The thermodynamic and emissive properties of the ocean thermal skin layer are crucial contributors to air‐sea heat flux. In order to properly observe ocean surface temperature without disturbing any ...delicate fluid mechanical processes, thermal infrared imaging is often used. However, wind impacting the ocean surface complicates the extraction of meaningful information from thermal imagery; this is especially true for transient forcing phenomena such as wind gusts. Here, we describe wind gust‐water surface interaction through its impact on skin layer thermal and emissive properties. Two key physical processes are identified: (1) the growth of centimeter‐scale wind waves, which increases interfacial emissivity, and (2) microscale wave breaking and shear, which mix the cool skin layer with warmer millimeter‐depth water and increase the skin temperature. As more observations are made of air‐sea interaction under transient forcing, the full consideration of these processes becomes increasingly important.
Plain Language Summary
When a wind gust impacts an air‐water interface, two separate processes work to increase the temperature sensed by an infrared camera. The shortwave‐roughened surface becomes more emissive, and the skin layer (upper tens to hundreds of micrometers) becomes warmer as it is mixed by microscale wave breaking. The present paper identifies the effects of both processes in a field observational data set. This work is important to the quantification of air‐sea heat flux from thermal infrared measurements.
Key Points
Wind gusts produce transient ocean skin layer thermal fronts that propagate near the observed wind speed
Wind gust fronts disrupt the ocean thermal skin layer due to microbreaking and increase emissivity due to capillary‐gravity wave growth
Following wind gust front passage, capillary‐gravity wave relaxation reduced surface emissivity faster than the cool skin was restored
•Velocity fields are measured in stratified flows from the wall to the interface.•Smooth stratified cases follow the laws established for single-phase pipe flows.•Detailed interface statistics show ...waves belong to the capillary–gravity type.•Comparison with linear wave theory is good; deviation is due to wave asymmetry.•Wave-induced velocity profiles show an effect of the oscillatory boundary layer.
Stratified two-phase pipe flow of air and water is studied in a horizontal transparent pipe of 50mm inner diameter with the goal of obtaining detailed velocity measurements and wave characteristics in the flowing liquid layer.
The velocity field below the moving interface was measured by combining planar PIV and a profile capturing technique. Depending on the flow rates of air and water, the liquid layer can be laminar, transitional or turbulent, and the interface can be smooth or wavy. Conditional averaging based on the wave phase was applied in order to decompose the velocity field into three components: time-averaged velocity, wave-induced fluctuations and turbulence-induced fluctuations.
Detailed interface statistics and phase-averaged velocity fields were obtained for laminar and turbulent wavy cases. The waves are shown to be asymmetric, with gravitational and capillary forces of similar magnitude. The linear wave theory provides a good approximation of the wave-induced velocity profiles. In the region close to the interface, however, the wave nonlinearity caused a deviation. In turbulent cases, a deviation was also observed close to the pipe wall, due to the existence of an oscillatory boundary layer. The separation of wavy and turbulent motion was only partly successful due to the wide range of wavelengths and heights occurring in all the wavy cases.
We consider here waves that propagate in equatorial oceanic regions. By means of Crandall–Rabinowitz bifurcation theory we prove existence of steady periodic two-dimensional equatorial geophysical ...water waves with capillary effects and stagnation points. We also show that if the vorticity is big enough these flows possess stagnation points. Moreover, we prove that the free surface has a priori regularity. The dispersion relation, i.e. a formula giving the speed at the free surface of the bifurcating laminar flow (in terms of the constant vorticity, mean depth, wave number, Earth’s rotation speed), is also provided.
Submesoscale fronts often become visible when the accumulation of biosurfactants in the water surface microlayer causes smooth surfaces, called frontal slicks, to develop. Based on in situ and ...remotely‐sensed data, a frontal slick was documented for the first time in a lake (Lake Geneva). A quasi‐stationary ∼10‐km long slick formed on the warm side of a surface temperature front with strong horizontal velocity strain. The slick width increased from ∼50 to ∼200 m in ∼1.5 hr due to “feeding” by wind‐driven, rapidly‐moving smaller slicks. Numerical modeling results, confirmed by satellite data, indicated that the boundary between mesoscale gyres isolated warm surface water from cold water associated with wind‐induced coastal upwelling. Measurements and modeling suggest that frontogenetic sharpening of the submesoscale temperature gradient created an active front with strong convergent flow. Such dynamics must be considered in buoyant material transport and the vertical exchange of surface water with deeper layers in lakes.
Plain Language Summary
Near‐surface currents in lakes are affected by interactions of structures of different scales, such as gyres, eddies and coastal upwelling. These interactions can lead to the formation of fronts that are zones of convergence and downwelling, leaving floating materials to concentrate on the surface. Fronts are important because they can modify the lateral transport of surface material and the exchange between near‐surface and deeper layers. Among the floating materials, biosurfactants create smooth surface areas (slicks) by suppressing capillary gravity waves, thus allowing visual detection of fronts in remote imaging. This study, carried out in Lake Geneva, documents for the first time the existence of a submesoscale (∼10 km) frontal slick in a lake. Three‐dimensional numerical modeling and satellite imagery showed that due to the interaction of mesoscale gyres and eddies with coastal upwelling, a thermal front was generated in the area where the frontal slick was observed. In situ field measurements provided evidence that the observed slick was located on the warm side of the predicted thermal front. Since frontal slicks are easy to observe and track, they can provide valuable information about mesoscale dynamics in lakes, which at present are poorly understood.
Key Points
A submesoscale frontal slick is documented in a lake for the first time; it evolved on the warm side of a surface temperature front
The temperature front was formed by mesoscale circulation that separated relatively warm surface water from colder upwelling‐induced water
The slick was 10‐km long and its width increased by a factor of four in 1.5 hr due to “feeding” by wind‐driven small‐scale slicks
In this paper we construct small-amplitude periodic capillary-gravity water waves with a piecewise constant vorticity distribution. They describe water waves traveling on superposed linearly sheared ...currents that have different vorticities. This is achieved by associating to the height function formulation of the water wave problem a diffraction problem where we impose suitable transmission conditions on each line where the vorticity function has a jump. The solutions of the diffraction problem, found by using local bifurcation theory, are the desired solutions of the hydrodynamical problem.
На основании метода, предложенного Келдышем, изучен случай образования волновых движений на поверхности идеальной однородной бесконечной жидкости, когда под поверхностью жидкости на конечной глубине ...помещен вихревой симметричный диполь. В рамках двумерной задачи рассматривалась плоская бегущая волна синусоидальной формы, в которой каждая ее частица будет двигаться по окружности, расположенной в вертикальной плоскости, то есть центр окружности будет совпадать с направлением распространения волны. В качестве источника возмущений выбран не одиночный вихрь, а волновой диполь. Получены два асимптотических решения для профиля волны на свободной поверхности: профиль капиллярно-гравитационных волн до источника возмущений, где ключевую роль в формировании волновых возмущений играет поверхностное натяжение; профиль капиллярно-гравитационных волн после источника возмущений, при формировании которого доминирующей является сила тяжести. Показано, что при разложении асимптотических решений в ряд Тейлора для профиля волны на свободной поверхности, для капиллярно-гравитационных волн характерны следующие закономерности: при сравнительно небольших расстояниях от источника возмущений профиль волны фактически линейный, приближения не вносят существенного влияния, то есть волна стремиться к предельной форме; но, по мере удаления от волнового диполя начинает формироваться синусоидальный профиль волны. На формирование профиля волны оказывает влияение изменение глубины источника возмущения. Так, например, при уменьшении h в капиллярно-гравитационной волне преобладает капиллярная составляющая, а при увеличении h более весомый вклад вносит гравитационная составляющая.
We investigate theoretically the effects of capillarity and gravity on interfacial Marangoni waves (M-waves). Previous studies which neglected these effects have shown the decay rate of M-waves to ...vary monotonically. In this work, we disprove it by deriving a new dispersion relation (accounting for these effects) for the M-waves, and show that their decay rate passes through a minimum, rather than varying monotonically. This result, obtained numerically elsewhere, was explained using a hypothetical directional packaging process. Our explanation here is consistent with the bigger picture, which includes both capillary-gravity waves (CG-waves) and M-waves, which coexist at the contaminated interface of two fluids. It is known that the CG-waves undergo maximum dissipation at a critical value of interfacial elasticity, which provides the restoring force for the M-waves. Here, we complete the story; we show that the M-waves undergo minimum dissipation at a critical value of capillarity/gravity, which provide the restoring forces for the CG-waves. Previously, it has been hypothesized that the maximum decay rate for CG-waves is caused due to their resonance with M-waves. Here, we show this hypothesis to be true for a temporally-damped wave system, and that the resonance (in addition to causing the maximum decay rate for CG-waves) also causes the minimum decay rate for M-waves.
Consideration in this paper is three-dimensional capillary–gravity water flows governed by the geophysical water wave equations with all the Coriolis terms being retained. It is proved that the ...merely possible flow exhibiting a constant vorticity vector captures vanishing vertical velocity, constant horizontal velocity and flat free surface.
A higher order current modified nonlinear Schrödinger equation (NLSE) in the case of broader bandwidth capillary-gravity waves travelling on the interface between two fluids extending to infinity is ...derived. This equation is extended by relaxing the narrow bandwidth restriction so that it will be more suitable for application to a realistic ocean wave spectrum. From the narrow and broader-banded evolution equations, the two-dimensional instability regions are plotted for different values of density ratio of two fluids, wave steepness, the non-dimensional velocity of the upper fluid and the surface tension coefficient. The instability regions corresponding to both an air-water interface and a Boussinesq approximation are also presented. It is important to note that the new broader-banded equation is found to predict an instability region in good agreement with the exact numerical results. The effect of surface tension is to expand the instability region in the perturbed wave numbers plane.
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