Laboratory investigation of the effect of sea foam on the scattering of microwave radiation

2020 ◽  
Author(s):  
Georgy Baydakov ◽  
Ermakova Olga ◽  
Vdovin Maxim ◽  
Sergeev Daniil ◽  
Troitskaya Yuliya
Keyword(s):  
Wind Speed ◽  
Surface Waves ◽  
Water Surface ◽  
Wind Waves ◽  
Short Wave ◽  
Doppler Spectrum ◽  
Momentum Exchange ◽  
High Frequency Region ◽  
Wave Flume ◽  
The Impact

<p>This paper models the impact of the presence of foam on the short-wave component of surface waves and momentum exchange in the atmospheric boundary layer at high winds. First, physical experiments were carried out in a wind-wave flume in which foam can be artificially produced at the water surface. Tests were conducted under high wind-speed conditions where equivalent 10-m wind speed, U10, ranged 12–38 m/s, with measurements made of the airflow parameters, the frequency-wavenumber spectra of the surface waves, the foam coverage of the water surface, and the distribution of the foam bubbles.</p><p>Microwave measurements were performed using a coherent Doppler X-band scatterometer with a wavelength of 3.2 cm and a sequential reception of linearly polarized radiation. It was shown that the presence of foam reduces the NRCS of the agitated water surface. Foam formations are concentrated mainly on the ridges and front slopes of wind waves, which make the main contribution to the scattering of radio waves. This may explain the effect of reducing the total NRCS: foam, which has less reflective properties, masks the main diffusers on the water surface. The second mechanism is associated with the effect of foam on short waves, by analogy with surfactant films.</p><p>The effect of foam on the shape of the Doppler spectrum of a microwave signal scattered by the water surface was investigated. In the case of weak wind, the presence of foam on the surface leads to a decrease in the short-wave part of the spectrum of surface waves and, as a result, to a decrease in the scattered signal. In addition, a mirror component appears in the Doppler spectrum corresponding to the fundamental frequency of the wave. In the case of a stronger wind, the grouping of additional scatterers (foam) on the crests of the waves leads to a shift of the Doppler spectra to the high-frequency region.</p><p>The work was supported by the RFBR (grants 18-35-20068, 19-05-00366, 19-05-00249) and the RF President’s Grant for Young Scientists (MK-144.2019.5).</p>

scholarly journals The Effect of Foam on Waves and the Aerodynamic Roughness of the Water Surface at High Winds

2019 ◽  
Vol 49 (4) ◽  
pp. 959-981 ◽  
Author(s):  
Yu. Troitskaya ◽  
D. Sergeev ◽  
A. Kandaurov ◽  
M. Vdovin ◽  
S. Zilitinkevich
Keyword(s):  
Boundary Layer ◽  
Wind Speed ◽  
Atmospheric Boundary Layer ◽  
Drag Coefficient ◽  
Surface Waves ◽  
Water Surface ◽  
Eddy Viscosity ◽  
Surface Drag ◽  
Momentum Exchange ◽  
High Winds

AbstractThis paper models the impact of the presence of foam on the short-wave component of surface waves and momentum exchange in the atmospheric boundary layer at high winds. First, physical experiments were carried out in a wind-wave flume in which foam can be artificially produced at the water surface. Tests were conducted under high-wind-speed conditions where equivalent 10-m wind speed ranged from 12 to 38 m s−1, with measurements made of the airflow parameters, the frequency–wavenumber spectra of the surface waves, the foam coverage of the water surface, and the distribution of the foam bubbles. Analysis of the resulting data indicates that the surface drag coefficient correlates with the fraction of foam coverage and the mean square slope (MSS) of the water surface, and that, at a certain wind speed, the MSS decreases with an increase in the fraction of foam coverage. Based on these results, we suggest a simple model for eddy viscosity in the turbulent boundary layer over a fractionally foam-covered wave surface. The measurements in a laboratory environment are shown to be in good agreement with the predictions of a quasi-linear model of the atmospheric boundary layer over a waved water surface that adopts this eddy viscosity. Adaptation of the proposed model to field conditions is discussed, and the synergetic effect of foam at the water surface and spray in the marine atmospheric boundary layer on ocean surface resistance at high winds is estimated so as to be able to explain the observed peaking dependence of the surface drag coefficient on the 10-m wind speed.

scholarly journals Suppression of Wind Ripples and Microwave Backscattering Due to Turbulence Generated by Breaking Surface Waves

2020 ◽  
Vol 12 (21) ◽  
pp. 3618
Author(s):  
Stanislav Ermakov ◽  
Vladimir Dobrokhotov ◽  
Irina Sergievskaya ◽  
Ivan Kapustin
Keyword(s):  
Remote Sensing ◽  
Surface Waves ◽  
Wave Breaking ◽  
Wave Train ◽  
Wind Waves ◽  
Breaking Waves ◽  
Doppler Spectrum ◽  
Strong Wave ◽  
Wave Maker ◽  
Radar Backscattering

The role of wave breaking in microwave backscattering from the sea surface is a problem of great importance for the development of theories and methods on ocean remote sensing, in particular for oil spill remote sensing. Recently it has been shown that microwave radar return is determined by both Bragg and non-Bragg (non-polarized) scattering mechanisms and some evidence has been given that the latter is associated with wave breaking, in particular, with strong breaking such as spilling or plunging. However, our understanding of mechanisms of the action of strong wave breaking on small-scale wind waves (ripples) and thus on the radar return is still insufficient. In this paper an effect of suppression of radar backscattering after strong wave breaking has been revealed experimentally and has been attributed to the wind ripple suppression due to turbulence generated by strong wave breaking. The experiments were carried out in a wind wave tank where a frequency modulated wave train of intense meter-decimeter-scale surface waves was generated by a mechanical wave maker. The wave train was compressed according to the gravity wave dispersion relation (“dispersive focusing”) into a short-wave packet at a given distance from the wave maker. Strong wave breaking with wave crest overturning (spilling) occurred for one or two highest waves in the packet. Short decimeter-centimeter-scale wind waves were generated at gentle winds, simultaneously with the long breaking waves. A Ka-band scatterometer was used to study microwave backscattering from the surface waves in the tank. The scatterometer looking at the area of wave breaking was mounted over the tank at a height of about 1 m above the mean water level, the incidence angle of the microwave radiation was about 50 degrees. It has been obtained that the radar return in the presence of short wind waves is characterized by the radar Doppler spectrum with a peak roughly centered in the vicinity of Bragg wave frequencies. The radar return was strongly enhanced in a wide frequency range of the radar Doppler spectrum when a packet of long breaking waves arrived at the area irradiated by the radar. After the passage of breaking waves, the radar return strongly dropped and then slowly recovered to the initial level. Measurements of velocities in the upper water layer have confirmed that the attenuation of radar backscattering after wave breaking is due to suppression of short wind waves by turbulence generated in the breaking zone. A physical analysis of the effect has been presented.

Experimental study of the initial stages of wind waves' spatial evolution

2011 ◽  
Vol 681 ◽  
pp. 462-498 ◽  
Author(s):  
DAN LIBERZON ◽  
LEV SHEMER
Keyword(s):  
Energy Transfer ◽  
Growth Rates ◽  
Water Surface ◽  
Wind Waves ◽  
Pressure Fluctuations ◽  
Theoretical Models ◽  
Small Scale ◽  
Wave Flume ◽  
Spatial Growth ◽  
The Mean

Despite a significant progress and numerous publications over the last few decades a comprehensive understanding of the process of waves' excitation by wind still has not been achieved. The main goal of the present work was to provide as comprehensive as possible set of experimental data that can be quantitatively compared with theoretical models. Measurements at various air flow rates and at numerous fetches were carried out in a small scale, closed-loop, 5 m long wind wave flume. Mean airflow velocity and fluctuations of the static pressure were measured at 38 vertical locations above the mean water surface simultaneously with determination of instantaneous water surface elevations by wave gauges. Instantaneous fluctuations of two velocity components were recorded for all vertical locations at a single fetch. The water surface drift velocity was determined by the particle tracking velocimetry (PTV) method. Evaluation of spatial growth rates of waves at various frequencies was performed using wave gauge records at various fetches. Phase relations between various signals were established by cross-spectral analysis. Waves' celerities and pressure fluctuation phase lags relative to the surface elevation were determined. Pressure values at the water surface were determined by extrapolating the measured vertical profile of pressure fluctuations to the mean water level and used to calculate the form drag and consequently the energy transfer rates from wind to waves. Directly obtained spatial growth rates were compared with those obtained from energy transfer calculations, as well as with previously available data.

scholarly journals Searching for chaotic deterministic features in laboratory water surface waves

2000 ◽  
Vol 7 (1/2) ◽  
pp. 37-48 ◽  
Author(s):  
M. Joelson ◽  
Th. Dudok de Wit ◽  
Ph. Dussouillez ◽  
A. Ramamonjiarisoa
Keyword(s):  
Surface Waves ◽  
Water Surface ◽  
Wind Waves ◽  
Theoretical Models ◽  
Resonant Interactions ◽  
Mechanical Waves ◽  
Random Character ◽  
Nonlinear Features ◽  
Low Dimensional ◽  
Laboratory Water

Abstract. The dynamic evolution of laboratory water surface waves has been studied within the framework of dynamical systems with the aim to identify stochastic or deterministic nonlinear features. Three different regimes are considered: pure wind waves, pure mechanical waves and mixed (wind and mechanical) waves. These three regimes show different dynamics. The results on wind waves do not clearly support the recently proposed idea that a deterministic Stokes-like component dominate the evolution of such waves; they are more appropriately described by a similarity-like approach that includes a random character. Cubic resonant interactions are clearly identified in pure mechanical waves using tricoherence functions. However, detailed aspects of the interactions do not fully agree with existing theoretical models. Finally, a deterministic motion is observed in mixed waves, which therefore are best described by a low dimensional nonlinear deterministic process.

scholarly journals Structure of the Airflow above Surface Waves

2016 ◽  
Vol 46 (5) ◽  
pp. 1377-1397 ◽  
Author(s):  
Marc P. Buckley ◽  
Fabrice Veron
Keyword(s):  
Surface Waves ◽  
Wind Waves ◽  
Quadrant Analysis ◽  
Momentum Exchange ◽  
Turbulent Structures ◽  
Mean Velocity ◽  
Law Of The Wall ◽  
Generate Surface ◽  
Turbulent Momentum ◽  
Sheltering Effect

AbstractIn recent years, much progress has been made to quantify the momentum exchange between the atmosphere and the oceans. The role of surface waves on the airflow dynamics is known to be significant, but our physical understanding remains incomplete. The authors present detailed airflow measurements taken in the laboratory for 17 different wind wave conditions with wave ages [determined by the ratio of the speed of the peak waves Cp to the air friction velocity u* (Cp/u*)] ranging from 1.4 to 66.7. For these experiments, a combined particle image velocimetry (PIV) and laser-induced fluorescence (LIF) technique was developed. Two-dimensional airflow velocity fields were obtained as low as 100 μm above the air–water interface. Temporal and spatial wave field characteristics were also obtained. When the wind stress is too weak to generate surface waves, the mean velocity profile follows the law of the wall. With waves present, turbulent structures are directly observed in the airflow, whereby low-horizontal-velocity air is ejected away from the surface and high-velocity fluid is swept downward. Quadrant analysis shows that such downward turbulent momentum flux events dominate the turbulent boundary layer. Airflow separation is observed above young wind waves (Cp/u*< 3.7), and the resulting spanwise vorticity layers detached from the surface produce intense wave-coherent turbulence. On average, the airflow over young waves (with Cp/u* = 3.7 and 6.5) is sheltered downwind of wave crests, above the height of the critical layer zc [defined by 〈u(zc)〉 = Cp]. Near the surface, the coupling of the airflow with the waves causes a reversed, upwind sheltering effect.

Microphysics of the air-sea coupling at high winds and its role in the dynamics and thermodynamics of severe sea storms

2020 ◽  
Author(s):  
Yuliya Troitskaya ◽  
Alexander Kandaurov ◽  
Daniil Sergeev ◽  
Olga Ermakova ◽  
Dmitrii Kozlov ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Atmospheric Boundary Layer ◽  
Water Surface ◽  
Synergetic Effect ◽  
Ocean Surface ◽  
Small Scale ◽  
Momentum Exchange ◽  
Momentum Fluxes ◽  
The Impact ◽  
High Winds

&lt;p&gt;Showing the record strengths and growth-rates, a number of recent hurricanes have highlighted needs for improving forecasts of tropical cyclone intensities most sensitive to models of the air-sea coupling. Especially challenging is the nature and effect of the very small-scale phenomena, the sea-spray and foam, supposed to strongly affecting the momentum- and heat- air-sea fluxes at strong winds. This talk will focus on our progress in understanding and describing these &quot;micro-scale&quot; processes, their physical properties, the spray and foam mediated air-sea fluxes and the impact on the development of marine storms.&lt;/p&gt;&lt;p&gt;The starting points for this study were two laboratory experiments. The first one was designed for investigation of the spray generation mechanisms at high winds. We found out 3 dominant spray generating mechanisms: stretching liquid ligaments, bursting bubbles, splashing of the falling droplets and &quot;bag-breakup&quot;. We investigated the efficiency spray-production mechanisms and developed the empirical statistics of the numbers of the spray generating events of each type. Basing on the &quot;white-cap method&quot; we found out the dependence of the spray-generating events on the wind fetch. The main attention was paid to the &quot;bag-breakup&quot; mechanism. Here we studied in detail the statistics of spray produced from one &quot;bag-breakup&quot; event. Basing on these developments, we estimated heat and momentum fluxes from the spray-generating events of different types and found out the dominant role of the &quot;bag-breakup&quot; mechanism.&lt;/p&gt;&lt;p&gt;To estimate the direct heat and momentum fluxes from the ocean surface to the atmosphere, we studied in the special experiment the foam impact on the short-wave part of the surface waves and the heat momentum exchange in the atmospheric boundary layer at high winds. Based on these results, we suggest a simple model for the aerodynamic and temperature roughness and the eddy viscosity in the turbulent boundary layer over a fractionally foam-covered water surface.&lt;/p&gt;&lt;p&gt;The synergetic effect of foam at the water surface and spray in the marine atmospheric boundary layer on ocean surface resistance at high winds is estimated so as to be able to explain the observed peculiarities of the air-sea fluxes at stormy conditions. Calculations within the nonhydrostatic axisymmetric model show, that the &quot;microphysics&quot; of the air-sea coupling significantly accelerate development of the ocean storm.&lt;/p&gt;&lt;p&gt;This work was supported by RFBR grant 19-05-00249 and RSF grant 19-17-00209.&lt;/p&gt;

The impact of atmosphere-ocean-wave coupling on extreme surface wind forecasts

2021 ◽  
Author(s):  
Emanuele Silvio Gentile ◽  
Suzanne L. Gray ◽  
Janet F. Barlow ◽  
Huw W. Lewis ◽  
John M. Edwards
Keyword(s):  
Wind Speed ◽  
Wind Waves ◽  
Surface Wind ◽  
Computational Cost ◽  
Sea Surface ◽  
Model Systems ◽  
Sea State ◽  
Extreme Surface ◽  
Wind Speeds ◽  
The Impact

&lt;p&gt;Accurate modelling of air-sea surface exchanges is crucial for reliable extreme surface wind forecasts.&amp;#160; While atmosphere-only weather forecast models represent ocean and wave effects through sea-state independent parametrizations, coupled multi-model systems capture sea-state dynamics by integrating feedbacks between atmosphere, ocean and wave model components.&lt;/p&gt;&lt;p&gt;Here, we present the results of studying the sensitivity of extreme surface wind speeds to air-sea exchanges at kilometre scale using coupled and uncoupled configurations of the Met Office's UK Regional Coupled Environmental Prediction (UKC4) system. The case period includes the passage of extra-tropical cyclones Helen, Ali, and Bronagh, which brought maximum gusts of 36 ms&lt;sup&gt;-1&lt;/sup&gt; over the UK.&lt;/p&gt;&lt;p&gt;Compared to the atmosphere-only results, coupling to ocean decreases the domain-average sea surface temperature by up to 0.5 K. Inclusion of coupling to waves decreases the 98th percentile 10-m wind speed by up to 2 ms&lt;sup&gt;-1&lt;/sup&gt; as young, growing wind waves decrease wind speed by increasing the sea aerodynamic roughness. Impacts on gusts are more modest, with local reductions of up to 1ms &lt;sup&gt;-1,&lt;/sup&gt; due to enhanced boundary-layer turbulence which partially offsets air-sea momentum transfer.&lt;/p&gt;&lt;p&gt;Using a new drag parametrization based on the COARE~4.0 scheme, with a cap on the neutral drag coefficient and decrease for wind speeds exceeding 27 ms&lt;sup&gt;-1 &lt;/sup&gt;, the atmosphere-only model achieves equivalent impacts on 10-m wind speeds and gusts as from coupling to waves. Overall, the new drag parametrization achieves the same 20% improvement in forecast 10-m wind skill as coupling to waves, with&amp;#160; the&amp;#160; advantage&amp;#160; of saving the computational cost of the ocean and wave models.&amp;#160;&lt;/p&gt;

Laboratory investigation of air-sea momentum transfer under severe wind conditions

2020 ◽  
Author(s):  
Maksim Vdovin ◽  
Georgy Baydakov ◽  
Daniil Sergeev ◽  
Yuliya Troitskaya
Keyword(s):  
Wind Speed ◽  
Data Processing ◽  
Drag Coefficient ◽  
Financial Support ◽  
Water Surface ◽  
Wind Wave ◽  
Aerodynamic Resistance ◽  
Sea Surface ◽  
Surface Drag ◽  
Wave Flume

&lt;p&gt;Wind-wave interaction at extreme wind speed is of special interest now in connection with the problem of explanation of the sea surface drag saturation at the wind speed exceeding 30 m/s. Now it is established that at hurricane wind speed the sea surface drag coefficient is significantly reduced in comparison with the parameterization obtained at moderate to strong wind conditions.&lt;/p&gt;&lt;p&gt;The subject of this work is investigation of aerodynamic resistance of the waved water surface under severe wind conditions (up to U10 &amp;#8776; 50 m/s). Laboratory experiments were carried out at the new high-speed wind-wave flume in the Large Thermally Stratified Tank (at the Institute of Applied Physics, Russia) built in 2019. The main difference between the new wind-wave flume and the old one is the absence of a pressure gradient along the main axis of the new flume. Aerodynamic resistance of the water surface was measured by the profile method with Pitot tube. A method for data processing taking into account the self-similarity of the air flow velocity profile in the aerodynamic tube was applied for retrieving wind friction velocity and surface drag coefficients. Simultaneously with the airflow velocity measurements, the wind-wave field parameters in the flume were investigated by system of wire gauges.&lt;/p&gt;&lt;p&gt;Analysis of the wind velocity profiles and wind-wave spectra showed tendency to decrease for surface drag coefficient for wind speed exceeding 25 m/s simultaneously with the mean square slope and significant wave height.&lt;/p&gt;&lt;p&gt;&lt;span&gt;Acknowledgments&lt;/span&gt; &lt;br&gt;This work was carried out with the financial support of the RFBR according to the research project 18-55-50005, 20-05-00322, 18-35-20068, 18-05-00265. Data processing was carried out with the financial support of Russian Science Foundation grant 19-17-00209.&lt;/p&gt;

Laboratory measurements of modulation of short-wave slopes by long surface waves

1991 ◽  
Vol 233 ◽  
pp. 389-404 ◽  
Author(s):  
Sarah J. Miller ◽  
Omar H. Shemdin ◽  
Michael S. Longuet-Higgins
Keyword(s):  
Surface Waves ◽  
Gravity Waves ◽  
Wind Waves ◽  
Short Wave ◽  
Wave Interactions ◽  
Wind Speeds ◽  
Short Waves ◽  
Hydrodynamic Modulation ◽  
Work Done ◽  
Good Agreement

Hydrodynamic modulation of wind waves by long surface waves in a wave tank is investigated, at wind speeds ranging from 1.5 to 10 m s−1. The results are compared with the linear, non-dissipative, theory of Longuet-Higgins & Stewart (1960), which describes the modulation of a group of short gravity waves due to straining of the surface by currents produced by the orbital motions of the long wave, and work done against the radiation stresses of the short waves. In most cases the theory is in good agreement with the experimental results when the short waves are not too steep, and the rate of growth due to the wind is relatively small. At the higher wind speeds, the effects of wind-wave growth, dissipation and wave-wave interactions are dominant.

Laboratory simulation of the pancake ice influence on the wind wave interaction

2020 ◽  
Author(s):  
Daniil Sergeev ◽  
Yuliya Troitskaya ◽  
Alexander Kandaurov
Keyword(s):  
Wind Speed ◽  
Wind Wave ◽  
Aerodynamic Drag ◽  
Average Distance ◽  
Wind Waves ◽  
Wave Interaction ◽  
Velocity Fields ◽  
The Arctic ◽  
Laboratory Simulation ◽  
Momentum Exchange

&lt;p&gt;Recently, much attention has been paid to the study and numerical simulation of wind waves in the Arctic regions of the oceans. Their distinctive feature is the presence of ice cover of various types, which can significantly affect the processes of wind wave interaction, including momentum exchange. A detailed study of such processes under natural conditions is very difficult, especially for the forming ice (including pancake ice), therefore, laboratory simulation is preferable. Previously studies of the influence of floating ice on the evolution of waves that were generated by wavemakers were carried out only. In this paper we present preliminary results of studies performed on the AELOTRON circular wind wave flume of the University of Heidelberg, where the interaction of air flow with a water surface was simulated for the first time in the presence of forming ice of the pancake type. Synchronous measurements of wave characteristics were carried out using a laser wavegauge, as well as airflow velocity fields were measured with PIV-methods. Shims made of rubber with a diameter of 7 cm and a thickness of 1 cm with a density of about 0.8 kg /m&lt;sup&gt;3&lt;/sup&gt; were used as elements of artificial ice. The measurements were carried out in clean water and at three concentrations of artificial ice: maximum, 2/3 of the maximum, 1/3 of the maximum. Ice covered about half the surface at maximum concentration. The measurements were carried out in the range of equivalent wind speeds U10 from 7 to 16&amp;#160; m/s. The threshold character of excitation of long waves was obtained (the length is much greater than the average distance between the elements of ice). The higher the density, the higher the threshold for wind speed. According to the results of processing the velocity fields, the dependence of the aerodynamic drag coefficient on the equivalent wind speed was constructed. It is shown that the presence of ice weakly affects the momentum exchange for all concentrations and over the entire wind speed range.&lt;/p&gt;&lt;p&gt;Investigation was supported by Russian Found Basic Research project # 18-05-60299 Arctic.&lt;/p&gt;

Sign in / Sign up

Export Citation Format

Share Document