbulent dissipation rates were also greatly reduced in partial ice cover. The reductions in waves and turbulence were balanced, suggesting that a wind-wave equilibrium is maintained in the marginal ice zone, though at levels much less than in open water. These results suggest that air-sea interactions are suppressed in the marginal ice zone relative to open ocean conditions at a given wind forcing, and this may act as a feedback mechanism in expanding a persistent marginal ice zone throughout the Arctic.

Transport at the Air-Sea Interface

Transport at the Air-Sea Interface
Author: Christoph S. Garbe
Publisher: Springer Science & Business Media
Total Pages: 330
Release: 2008-06-30
Genre: Science
ISBN: 3540369066

One key uncertainty in predictions of future climate is caused by the lack of knowledge of transport processes in the air-water interface; this poses the main transfer resistance between oceans and atmosphere. This book reviews recent progress in the domains of experimental process studies as well as computer stimulation. It represents an early approach of merging insights gained in both fields and broadens our understanding of air-water gas and heat exchange.

The Interaction of Ocean Waves and Wind

The Interaction of Ocean Waves and Wind
Author: Peter Janssen
Publisher: Cambridge University Press
Total Pages: 310
Release: 2004-10-28
Genre: Science
ISBN: 0521465400

This book was published in 2004. The Interaction of Ocean Waves and Wind describes in detail the two-way interaction between wind and ocean waves and shows how ocean waves affect weather forecasting on timescales of 5 to 90 days. Winds generate ocean waves, but at the same time airflow is modified due to the loss of energy and momentum to the waves; thus, momentum loss from the atmosphere to the ocean depends on the state of the waves. This volume discusses ocean wave evolution according to the energy balance equation. An extensive overview of nonlinear transfer is given, and as a by-product the role of four-wave interactions in the generation of extreme events, such as freak waves, is discussed. Effects on ocean circulation are described. Coupled ocean-wave, atmosphere modelling gives improved weather and wave forecasts. This volume will interest ocean wave modellers, physicists and applied mathematicians, and engineers interested in shipping and coastal protection.

Advances in Coastal and Ocean Engineering

Advances in Coastal and Ocean Engineering
Author: M. Brocchini
Publisher: World Scientific
Total Pages: 160
Release: 2002
Genre: Science
ISBN: 9812705295

This book is a useful source of ideas and information for scientists whose work involves understanding and modelling turbulent flows with free surfaces. It has the following merits: (1) It provides a framework for developing the analysis of this field, which, although important, has received only limited study; (2) It recognizes the importance of the two-phase nature of strongly disturbed free surface flows, with both natural and technological applications; (3) It suggests possible lines of future research (especially experimental) to quantify the characteristics of flow regimes which are mainly known qualitatively at present.

On the Stress-Shear Relation Near a Turbulent Air-Sea Interface

On the Stress-Shear Relation Near a Turbulent Air-Sea Interface
Author:
Publisher:
Total Pages: 0
Release: 1982
Genre:
ISBN:

The average wind profile versus height for a nearly flat water surface roughened by capillaries is logarithmic down to a certain point, and must then approach the surface velocity smoothly at the time averaged surface. For lack of data, we hypothesize the form of the interfacial sublayer to be that of Liu, et al. (1979) for smooth flow with a modification in the dominant scale size to accommodate the transition from smooth to rough flow. The result implies that the surface shear increases with applied stress until roughness sets in. Then owing to increased turbulence at the interface, the shear may reach a maximum and decrease. If this were to hold true, there would be important implications for air-sea coupling in general, and wind wave generation mechanisms in particular. (Author).