Marine eddies observed in radar, optical and radiometric panoramas of the sea surface play an important role in the processes of ocean local and global mixing, suspended and organic matter redistribution, propagation of pollutant waters, biological processes et al. Meanwhile, in-situ investigations of the statistical characteristics of generating vortices in the water areas are difficult to conduct. The widespread method of marine eddies investigation is based on the remote sensing data. However, the relation between the characteristics of spiral structures and the marine eddies which lead to their manifestation is poorly studied. In order to investigate this relation, we apply the well-known MCC-method. Investigation of the horizontal fields related to the manifestation of spiral structures revealed new peculiarities which can be described analytically. The analytical model describes the velocity field composed of a marine eddy and a homogeneous current, the results are confirmed by the velocity fields obtained using satellite data, the new features of upper ocean currents associated with marine eddies are discussed. The described approach allows to retrieve inner structure of the genuine velocity field related to marine eddy.
The paper is focused on investigation of microwave backscattering from wind waves on a clean water surface. Field experiments were carried out in the coastal zone of the Black Sea using dual co-polarized Doppler X-band scatterometer and a three-band Doppler dual co-polarized radar (X-, С-, S-bands). The radar incidence angles were about 50 - 60 degrees, the wind changed in a wide range of speeds. We assumed that microwave backscattering at VV and HH polarizations is composed by a Bragg (polarized) component associated with Bragg waves and a non-polarized component (NBR). Analysis of Doppler spectra of NBR allowed us to remove the effect of strong wave breaking (overturning wave crests) from the time series and to study the backscatter associated only with dm-scale waves. Measurements of wind waves with a wire gauge were carried out simultaneously with the radar monitoring. It is shown that the velocities of non-Bragg scatterers not associated with strong wave breaking in X-, С-, S-bands correspond to the velocities of short dm waves and weakly depend on radar wavelength. The speeds of the scatterers in X-, С-, S-bands associated with overturning wave crests are also close to each other (within the measurement error). The intensity of NBR in X-, С-, S-bands grows with wind speed as well as with the intensity of dm-waves measured by the wire gauge. Strong suppression of NBR and simultaneously measured decrease of short dm-wave intensity are demonstrated, thus confirming the assumption that the intensity of the NBR in X-, С-, S-bands is determined by dm waves.
Analysis of possibilities of identification and characterization of marine processes using their signatures in radar and optical imagery of the sea surface is a very important problem of the ocean remote sensing which has not been solved yet completely by now. Marine slicks which are the areas of suppressed wind waves can be recorded by different sensors and can be indicators of internal waves, non uniform currents, atmospheric convective cells, etc. Field studies including those simultaneous and co-located with remote observations is the most perspective way to the problem solution. An expedition of the Institute of Applied Physics RAS was organized to study the nature of slick bands and its dynamics in the field of various subsurface processes. Field experiments were carried out in the coastal zone of the Black sea from the Oceanographic Platform of Marine Hydrophysical Institute RAS and from the shore. The structure of the currents in the studied area is characterized by significant heterogeneity, so we were able to register different slick structures in the flow field and wind and the slick dynamics. In some experiments, marine slicks were recorded simultaneously in satellite Sentinel images. Observations of surface manifestations of internal waves were carried out using a digital radar station MRS-1000 and multi-frequency radar complex of IAP RAS. At the same time the measurements of currents in the water column were carried out using the ADCP WH Monitor 1200 kHz, wind speed and direction at a height of 30 meters using WindSonic acoustic anemometer. During the passage of internal waves a system of slick bands with a reduced intensity of small-scale waves were observed. Slick bands were observed mainly over the rear slopes of the internal waves; the data from the accompanying measurements showed that the phase velocity was close to the surface current velocity. Theoretical analysis has shown that in this case the convergent zones, where surfactants are accumulated were formed at the rear slopes of the internal waves. This mechanism of slick formation was predicted earlier theoretically and then was modeled in laboratory experiment.
Recent studies of microwave radar return at moderate and large incidence angles have shown the backscattering is determined by resonance (Bragg) surface waves of cm-scale wavelength range, and by non polarized (non Bragg) component which is associated with wave breaking and quasi specular reflection. This paper is focused on results of field studies of non-Bragg backscattering from the clean water surface and from the water surface covered with surfactant films. The study was carried out using dual polarized X-band radars in the coastal zone of the Black Sea in 2017-2019 at an incidence angle of about 60 degrees. It was found that the radar return contains a Non Bragg component not related to the breaking crests and specular tilt areas at wind speeds from a threshold of the wind wave generation up to wind velocities of the order of 10 m/s. The part of the non-Bragg component not related to the wave breaking crests decreases strongly in the areas of film slicks. At high wind velocities the non-Bragg component out of the spikes is strongly modulated (several times larger than the Bragg component) in the long-wave field, in film slicks the modulation of the non-Bragg component increases. Analysis of the Doppler shifts showed that the velocities of the non-Bragg scatterers correspond to the dm-scale free surface waves and vary slightly in the areas of film slicks. Thus, we concluded that nonlinear features associated with the dm-scale wind waves cause the non-Bragg scattering.
Traditionally spiral slick structure is interpreted as a manifestation of marine eddy, which length scale is taken equal to the scale of spiral. This approach is based on the assumption that wind has no effect on the kinematics of forming slick bands which, according to our estimation, is incorrect in real sea conditions. This can lead to misinterpretation of remote sensing data in the field of definition of the characteristics of marine eddies, particularly, in radar images. The system of equations for the description of kinematics of a Lagrangian particle (element of surface active substance) in stationary fields of axisymmetric eddy with non-zero radial velocity component and homogeneous wind was obtained. It was shown that the center of the spiral is not collocated with the center of the eddy, and the distance between them can achieve the scale of eddy core. It was shown that the displacement of the spiral center is quasi perpendicular to the wind direction in case of small radial velocity component compared to the tangential one. It was shown analytically that there is a threshold wind velocity which corresponds to the breakdown of the spiral structure. Simulation based on the discrete-time approximation of particle trajectories, as well as radar observation of marine eddies, demonstrates the possibility of appearance of a “focus” and a “saddle” in the characteristic shape of slick bands. The perspectives of correct retrieval of length scales and character velocities of observed sub mesoscale marine eddies are discussed.
River confluence is a ubiquitous phenomenon which plays an important role in river dynamics, mixing processes, pollution transport etc. It can be often visually observed that two converging rivers continue to flow as two parallel weakly mixing streams separated by a relatively thin transition region – a mixing zone (MZ), which can reach out for some distance downstream the junction apex. A typical example of a river confluence is the merging of the Volga and Oka rivers (Russia). This paper presents some new results on the Volga/Oka rivers confluence based on both ‘in situ’ measurements of hydrological characteristics in the MZ and on satellite observations. During the ‘in situ’ measurements a large set of data regarding velocity fields, chlorophyll-a concentration, water temperature, turbidity, parameters of organic surface films etc. was obtained. It is found that significant differences between the hydrological characteristics of the Volga and Oka flows are observed at sufficiently large distances downstream the junction apex (about 10 river widths or more); and the mixing zone remains quite narrow. Film sampling and further analysis of the surfactant films in the MZ were carried out. It is shown that the surface tension decreases and the film elasticity grows in the foam/slick bands separating the Volga and Oka flows thus indicating the increased concentration of surfactants in the bands. Satellite images of the Volga-Oka MZ are collected indicating that the confluence area is clearly seen in satellite optical imagery (MSI Sentinel-2 satellite) due to strong difference in color between the flows. The radar imagery shows the mixing zone in the form of a slick/foam band (SAR Sentinel-1) which manifests on the water surface due to enhanced damping of short wind waves.
Understanding of physical mechanisms of gravity capillary waves (GCW) damping due to an impact of turbulence is important for developing methods of ship wake remote sensing, especially for tracking and characterization of ships. Analysis of literature reveals the necessity of setting a reliable experiment to study the effect of damping of GCW due to turbulence. Available laboratory studies are based on significantly different experimental methods with some disadvantages which have caused a large scatter of experimental data. The previously proposed method, which is based on the simultaneous independent generation of surface waves and turbulence in a wave tank, is free from inherent disadvantages of previously used methods. The method is used to conduct a series of experiments in order to measure the dependence of the damping coefficient of GCW on their frequency at different intensities of turbulence in a wide frequency range. Due to the proposed technique the range of surface wavelengths is extended in order to investigate the case when the surface wavelength is comparable to the scale of turbulence. It is shown that the frequency dependence of the eddy viscosity coefficient is characterized by the presence of a maximum for both turbulent regimes. The maximum value of eddy viscosity coefficient is proportional to the velocity of turbulent pulsations. The results are discussed in application to radar imaging of turbulent ship wake.
Surfactant films on the sea surface can appear due to pollutions, river and collector drains, as well as biological processes. Film slicks can indicate different processes in the upper ocean and in atmosphere. In particular, slick signatures in SAR-imagery of the sea surface at low and moderate wind speeds are often associated with marine currents. Other factors such as wind and physical characteristics of films can significantly influence the dynamics of slick structures. A perspective approach aimed at measuring surface currents is developed. Based on the approach an impact of wind on the kinematics of artificial slick bands is determined. Simulation of slick band propagation from the localized source of surfactant in the field of wind and eddy-shape current is performed. As a result of simulation the shape of surface slick structure, which is close to the observed on SAR image of water surface, is obtained. It is shown that the possibility of spiral bands formation due to presence of marine submesoscale eddies is determined by near-surface wind. Moreover, it is declared that a traditional estimation of scales of marine eddies based on the scales of spiral slick structures is not basically correct.
The present paper reveals the practical possibilities of the solar path observation from underwater as an instrument for remote sensing of wavy surface. Paper includes: a theoretical model of underwater solar path image and its statistical moments; algorithm for solving inverse problem; description of full-scale experiment and results of slope frequency spectrum retrieval. Presented spectra were obtained for random selected data from continues field measurements in the coastal zone of the Black Sea. On their example it was shown that spectra obtained by image processing and wave gauge data are close over the entire frequency interval. At frequencies corresponding to capillary waves, where wave gauge not applicable, underwater vision system still continues to measure wave spectrum. Obtained results complement previously published results of retrieval wind wave characteristics and water optical properties using underwater solar path images, thereby confirming advantages of applying underwater optical systems for remote sensing of different waters.
Dual-polarized microwave radars are of particular interest nowadays as perspective tool of ocean remote sensing. Microwave radar backscattering at moderate and large incidence angles according to conventional models is determined by resonance (Bragg) surface waves typically of cm-scale wavelength range. Some recent experiments have indicated, however, that an additional, non Bragg component (NBC) contributes to the radar return. The latter is considered to occur due to wave breaking. At present our understanding of the nature of different components of radar return is still poor. This paper presents results of field experiment using an X-/C-/S-band Doppler radar operating at HH- and VVpolarizations. The intensity and radar Doppler shifts for Bragg and non Bragg components are retrieved from measurements of VV and HH radar returns. Analysis of a ratio of VV and HH radar backscatter – polarization ratio (PR) has demonstrated a significant role of a non Bragg component. NBC contributes significantly to the total radar backscatter, in particular, at moderate incidence angles (about 50-70 deg.) it is 2-3 times smaller than VV Bragg component and several times larger that HH Bragg component. Both NBC and BC depend on azimuth angle, being minimal for cross wind direction, but NBC is more isotropic than BC. It is obtained that velocities of scatterers retrieved from radar Doppler shifts are different for Bragg waves and for non Bragg component; NBC structures are “faster” than Bragg waves particularly for upwind radar observations. Bragg components propagate approximately with phase velocities of linear gravity-capillary waves (when accounting for wind drift). Velocities of NBC scatterers depend on radar band, being the largest for S-band and the smallest at X-band, this means that different structures on the water surface are responsible for non Bragg scattering in a given radar band.
Retrieving the water-leaving reflectance from airborne hyperspectral data implies to deal with three steps. Firstly, the radiance recorded by an airborne sensor comes from several sources: the real radiance of the object, the atmospheric scattering, sky and sun glint and the dark current of the sensor. Secondly, the dispersive element inside the sensor (usually a diffraction grating or a prism) could move during the flight, thus shifting the observed spectra on the wavelengths axis. Thirdly, to compute the reflectance, it is necessary to estimate, for each band, what value of irradiance corresponds to a 100% reflectance. We present here our calibration method, relying on the absorption features of the atmosphere and the near-infrared properties of common materials. By choosing proper flight height and flight lines angle, we can ignore atmospheric and sun glint contributions. Autocorrelation plots allow to identify and reduce the noise in our signals. Then, we compute a signal that represents the high frequencies of the spectrum, to localize the atmospheric absorption peaks (mainly the dioxygen peak around 760 nm). Matching these peaks removes the shift induced by the moving dispersive element. Finally, we use the signal collected over a Lambertian, unit-reflectance surface to estimate the ratio of the system's transmittances to its near-infrared transmittance. This transmittance is computed assuming an average 50% reflectance of the vegetation and nearly 0% for water in the near-infrared. Results show great correlation between the output spectra and ground measurements from a TriOS Ramses and the water-insight WISP-3.
At present a sufficient amount of methods is offered for determining the characteristics of sea roughness in accordance with optical images of wavy water surface obtained from different near-shore constructions, sea platforms, vessels, aircraft and satellites. The most informative elements in this case are solar path and peripheral areas of the image free from sun glitters. However, underwater images of the surface obtained with the help of optical receiver located at a certain depth contain apart from the mentioned elements one more informative element– Snell’s window. It is an underwater sky image which distortions of border contain information on roughness characteristics and serve as the indicator of its variability. The research offers the method for determining energy spectra of wind waves in accordance with the second statistical moment of Snell’s window image. The results of testing of the offered method are provided based on natural images registered in the course of trip to the Black Sea under conditions of different wind and wave environment for clear surface and surface covered by surfactant films. For both cases frequency spectra of surface slopes are recovered and their good coincidence to the spectra received by processing of signals from a string wave recorder is established. Efficiency of application of the offered method for tasks of remote monitoring and environmental control of natural reservoirs is shown.
Investigation of the Doppler shift of radar return from the sea surface is very important for better understanding of capabilities of exploitation of microwave radar for measuring velocities of marine currents. Here new field experiments carried out from a Platform on the Black Sea with a coherent X-band scatterometer, and a Doppler multifrequency (X- /C-/S-band) dual-polarized radar recently designed at IAP RAS are discussed. It is shown that the radar return contains both Bragg (polarized) and non polarized scattering components, presumably giving different contributions to radar Doppler shifts. Radar Doppler shifts were estimated using two different definitions as a) a frequency of the “centre of gravity” of an instantaneous radar return spectrum (ASIS) averaged over periods of dominant wind waves and b) the “centre of gravity” of the averaged over dominant wave periods spectrum (SAS). The ASIS and SAS values for both VV and HH-polarizations are shown to be different due to effects of radar backscatter modulation by dominant (long) wind waves. The radar Modulation Transfer Function (MTF) has been analyzed from experimental data and difference between SAS- and ASIS-values has been satisfactory explained using the measured MTF-values. It is obtained that experimental values of ASIS can be satisfactory described by the Bragg model despite the significant contribution of NP component to the radar backscatter. A physical explanation of the effect is given.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.