Euromech 387

"Surface Slicks and Remote Sensing of Air-sea Interactions"

(7-9 April 1998) Windmill Village, Birmingham Road, Coventry CV5 9AL, UK

Overview
Abstracts
Addresses
Information for Authors
Letter from Neale Thomas
Euromech 387 programme
Mail to Martin Gade


Editorial Board:

1.          Martin Gade (Universität Hamburg, Germany)

2.          Gerald M. Korenowski (Rensselaer Polytechnic Institute, NY, USA)

3.          John C. Scott (QinetiQ Ltd., UK)

4.          Neale H. Thomas (FRED Ltd., UK)


Overview

  1. Submitted Papers
    1. Mohamed Atmane (interfacial intermittence, 6 pp.) 
    2. V. V. Bakhanov (current effect on nonlinear surface waves, 5 pp.) 
    3. Simon Boxall (visible and near-infrared measurements, 6 pp.) 
    4. Nicole Braun (artificial rain & slicks, 8 pp.) 
    5. Peter Dovey (radar observations of surface films, 13 pp.) 
    6. Kristian Dysthe (surface renewal and sea slicks, 8 pp.) 
    7. Stanislaw Ermakov (two papers: damping of gravity-capillary waves, 7+6 pp.)  
    8. Nelson Frew (2 papers: variability in chemical composition, 11+9 pp.) 
    9. Martin Gade (two papers: radar measurements of slicks, 5+5 pp.)  
    10. Christine Gommenginger (LGA monitoring of slicks, 9 pp.) 
    11. Harm Greidanus (radar modelling of slicks, 13 pp.) 
    12. Jochen Horstmann (wind-speed from SAR imagery, 11 pp.) 
    13. Heinrich Hühnerfuss (slick morphology, 7 pp.) 
    14. Paul Hwang (wavenumber spectra, 5 pp.) 
    15. Bethan Jones (Sea Empress oil, 11 pp.) 
    16. Gerald Korenowski (two papers: surfactant concentration distribution, 8+7 pp.)  
    17. Susanne Lehner (global statistics from ERS SAR, 17 pp.) 
    18. Konstantin Litovchenko (oil spills on ALMAZ and ERS SAR, 11 pp.) 
    19. Giuseppe Loglio (static and dynamic surface tension, 6 pp.) 
    20. Sean McKenna (air-sea gas transport, 11 pp.) 
    21. Boris Noskov (multiple scattering of surface waves, 7 pp.) 
    22. Efim Pelinovsky (models of internal tides, 5 pp.) 
    23. Stanislaw Pogorzelski (structural variability, 22 pp.) 
    24. John Pullen (two papers: PIV + Lagrangian motions, 8+7 pp.)  
    25. Uwe Schimpf (air-sea gas transfer, 9 pp.) 
    26. Ted Schlicke (air-sea gas transfer, 8 pp.) 
    27. Christian Senet (GKSS; surface tension measurements, 8 pp.) 
    28. José da Silva (slicks & internal waves, 10 pp.) 
    29. David Woolf (thermal imagery, 10 pp.) 

 

  1. Additional Contributions
    1. Ian Robinson (summary of final discussion
    2. Nelson Frew (tribute to Erik Bock)
    3. Giuseppe Loglio (Carlo Marangoni's biography, 2 pp.)
    4. ... plus foreword(s)

 

Sum / papers : 35
Sum / existing pages: 301
Sum / existing pages (starting right-hand): 322 

 

(   : paper submitted;
   
: one review received;
   
: both reviews received;
   
: reviews sent to corresponding authors;
   
: updated paper submitted;
   
: paper accepted   )


Abstracts

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Atmane, M. A. and J. George:
Interfacial intermittence characterised by scalar and dynamic fields

Mass transfer at a shear-free air-water surface in an agitated tank is investigated in this study. The free surface is submitted to a turbulent flow generated in the bottom of the tank using an array of micro jets. The mass transfer is simulated by creating a gradient of oxygen concentration in the water phase. The vertical and horizontal velocity components are measured, in the water phase, close to the surface using the LDA technique. The oxygen concentration profiles below the free surface are evaluated using a polarographic probe. Our measurements show that the thickness of the oxygen boundary layer below the surface can be evaluated allowing a first estimate of the mass flux. A physical model based on the turbulent flux measurement, the surface renewal model as well as the interplay between different scales of the turbulent flow has been proposed.

Bakhanov, V. V.:
Current effect on nonlinear surface waves

The transformation of counterstream nonlinear surface waves on a local perturbation of the stream is investigated. It is theoretically obtained, that surface-wave nonlinearity causes a sharp increase of surface-wave amplitude variability in time within a broad frequency band. Laboratory investigation results of various amplitude and frequency surface wave transformation in an inhomogeneous current field are given. It is found out, that the growing amplitude of a surface wave generated by a wave maker enhances variability of its parameters in the inhomogeneous current field. Theoretical calculations results are shown to agree well with the experimental basic singularities of the nonlinear surface wave transformation in the inhomogeneous current field.

Boxall, S. R. and V. Byfield:
Oil detection and thickness estimates from radiance measurements at visible and near infrared wavelengths

This study develops the technique of optical measurement for the detection of oil in the marine environment. A theoretical model is proposed, which relates upwelling radiance from surface oil to the optical properties of the oil, the thickness of the oil layer, and a number of ancillary environmental parameters. The model is used to interpret optical image data from field experiments, and to provide estimates of relative oil thickness.

Braun, N., M. Gade, and P. A. Lange:
Laboratory measurements of artificial rain impinging on slick-free and slick-covered water surfaces

Laboratory measurements with artificial rain have been carried out in the wind-wave tank of the University of Hamburg, in order to gain better understanding of the radar backscattering from a (slick-free and slick-covered) water surface, particularly when it is agitated by strong rain. We used a coherent 9.8 GHz (X band) scatterometer at different polarizations and at an incidence angle of 28 degrees, a resistance-type wire gauge, and a two-dimensional laser slope gauge. A water surface area of 2.3 m2 was agitated by strong artificial rain with a rain rate of 160 mm/h and rain drops of 2.9 mm diameter. The wind-speed range used in the present investigation was between 2 and 10 m/s. A monomolecular surface film was produced by deploying oleyl alcohol on the water surface. Our first results of the analyses of the measured radar Doppler spectra and wave amplitude and slope spectra are presented. We show that (at a high rain rate of 160 mm/h) the surface slick affects the rain-induced increase of the surface roughness (i. e. the generation of crowns, cavities, stalks, ring waves, and secondary drops) less strongly than the wind-induced surface roughness (which is strongly reduced in the presence of the slick).

Dovey, PR., N.R. Stapleton, and J.C. Scott.:
Radar observations of natural and artificial slicks

The current DERA slick research programme is aimed at understanding of the way slicks affect satellite radar images at low wind speed, principally to understand their role in allowing the detection of mesoscale ocean features.  The work examines the ways in which the ocean features affect the biogenic film distribution, and also involves `active' radar imaging experiments, by spreading artificial ‘Emkarox’ slicks in a variety of ocean surface conditions prior to imaging. This paper presents results covering a selection of the observations and experiments carried out utilising radar sensors onboard ERS-1 and SIR-C, and a multi-frequency clifftop glancing angle radar.

Dysthe, K. B.:
On surface renewal and sea slicks

[Excerpt from the autor's introduction in place of an abstract.]  In this paper we consider some simple case studies with surface film and substrate motion in search for some criteria for construction and destruction of a coherent film-covered surface. [...] We show in the following [...] that a slick can easily suppress [...] convective surface motion. Thus the heat transport through the sea surface is affected by surface films.

Ermakov, S. A., S. V. Kijashko, and J. C. Scott
Damping of capillary-gravity waves due to films of insoluble ordinary surfactants and polymers

Laboratory measurements of damping of capillary-gravity waves due to organic films were carried out in a wide range of surfactant concentrations using a method of parametrically excited surface waves at frequencies from about 10 Hz to 30 Hz. The wave damping coefficient was measured for films of ordinary surface-active substances (oleyl alcohol and oleic acid), and for polymer films (polyoxyalkylene glycol - “Emkarox”). It is shown that the damping coefficient curves for the ordinary surfactants exhibit one peak at surface concentrations smaller than the concentration of a saturated monolayer; at larger concentrations the damping coefficient remains practically constant. The dependencies of the damping coefficient on surfactant concentration for polymer films are strongly different. They are characterized by the existence of two maxima, one is a narrow peak at very low concentrations smaller than the concentration of a saturated monolayer and another is a plateau-like maximum at larger concentrations. The elasticities of the ordinary and polymer films are estimated from the measured damping coefficient using a theory of wave damping for purely elastic films. The estimated dynamic elasticities are shown to differ strongly, especially for saturated monolayers, from static elasticities, obtained from Langmuir trough measurements.

Ermakov, S. A., , I.A.Sergievskaya, E.M.Zuikova, V.Yu.Goldblat, Yu.B.Shchegol’kov, N.Stapleton, and J.C.Scott
Wave  tank study of phase velocities and damping of gravity-capillary wind waves in the presence of surface films

The wavenumber-frequency spectrum of gravity-capillary waves were measured using two optical spectrum analysers and an artificial gradient illuminator. It was found that the phase velocities for wavelengths smaller than 2-3 cm on clean water remain nearly constant and do not follow the dispersion relation of free surface waves. The phase velocities were found to increase with fetch, in the way that the phase velocities of dominant decimetric wind waves increase with fetch. This evidences that the wind wave spectrum contains nonlinear cm-mm-scale harmonics bound to the dominant waves and propagating with the phase velocities of the dominant waves. In the presence of films the observed phase velocities are closer to those of free linear gravity-capillary waves than they are in the case of clean water. This can be explained by the bound waves being more strongly damped than the free waves. The relation between bound and free waves is estimated from measurements of phase velocity. Wind ripple damping has been found to be maximum at wavelengths around 5-7 mm. The last effect agrees with our field experiments using artificial slicks and can be explained qualitatively by a nonlinear „cascade“ damping mechanism, when the damping of decimetric dominant waves leads to strong damping of their cm-mm-scale nonlinear harmonics including damping of „parasitic“ capillary ripples.

Frew, N. M., R. K. Nelson, and C. G. Johnson:
Sea slicks: variability in chemical composition and surface elasticity

Surface-active organic matter collected from the air-sea interface is examined using mass spectrometry. The mass spectra reveal significant spatio-temporal variability in the chemical composition of sea slick materials. This compositional variability leads to differences in observed surface pressure-elasticity relationships.

Frew, N. M., R. K. Nelson, and C. G. Johnson:
Correlation studies of mass spectral patterns and elasticity of sea slick materials

Variations in the chemical composition of surfactants from natural sea slicks are compared to variations in surface elasticity using mass spectrometry, Langmuir film balance measurements, and multivariate statistical techniques. It is shown that the information on chemical class and molecular structure contained in the mass spectra is strongly correlated with measured static elasticity and can be used to estimate film elasticity at a given surface pressure.

Gade, M.:
On the imaging of biogenic and anthropogenic surface films on the sea by radar sensors.
Part I: damping ratio measurements by SIR-C/X-SAR and by an airborne scatterometer

Radar signatures of sea surface films of different origin are studied, which have been acquired by the airborne multi-frequency/multi-polarization scatterometer HELISCAT as well as by the multi-frequency/multi-polarization Spaceborne Imaging Radar-C / X-Band Synthetic Aperture Radar ("SIR-C/X-SAR") during the two shuttle missions in 1994. During both missions, controlled surface film experiments were performed in the German Bight, where various artificial biogenic substances as well as mineral oil were deployed on the sea surface, in order to study the radar signatures caused by the surface films of different visco-elastic properties. We show that, under high wind conditions (> 10 m/s), a discrimination between the different kinds of surface films is very difficult, whereas at low to moderate wind speeds (≤ 5 m/s) a discrimination seems to be possible.

Gade, M.:
On the imaging of biogenic and anthropogenic surface films on the sea by radar sensors.
Part II: results from modelling and from laboratory measurements

Radar signatures of sea surface films of different origin are studied, which have been acquired by the airborne multi-frequency/multi-polarization scatterometer HELISCAT during the second Spaceborne Imaging Radar-C / X-Band Synthetic Aperture Radar ("SIR-C/X-SAR") mission in 1994, where a controlled surface film experiment was performed in the German Bight. Using results from the HELISCAT measurements a new model is developed for the wavenumber-dependent radar contrast at high wind speeds (> 10 m/s) caused by the surface films of different visco-elastic properties. In addition, results of laboratory experiments which were carried out in the wind-wave tank of the University of Hamburg are presented. We show that a different damping of bound and freely propagating surface waves by a monomolecular surface film occurs at certain wind-speed ranges.

Gommenginger, C. P. and S. R. Boxall:
Long-term microwave radar monitoring of ocean slicks at low grazing angles

This paper proposes the use of a digital microwave marine radar system for the detection and monitoring of surfactant slicks at sea. The spatial, temporal and radiometric resolutions of this low grazing angle instrument are compared to those of satellite and airborne radars traditionally used for oil pollution monitoring. Experimental evidence is presented in two artificial surfactant releases in the coastal zone which were successfully detected and tracked with a shore-based marine radar. The information contained in a long-term time-series of NRCS images is explored and its synergistic use with traditional satellite- and air-borne radar data discussed.

Greidanus, H.:
Radar modelling of slicks based on the "VIERS-1" wave spectrum

The VIERS-1 model (Janssen et al., 1998) was originally derived to describe ERS scatterometer measurements over the sea on a physical basis. This model is extended to include the saturation range, and it is used to derive equilibrium wave spectra and formulations for the relaxation toward that equilibrium, with and without slicks present. It was attempted to derive actual relaxation rates from the model, but this did not seem to lead to acceptable results. The extended VIERS model is combined with a radar imaging model originally designed for radar bathymetry, to yield computations of radar transects through slicks. The resulting model is capable of quantitatively computing radar contrasts over a slick, showing effects of edge smoothing and apparent size increase due to finite relaxation times. The present work offers, however, no validation with actually measured data.

Horstmann, J., W. Koch. S. Lehner, and W. Rosenthal:
Mesoscale wind fields and their variation retrieved from Synthetic Aperture Radar aboard ERS-1/2

The synthetic aperture radar (SAR) aboard the European remote sening satellites (ERS-1 and ERS-2) acquires images that can be used to derive wind fields over the ocean surface. For this purpose an algorithm, based on Fast Fourier transformation, was developed to retrieve wind direction from wind induced streaks present in most SAR images. For computation of wind speed an algorithm based on the C-band models CMOD4 and CMOD_IFR2 was evolved. To show the applicability of the algorithms, images at different geographical locations and under different meteorological and oceanographical conditions were selected and compared to ground truth measurements. At the German coast of the Baltic Sea a mesoscale wind field was retrieved from an ERS-1 SAR image and compared to the mesoscale atmospheric model GESIMA. Furthermore the different errors in wind speed due to effects of resolution, wind direction, and the accuracy of normalized radar cross section (NRCS) were estimated. The large coverage together with the high resolution of the ERS SAR-retrieved wind fields offer the opportunity to investigate the spatial variation of the boundary layer wind between scales of 2 to 100 km. It is shown that in this range there is no evidence of a spectral gap and the spectral energy decreases with increasing wavenumber. The decrease of spectral energy is according to the power law k-5/3 which is also predicted by the theory of two-dimensional isotropic turbulence.

Hühnerfuss, H., F. Hoffmann, J. Simon-Kutscher, W. Alpers, and M. Gade:
New chemical insights into structure and morphology of sea slicks and their geophysical implications

The different morphology of palmitic acid methyl ester (PME) slicks spread from n-hexane and ethanol, respectively, was studied by ‘Brewster Angle Microscopy’ [BAM]. In the latter case, already at large areas per molecule a foam-like structure is being formed on the water surface, i.e., a two-dimensional network which appears to be comparable with the morphological structure of biogenic sea slicks. In line with this assumption both the magnitudes of the radar backscatter damping ratios as well as the characteristics of the damping ratio/wavenumber curves determined for the PME slick spread from ethanol and for the biogenic slicks were very comparable, while in the presence of the PME slick spread from n-hexane lower damping ratios were determined. In the first instance, we were able to simulate the water wave damping characteristics of the biogenic sea slicks very well. Furthermore, the relaxation of alkanoic acid esters, which are often being found in biogenic sea slicks, was investigated by ‘Infrared Reflection-Absorption Spectroscopy’ [IRRAS]. It turned out that the ester group is continuously hydrated and dehydrated during compression and dilation on an undulating water wave field. It can be safely assumed that the strong water wave damping induced by these chemical compounds is centrally related to this phenomenon.

Hwang, P. A.:
Wavenumber spectra of short water waves in the ocean - a field case with internal waves and natural slicks

Spatial measurements of capillary-gravity waves in the ocean were collected using a scanning slope sensor mounted on a free-drifting, wave-following buoy. The spectral analysis of spatial measurements yields wavenumber resolution directly. The range of wavelengths resolved from the optical sensor is from 0.4 to 6.2 cm. This paper investigates the variability of the spectral intensities of these capillary-gravity waves under natural field conditions. For mild to medium wind speeds, the variation factor is typically between 1.5 to 2.5 in the short gravity wave regime, and between 2 to 4 in the capillary wave regime. The quantitative enhancement of the roughness contrast by natural slicks and surface current strains induced by internal waves is also presented.

Jones, B.:
SAR imagery and visual observations of the Sea Empress oil spill

A comparison has been made between the visual observations of surface oil and four synthetic aperture radar (SAR) images taken during the 'Sea Empress' oil spill. The study suggests that the optimum wind speed for the identification of thick surface oil is 10 - 12 knots. At this wind speed, there is a distinction between the backscatter reduction due to sheen and thick oil. In lighter winds, sheen becomes increasingly evident in the images and ist signal becomes indistinguishable from that of thicker oil.

Korenowski, G. M., E. VanWagenen, and A. Hirsa
Imaging Surfactant Concentration Distributions at the Air/Water Interface
Part I: Surfactant Concentration Gradient on a Laminar Channel Flow

The surface specific spectroscopic probes of reflected second harmonic generation (SHG) and reflected sum frequency generation (SFG) have been successfully employed to measure surfactant monolayer concentration gradients on the ocean surface. Reflected SHG was adapted for areal measurements of monolayer concentrations using an intensified pulse gated CCD camera for detection and a dichromatic image splitter to allow for signal normalization and scaling. As a preliminary test prior to field use, the imaging probe was used to measure the surfactant concentration gradients for an insoluble monolayer on a channel flow down stream of the Reynolds ridge. The resulting concentration measurements had a temporal resolution of 3 nanoseconds and spatial resolution of 0.21 millimeters or less per pixel.

Korenowski, G. M., J. R. Saylor, E. VanWagenen, J. S. Kelley, M. E. Anderson, and E. J. Edwards:
Imaging Surfactant Concentration Distributions at the Air/Water Interface
Part II: Insoluble Monolayer Concentrations on Standing Capillary Waves

Surfactant concentrations at the peaks and troughs of a standing capillary wave field were measured using a reflected second harmonic generation (SHG) imaging technique. The results revealed behavior that is consistent with a compaction of the insoluble monolayer at the crests of the capillary waves and a dilation of the monolayer in the troughs. Each measurement was obtained using a single 3 nanosecond laser pulse, providing essentially instantaneous measurements of surfactant concentration.

Lehner, S., J. Schulz-Stellenfleth, and J. Horstmann:
Global distribution of sea surface features from ERS-2 SAR wave mode data

Since the launch of the European remote sensing satellites ERS-1 and ERS-2 in 1991 and 1995, synthetic aperture radar (SAR) images have been acquired over the oceans on a continuous basis. A global dataset of 5 km x 10 km complex SAR images, so called imagettes, which are acquired every 200 km along the orbit by the ERS-1 and ERS-2 satellites, are used to study sea surface features on a global basis. So far the European Space Agency (ESA) only provides coarsely gridded image power spectra (UWA spectra) which are used for ocean wave measurements at the European Center for Medium Range Weather Forecast (ECMWF). Using imagettes instead of image power spectra allows to study ocean surface features caused by natural slicks, sea ice or atmospheric processes. Different image parameters like mean intensity, coefficient of variation as well as energy, entropy of the 2-dimensional power spectrum and intensity variation in range direction are used to classify these phenomena. The results are compared to model data of the ECMWF. Furthermore the impact of these phenomena on scatterometer (SCAT) wind speed measurements is considered.

Litovchenko, K., and A. Ivanov:
Detection of Oil Slicks Parameters from ALMAZ-1 and ERS-1 SAR Imagery

This paper presents the results of the oil spill observations using the Almaz-1 and ERS-1 SAR collected in the Norwegian Sea during the Dedicated Oil Spill Experiment in 1991. Three artificial slicks were released from vessel on August 1991 and data on sea and weather conditions near the test area were collected. The test area was imaged by the Almaz-1 SAR and ERS-1 SAR. The analysis of acquired SAR images shows that the reduction of the backscatter from oil-covered sea surface ranged in wide limits: between few tens and 15 dB. For the first time an effect of an intensification of wind waves both in the area of the slick and at the windward edge of the oil spill expressed as a magnification of the SAR image brightness has been detected. The increase of relative backscatter power was up to 2.0 dB. It is concluded that both Almaz-1 and ERS-1 SARs are the valuable tools for oil spill detection and localization but the detectability essentially depended on wind speed, sea state and age of spills.

Loglio, G., P. Pandolfini, R. Miller, B. Noskov, U. Tesei, A. M. Stortini, and R. Cini:
Static and dynamic surface tension of marine water: onshore or platform-based measurements by the oscillating bubble tensiometer

We illustrate functionality and performances of an oscillating bubble tensiometer, designed for the measurement of static and dynamic surface tension of marine water samples. The instrumentation works in completely automatic mode. Semicontinuos operation can also be easily implemented. By virtue of its geometrical configuration, the apparatus shows a relative insensitivity to external disturbances, in comparison to other measurement techniques, and consequently it can be employed even under severe environmental conditions, such as those ones encountered onshore or on platform-based locations. We report experimental observations of the dynamic surface tension response to harmonic surface area changes, for samples of sea water and for a dilute aqueous solution of a standard surfactant. The observed results are considered useful for a physical characterisation of marine water in respect to interfacial ocean-atmosphere phenomena and processes. Also, the oscillating-air-bubble tensiometer mimics the real behaviour of the bubbles (ubiquitously present in the near surface of the oceans) and appears a suitable instrument for the assessment of the "availability to the air-water interface" of natural and man-made adsorption-film-forming organic materials.

McKenna, S. P. and E. J. Bock:
Physicochemical effects of the marine microlayer on air-sea gas transport

Physicochemical mechanisms of enhanced gas transport across the air-sea interface are still not fully understood, however, they are of critical relevance to many global climatological processes. For example, the flux of CO2 (a known greenhouse gas) from the atmosphere into the ocean is important to the environmental balance of the planet. Further, the need to interpret air-sea gas transfer on global scales requires that the key mechanisms of transport be relatable to remotely sensed information, such as imagery obtained from satellite-borne radars. The ubiquity of surfactants in the ocean, especially in coastal zones (areas of significant phytoplankton concentration and biological productivity), strongly suggests their relevance to air-sea interactions. Surfactants are well known to impact surface and near-surface hydrodynamics and their role in reducing gas-transfer rates has been well documented. This paper reviews some of the history of these closely linked topics, focusing on the progress made in laboratory and field measurements. Sample results from a theoretical study in short wave propagation and laboratory experiments involving near-surface vortical flows are included to illustrate the important role of surfactant films on free- and near-surface hydrodynamics and the implications for air-sea gas exchange.

Noskov, B. A. and G. Loglio:
Multiple scattering of surface waves by two-dimensional colloid systems

Insoluble surface films at the air-water interface have been investigated by the capillary wave method for many years. However, it is only recently that a possibility to extract some characteristics of their macroscopic structure by this technique has been realised. This work presents the results of a theoretical study of surface wave propagation along the liquid surface covered by a heterogeneous insoluble film.

Pelinovsky, E. and T. Talipova:
Models of Intense Internal Tides Observed by Remote Sensing

[Excerpt from the autors' introduction in place of an abstract.]  Internal tides, generated through the action of the barotropic tide forcing stratified water over sloping topography, are a common feature of many continental slope and shelf regions. These internal tides are long internal waves most commonly seen at semi-diurnal periods and with wavelengths of 10's of km. In many instances these long internal waves are observed to form instabilities seen as internal bores, short period waves, and soliton like waves. These short waves have periods on the scale of 10 minutes and wavelengths of 100's m. In most cases it appears that internal solitary waves are generated through topographic effects (sills and continental slopes) on tidal flow or on long internal waves such as internal tides. These energetic waves are important dissipation mechanisms for the long waves and provide a source of mixing energy that can move sediment and increase biological productivity. [...] In many cases the remote sensing was used to detect the internal wave field. Usually the Korteweg - de Vries equation is applied to explain the evolution of internal waves in the coastal zone and its manifestation on the sea surface. Last observations shows that internal waves reach large amplitudes and a weak nonlinear theory should be improved. Here the mathematical model of intense internal tide transformation is described. It is applied to calculate the orbital motions in the subsurface layer induced by the internal wave field.

Pogorzelski, S. J. and A. H. Kogut:
Structural variability of natural marine and inland waters films.
Part I: theoretical aspects

Theoretical aspects of interpreting force-area isotherms, and dynamic surface tension experiments on natural water surfactant films are discussed. The 2D virial equation of state applied to the isotherm plots, taken at an equilibrium film state, leads to the static averaged film parameters: a number of moles, the limiting specific area, and mean molecular weight of surfactants composing the film. The 2D polymer film scaling theory can be adopted to the multicomponent film, by measuring the high-frequency limit of the surface modulus E versus surface pressure (E = yp) with the scaling parameter y closely related to the surfactant miscibility and vertically-segregated film structure. The film structure evolution is also predicted from the p - A - isotherm hysteresis vis the entropy change DSc atributed to a certain loss of the degrees of feedom of the interfacial system during a compression-expansion cycle. A time scale and the nature of the film relaxation processes are derived from dynamic surface pressure-time plots applying the rheokinetic analysis for a mixed diffusion-kinetic adsorption model. Relaxation processes of the interfacial system lead to the surface viscoelasticity that can be expressed in the framework of the diffusional viscoelasticity model.

Pogorzelski, S. J. and A. H. Kogut:
Structural variability of natural marine and inland waters films.
Part II: experimental aspects

Natural surface film experiments in inland waters and shallow offshore regions of the Baltic and Mediterranean Seas were carried out in the time period 1990 - 1999 under calm sea conditions using a novel device for sampling and force-area studies. The sampler-Langmuir trough-Wilhelmy filter paper plate system "cuts out" an undisturbed film-covered sea area (8 cm thick) to perform p - A studies without any initial physico-chemical sample processing. The limiting specific area Alim (268 - 3157 A2/molecule) and mean molecular weight (0.65 - 9.7 kDa) of microlayer surfactants were determined from the 2D virial equation of state applied to the isotherms. Additional information on the film structure were derived from the entropy change DSc attributed to the p - A hysteresis, and the 2D polymer film scaling theory adopted to marine films. The stress-relaxation measurements revealed a two-step relaxation process at the interface with characteristic times t1 (1.1 - 2.8) and t2 (5.6 - 25.6 seconds) suggesting the presence of diffusion-controlled and structural organisation relaxation phenomena. The obtained results demonstrate that natural films are a complex mixture of biomolecules covering a wide range of solubilities, surface activity and molecular weight with an apparent structural organisation exhibiting a spatial and temporal variability.

Pullen, J., A. Arnott, and C. Greated:
Part I: Particle image velocimetry study of the effect of surface active films on capillary gravity waves
Part II: Lagrangian motions near the surface of capillary gravity waves with surface active films

[Excerpt from the autors' introduction in place of an abstract.]  The calming effect which a film of oil has on a water surface has been known for many centuries. Numerous laboratory and field studies have shown that even films the thickness of a single molecule can have a significant effect on the initiation and propagation of surface waves. The general result of these films, whether formed by natural biological processes or mans' contamination, is to suppress wave formation, the practical significance of which is to alter the surface roughness of the body of water in question. In order to understand the effect that the surface contaminant has on the wave generation process, an experimental programme was undertaken to examine in detail the motion of the water particles near the surface using the techniques of Particle Image (PIV) and Particle Tracking Velocimetry (PTV). The investigation was carried out along two lines:

1.     The investigation of the damping of wind generated waves by a thin soap film using cross-correlation digital PIV based on a novel, twin CCD camera system.

2.     Quantitatively measure the effect of several films on the elliptical trajectories of surface particles beneath waves generated by a hinged paddle.

Schimpf, U., H. Haußecker, and B. Jähne:
Measurements of air-sea gas transfer using active and passive thermography

In recent years, the global climate change and the growing pollution in atmosphere and oceans have drawn the attention to small-scale transfer processes at the air/water interface. Knowledge about the mechanisms of mass transfer is still marginal because of the lack of suitable experimental techniques. To obtain an insight into these processes new techniques for the quantitative investigation of gas exchange have been developed. In the following the Controlled Flux Technique (CFT) is presented, using heat as a proxy tracer for gases. The high spatial and temporal resolution of this unique technique allows studying key questions of transfer processes. Detailed laboratory measurements in the Heidelberg wind wave facility are reported. By switching the heat a the water surface periodically on and off , the temperature gradient across the interface is controlled and estimated. Given the net heat flux and the temperature gradient, the transfer velocity for heat is calculated and scaled to the gas transfer velocity for an arbitrary gas. The results are discussed and compared with conventional mass balance methods used in the facility.

Schlicke, T., J.M. Buick, A.D.Arnott, C.A. Greated, and N.H. Thomas:
Effect of Surface Films on Wave Amplitude and Dispersion of a Film by Breaking Waves

The presence of a surface film has been shown to alter the motion of surface capillary gravity waves [1,2]. The interaction between waves and a surface film depends on factors such as: chemical composition of the film, its thickness, wave parameters coupled with bed slope and wind. A film acts to damp small scale waves, preventing their build up into larger waves which may break. Ironically, the beaking process is the primary mechanism for rapid dispersal of the film itself, therefore an understanding of the interaction is important. The work of [1,2] is extended using 2 approaches:
a) The mixing of a surface film after a break event, using the technique of Laser Induced Fluorescence (LIF) and
b) A study of how the effect of a film affects steep gravity waves with parasitic capillary waves riding on the leading face.
The work is ongoing, in this paper some of the effects of different breaking wave amplitudes on the mixing process are presented, along with preliminary work on how the surface film damps waves and affects the energy transfer between different length scales in the waves.

Senet, C. M., J. Seemann, F. Ziemer:
Hydrographic parameter maps determined from optical image sequences of dynamic water surfaces

An experiment at the wind wave of the University of Hamburg was performed to retrieve optical image sequences of the water surface. The analysis of the image sequences based on a three-dimensional Fast Fourier Transformation (3D FFT) is limited to homogeneous and stationary wave fields. This spectral description is limited to stationary and homogeneous wave fields and therefore other techniques which are designed for local scales are applied. For the examination of inhomogeneous wave fields a new method based on inverse and directional filter techniques is applied to retrieve hydrographic parameters on a local spatial scale. The results show the ability of the both methods to analyse and interpret multi-directional homogeneous and inhomogeneous sea states.

da Silva, J. C. B., S. A. Ermakov, and I. S. Robinson:
SAR imaging of slick modulated dynamic features

A method for remote determination of the presence of surfactants in the sea surface microlayer based on observations of internal wave (IW) signatures on satellite radar imagery is proposed. It is shown that small variations of surfactant concentration that occur on shelf zones caused by tidal flow can modulate the surface signatures of IWs, producing different signatures according to the local film concentration and wind speed. An analysis and classification of ERS SAR signatures of short-period IWs on the Iberian shelf is presented. Here it is shown that onshore propagating short-period IWs exhibit different SAR signatures according to their relative position to the phase of the internal tidal wave. These transitions of signature mode can be explained by large scale variations of film parameters produced by the internal tidal waves. at low -to- moderate winds leading waves in the IW packets usually exhibit double sign signatures, which evolve presumably to negative sign signatures for the following waves in the packets. At very low near threshold winds the IW signatures can be in the form of the double sign type undergoing transition to positive sign signatures when compared to the background behind the IW packet. This is a consequence of different threshold wind speeds for excitation of Bragg waves at different film pressures. We present experimental evidence of an increase of unperturbed film pressure in the direction of the ranking order of solitons in a packet of short-period IWs which is predicted by the model and in agreement with measurements of film elasticity.

Woolf, D. K. and N. Ward:
Thermal imagery of surface renewal phenomena

Processes within the sea surface microlayer have been relatively little studied considering their major role in air-sea interaction, especially gas exchange. This state of affairs can be explained by the inaccessibility of this very thin layer to most methods. One of the few suitable tools is radiometry which can probe a surface layer of ~1 - 1000 micrometres according to the wavelength chosen. In particular, thermal imagery can be used to investigate turbulence impinging on the sea surface - "surface renewal patterns". Field measurements with thermal imaging cameras show evidence of fairly large-scale (~1 metre) organised turbulent structures. Thermal imagery is also a powerful tool in the study of breaking waves. Thermal images of breaking waves are characterised by a "hot" high-emissivity crest ahead of a warm patch. Interpretation of field observations remains difficult. Greater control on variables is possible in the laboratory. Laboratory experiments show that in dynamically weak situations (e.g., free convection), though the influence of eddies is readily apparent, these eddies do not appear to bring bulk water to the absolute surface. Even quite weak bubble plumes are highly effective in renewing the surface, suggesting that they will influence the thermal signature of the sea surface following breaking waves, and may be an effective agent of air-sea gas exchange. Surface-active materials both influence the nature of surface renewal and the measurement process (through alteration of emissivity).


Addresses

Arnott, Alistair, Abteilung EV, Deutsches Zentrum fuer Luft- und Raumfahrt (DLR), Göttingen, Germany
Atmane, Mohamed A., Applied Physics Laboratory, University of Washington, Seattle, WA, USA
Asher, William, Applied Physics Laboratory, University of Washington, Seattle, WA, USA
Bakhanov, Victor V., Applied Physics Laboratory Russian Academy of Sciences, Nizhny Novgorod, Russia
Boxall, Simon R., School of Ocean and Earth Science, Southampton Oceanography Centre, Southampton, UK
Braun, Nicole, Institut für Küstenforschung, GKSS Forschungszentrum, Geesthacht, Germany
Buick, James M., CEAC; Aston University, Birmingham, UK
Byfield
, Valborg, School of Ocean and Earth Science, Southampton Oceanography Centre, Southampton, UK
Dovey, Peter R., QinetiQ Inc., Winfrith Newburgh, Dorsey, UK
Dysthe, Kristian B., Dept. of Mathematics, University of Bergen, Bergen, Norway
Ermakov, Stanislav A., Applied Physics Laboratory Russian Academy of Sciences, Nizhny Novgorod, Russia
Frew, Nelson M., Woods Hole Oceanographic Institution, Woods Hole, MA, USA
Gade, Martin, Institut für Meereskunde, Universität Hamburg, Hamburg, Germany
Gommenginger
, Christine P., School of Ocean and Earth Science, Southampton Oceanography Centre, Southampton, UK
Greidanus, HSF van Wimersma, TNO Physics and Electronics Laboratory, JG Den Haag, The Netherlands
Horstmann, Jochen, Institut für Küstenforschung, GKSS Forschungszentrum, Geesthacht, Germany
Hühnerfuss, Heinrich, Institut für Organische Chemie, Universität Hamburg, Hamburg, Germany
Hwang, Paul A., Naval Research Laboratory, Stennis Space Center, MS, USA
Ivanov, Andrei, P.P. Shirshov Institute of Oceanology, Russian Academy of Sciences, Moscow, Russia
Jenkins, Alastair D., DNMI Marine Forecasting Centre, Bergen Norway
Jones, Bethan, Marine Science Laboratories, University of Wales, Bangor, UK
Klinke, Jochen, Scripps Institution of Oceanography, University of California, San Diego, CA, USA
Korenowski, Gerald M., Dept. of Chemistry, Rensselaer Polytechnic Institute, Troy, NY, USA
Lange, Philipp A., Windwellenkanal der Universität Hamburg, Bundesanstalt für Wasserbau, Hamburg, Germany
Lehner, Susanne, Deutsches Zentrum fuer Luft- und Raumfahrt (DLR), IMF, Oberpfaffenhofen, Germany
Litovchenko, Konstantin, Space Research Institute, Russian Academy of Sciences, Moscow, Russia
Loglio, Giuseppe, Dept. of Organic Chemistry, University of Florence, Florence, Italy
McKenna, Sean P., Dept. of Applied Ocean Physics and Engineering, Woods Hole Oceanographic Insitution, Woods Hole, MA, USA
Noskov, Boris A., Research Institute of Chemistry, St. Petersburg State University, Staryj Petergof, Russia
Pelinovski, Efim, Institute of Applied Physics, Russian Academy of Sciences, Nizhny Novgorod, Russia
Pogorzelski, Stanislaw J., Institute of Experimental Physics, University of Gdansk, Gdansk, Poland
Robinson, Ian S., School of Ocean and Earth Science, Southampton Oceanography Centre, Southampton, UK
Scott, John C., QinetiQ Inc., Winfrith Newburgh, Dorsey, UK
Schimpf, Uwe, Interdisciplinary Center for Scientific Computing, University of Heidelberg, Heidelberg, Germany
Senet, Christian M., Institut für Küstenforschung, GKSS Forschungszentrum, Geesthacht, Germany
da Silva, José C.B., Instituto de Oceanographia, Universidade de Lisboa, Lisboa, Portugal
Stapleton, Neil R., QinetiQ Inc., Winfrith Newburgh, Dorsey, UK
Thomas, N.H., FRED Ltd., Birmingham, UK
Woolf, David K., James Rennell Division, Southampton Oceanography Centre, Southampton, UK


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