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.
The present paper considers the possibility of a system for environmental monitoring of film pollution in the Gorky reservoir. A combination of the well-known approachs for calculating the drift trajectories of passive particles on the sea surface with the data of remote sensing, which provides primary detection of a pollution spill in the scanned area is proposed. X-band digital coherent radar was the source of remote data. Based on this radar, an automated radar system is being developed for the automatic detection of film contaminants on the water surface. The results of tests of the algorithm for automatic detection of film pollutions based on field measurements are presented. The model component of the system is based on the results of measurements of current velocities and the physical dependence of the slick drift. As a result of comprehensive studies, the developed system has demonstrated operability for detecting and predicting the spread of film pollution in the Gorky reservoir.
Investigation of the mixing processes in river confluence zones is an important problem, particularly in application to ecological monitoring of rivers and inland waters. Such processes can be very complex and can affect riverbed deformation, mixing and pollution transport, etc. A typical example is a confluence zone (CZ) of the Volga and Oka rivers (Russia). In this paper, the Volga/Oka CZ signatures in satellite optical (Sentinel-2 MSI) and radar (Sentinel-1 SAR) images are analyzed for the period of ice/snow melting in early spring. First, the ice melting appears immediately downstream of the meeting point of the rivers. At the early stages of the ice cover melting the CZ is seen as a narrow dark band in the optical images and as a bright band in radar images because of wet snow/ice cover within the CZ. The faster melting in the CZ than outside can be associated both with the emissions of warm water from factories/thermal power plants and with water transportation towards the CZ by weak transverse currents in the river flows. The enhanced radar backscatter in the CZ can be associated with an increase of the snow/ice moisture and thus an increase of snow/ice cover roughness. With air temperature increase, an open water patch with fragmented ice appears transformed later into a long polynia. The polynia is manifested as a dark band in radar imagery presumably due to the suppression of wind waves associated with fragmented ice. This happens even at moderate wind velocities normally exceeding the threshold of wind wave excitation.
The present paper is devoted to the development of a multisensory approach to hydrophysical measurements of the ocean surface layer. The combination of coherent acoustic sounding, coherent radar sounding, and optical stereo photography is considered. Each of the methods separately has several advantages, but it is not free from disadvantages. Coherent acoustic sounding, described by resonant scattering, has a large observation area but has a poor spatial resolution. Coherent radar sensing, also described by resonant scattering, has a smaller observation area but a higher spatial resolution. Both methods allow us to recover the velocity characteristics of scattering associated with the velocities of resonant scatterers. Using the relationship between sea surface elevations and orbital wave velocities, it is possible to determine the spatial spectra of wind waves in a wide range of wind wavelengths up to several meters. To determine the characteristics of shorter-scale wind waves, it is proposed to use data from optical stereo photography. As a calibration for the optical system, information on the elevation of the sea surface obtained from coherent acoustic and radar sounding data can be used. Thus, the spatial spectrum of wind waves can be reconstructed in the range of energy-carrying to gravitational-capillary waves. In addition to the spectral characteristics of waves, it is possible to determine the velocity of the near-surface flow by measuring the Doppler shift of acoustic and radar signals. High-resolution data obtained from stereo photography of the sea surface allow clarifying the relationship between the velocities of acoustic and radar scatterers and hydrometeorological parameters.
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.
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.
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.
The investigation of surface film pollution is extremely important for ocean ecology and developing the methods of ocean remote sensing. The goal of this work is an experimental study of parameters of surfactant films in real conditions on the sea surface and their effect on measured radar return contrast in the film slicks. The properties of films of oleic acid in real conditions were studied under moderate winds in the Gorky Reservoir. Previously, the dependence of the elasticity and the surface tension coefficient of the oleic acid film (the parameters that determine the wave damping) on the surface of the distilled liquid were studied in detail in our laboratory. To study the properties of the film on the water surface under real conditions, a surfactant was sprayed onto the water surface, after which surface samples were taken using a net method. The film concentration and elasticity of the film were retrieved in the IAP laboratory. It is shown that the mean surface concentration of the film is several times higher than the concentration of the monomolecular layer of oleic acid. In different areas of the film slick, the concentration can vary by 2-3 times. The elasticity of the film formed by oleic acid on the water surface in real conditions is approximately two times less than the elasticity of the oleic acid film previously measured in laboratory conditions. The retrieved elasticity was used to explain the suppression of the X-band radar signal operating at VV polarization at an incidence angle of 60 degrees. To calculate the damping, a model was used that takes into account nonlinear sources of wind wave generation. Using the new elasticity value improves the agreement between the measured and calculated data.
The paper considers the results of a promising method for remote determination of parameters of dynamic processes in the ocean – coherent ocean radar sensing. This work proposes a methodology for determining the speed and direction of the sea surface current and wind waves spectrum by measurements of X-band Doppler radar. Based on the Doppler Effect, it is possible to measure the orbital velocities of wind waves on the sea surface, the velocities of breaking waves, the velocities of the sea surface current and the speed parameters of other oceanic dynamic processes. Using the basic expressions of the wave theory of free waves on water, it is possible to restore the heights of wind waves without additional calibration. Theoretical numerical simulation of the Doppler velocity of the Bragg waves in the field of wind waves and currents were carried out. The simulation used a two-scale model of microwave scattering on an wavy water surface, taking into account the shading of the sea surface by wave crests. A correlation analysis of the surface current, calculated through hydro meteorological parameters and Doppler radar panoramas, showed a maximum correlation coefficient for a velocity value is about 0.88 with a root mean square error of 8 cm/s, and for a direction is about 0.98 with a root mean square error of 14 degrees. The work shows the possibility to recover wind wave spectra from the data on the Doppler shift of microwave radio waves. Features that are not described by the two-scale model are found and discuss.
The role of wave breaking in microwave backscattering from the sea surface is a problem of great importance for development of theories and methods of the ocean remote sensing. Recently it has been shown that the microwave radar return is determined by both Bragg and non Bragg scattering components, and some evidences have been given that the latter is associated with wave breaking. However, our understanding of different mechanisms of the role of wave breaking on small-scale wind waves (ripples) and thus on the radar return is still insufficient. This paper presents results of laboratory experiments on the influence of wave breaking on Ka-band radar signals. An effect of the radar return suppression after wave breaking has been revealed and attributed with wind ripples suppression by breaking waves. The experiments were carried out in an oval wind wave tank where intense m/dm-scale surface wave trains were generated by a mechanical wave maker, in particular using a method of dispersive wave focusing. Wind waves were independently generated in the wave tank. A Ka-band radar was mounted at a height of about 1 m above the water level the incidence angle of microwave radiation was about 50 degrees. The experiments were performed both for a clean water surface and in the presence of an oleic acid monomolecular film. It has been obtained that the radar return before the wave train was determined by wind ripples, the radar Doppler spectrum was centered close to the Bragg wave frequencies. The radar signal intensity was strongly enhanced in a wide frequency range when the train was passing by the study area. After the intense wave train the radar return dropped and then slowly recovered to the initial level. We believe that the attenuation of radar backscattering after the wave train is due to suppression of wind ripples by turbulence and surfactants associated with wave breaking.
The work is devoted to the experimental study of Doppler velocity in artificial surfactant films on the sea surface. Such
films simulate the oil spills. The paper develops a method of remote detection of oil spill pollution on the water surface.
The method is based on a joint analysis of amplitude and velocity radar images of the water surface. It is shown that the
surfactant films lead to a significant change in the Doppler velocity, which can be used to increase the detection
probability of pollution on the sea surface. A statistical analysis of the two-dimensional distribution of RCS and Doppler
velocity in artificial surfactants films and pure water in a wide range of weather conditions is performed. It is
demonstrated that the difference between the measured Doppler velocity in the upwind direction and the phase velocity
of the Bragg waves corresponds with the slick drift velocity.
The possibilities of the definition of physical characteristics of organic films on the water surface, including oil thickness estimation, were studied in laboratory conditions based on the novel opto-acoustical approach. It was shown that the continuous infrared irradiation of film on the water surface leads to the generation of ultrasonic wave with frequencies of the order of several kHz which is the result of local heating of subsurface layer of water. The preliminary results regarding a relation between physical characteristics of surfactant films and the characteristics of acoustic wave are presented. A promising approach of remote sensing definition of surfactant films characteristics in real sea conditions was proposed on the base of these results. The development of this approach will contribute to the development of modern systems of strategic remote sensing of the ocean and inland basin.
The article discusses the results of theoretical and experimental studies of Doppler velocity on the sea surface. Doppler
velocity is measured by radar sensing at moderate and low grazing angles. The experiments were performed using
coherent X-band and Ka-band panoramic digital radars operating with the horizontal polarization for transmission and
reception. These radars have high spatial resolution. An algorithm for reconstructing Doppler velocities is proposed and
estimates of the fluctuation sensitivity of the method are carried out. The method is applied to the conditions of the fetch-limited
wind wave growth, which is typical for enclosed waters and the sea nearshore, where the dominant wavelength is
of the order of ten meters. Based on the two-scale model, the dependences of the Doppler velocity on the parameters of
the ocean-atmosphere interface are considered. The effect of shadowing by the crests of wind waves on the magnitude of
the Doppler velocity at low grazing angles is discussed. The manifestation of the orbital velocity of wind waves is
demonstrated.
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.
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.
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.
The presented paper is aimed at the possibilities of detecting surfactants fluorescence on a water surface using a
portable UV diode light source and spectrometer. Under controlled conditions of film thickness, a series of laboratory
fluorescence measurements for some surfactant most commonly found in inland waters like crude oil and diesel were
performed. Obtained fluorescence spectra were registered for thin films with a thickness from 0.6 to 33 μm. Estimates of
the fluorescence intensity in relation to film thickness are obtained.
The problem of characterization of marine currents using ocean remote sensing data is very challenging and has not been completely resolved by now. Optical and IR satellite images of the ocean have been traditionally used to estimate the current velocities when comparing color or temperature inhomogeneities in co-located subsequent scenes. SAR as all-weather and all-day instrument with high spatial resolution is very perspective for ocean remote sensing, and the procedure similar to the optical/IR observations seems to be usable for SAR for the current velocity estimation, too. Marine biogenic film slicks often observed on the sea surface as systems of “filamentary” structures at low/moderate wind conditions can be considered as appropriate features for marine current tracking. However, very few attempts have been made to study the current velocity field when studying slick features in SAR images acquired from different satellites at a comparably short time interval. In this paper two sequential satellite SAR images acquired with Envisat ASAR and ERS-2 SAR have been analyzed in order to estimate the surface marine currents. The acquisition time difference between the images was nearly 30 min. The images were characterized by a number of slick features which were nearly identical within the 30 min time shift, so that it was rather easy to track any chosen slick structure and to retrieve the velocity field. A Maximum Cross-Correlation (MCC) method has been used for the current retrieval, when analyzing correlation between the sequential images. It has been obtained that for some slick filamentary structures or for their parts the retrieved current velocities were directed nearly along the filaments, so that the slicks can be considered as the current streamlines. On the contrary, for some other slicks the retrieved current velocity vectors were directed at quite large angles to the filament tangent lines. We believe that the latter effect appears for varying currents due to the “memory” of slicks which cannot change their orientation or appear/disappear instantaneously according to fast changes of environmental conditions, in particular according to wind speed velocity/direction changes.
One of the urgent problems of gas pipeline transport today is the detection of gas leaks in underwater sections of gas pipelines and reducing their negative impact on the environment. This paper is devoted to the development of remote detection methods of gas leaks from underwater gas pipelines by images of slicks above them. A series of laboratory experiments was carried out, in which the structure of average flows in the water column created by bubble flows with different gas flow rates was obtained. The simulation of surfactant removal to the water surface by bubbles was performed by adding sodium dodecyl sulfate (SDS) to the laboratory cuvette at the concentration of 0.5 mg/l. For the surfactant film on the water surface, experimental dependences of its characteristics on the operation time of the "bubble pump", such as the surface tension coefficient, elasticity and the relative damping coefficient of small- scale waves under the film, were obtained. It was shown that there are flow convergence zones at some distance from the gas outlet area, which vary depending on the intensity of the gas flow. Surfactant film samples taken in these areas confirmed that over time, the accumulation of surfactants occurs on the water surface. Field experiment conducted in one of the backwaters of the Oka river in the city of Nizhny Novgorod allowed to spot and evaluate the size and shape of the film slick formed around the gas outlet area. The conducted experiments confirmed the formation of a surfactant film spot near the removal area, which indicates the possibility of its remote detection in full-scale conditions, provided the wind near water surface is not more than 2-10 m/s. The obtained data made it possible to get a general idea of the morphology and characteristics of the surfactant spot around the gas outlet from the water.
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.
Evolution of film slicks on the sea surface is a very important problem, particularly in application to monitoring of pollution transport in the ocean and in the coastal areas. The geometry of film slicks is determined by physical characteristics of surface films and by environmental parameters. At present our understanding of the role of geophysical processes controlling the slick dynamics is still insufficient. This is partly because of the luck of systematic studies of film slicks in controlled experiments. A perspective approach to the problem solution is organization of special experiments with artificial slicks formed by films with known characteristics. Previously we reported on the results of some experiments and proposed a simple model of spreading of surfactant spills accounting for the surface stresses induced by wind waves. In this paper new results of experiments on oil spill evolution are presented and physical mechanisms of this evolution are discussed. The experiments were conducted on the Gorky Water Reservoir. Surfactants (oleic acid) were spilled out from a motor boat. The slick shape was studied using aerial photography and a methodology of contouring slicks using a motor boat with a GPS receiver onboard. It is obtained that the cross-wind slick size grows quite slowly with time being almost independent on wind, while the growth rate of the along-wind axis increases rapidly with wind velocity. To explain the observed effects a mechanism of “oil overflow” is hypothesized. It describes the movement of surfactants from an upwind slick side to its downwind side due to the action of along-wind wave induced stresses. This process should contribute also to some additional drift velocity of slicks. The conclusion about the drift is consistent with a hypothesis, discussed in the literature that oil slicks move faster than the surrounding water surface microlayer.
The first results in development of method for satellite monitoring of the bio-optical water properties of Gorky reservoir as an example of an inland freshwater eutrophic water body are presented. The method is based on the semi-analytical algorithm for the Black Sea and uses the data on the reflectance coefficient of the water column, allowing to calculate the concentrations of optically significant substances (phytoplankton pigments, dissolved organic matter and mineral suspended matter). Field measurements of spectral reflectance were carried out in years 2016 – 2017. Spatial variability of reflectance and factors affecting it were analyzed. Reflectance model used in Black Sea algorithm was adapted to biooptical features of the studied water body. Model calculations of pigment concentration were compared with chlorophyll a content data obtained from water samples analyses. The pigment absorption spectra were calculated, showing the spectral features characteristic of photosynthetic pigments. The ways of further research for algorithm development are determined.
The goal of current investigation was a study of seasonal variability of remote sensing reflectance in the lake part of the Gorky reservoir, characterized by intense bloom of blue-green algae. The basis of this study includes the data of ship measurements of remote sensing reflectance, euphotic zone and chlorophyll a and dissolved organic carbon concentrations, performed from May 14, 2018 to August 27, 2018 in 7 points of the reservoir, two of which are located in shallow areas with a slow current (floodplain area), two - in the channel and the remaining three - in estuaries of three inflowing rivers. The spatial and temporal variability of remote sensing reflectance is analyzed, its variations are calculated, vertical profiles of chlorophyll a and dissolved organic carbon are constructed, variations of euphotic depth are determined and correlation between variations of remote sensing reflectance and concentrations of optically active components is established
An experimental study of statistical characteristics of fluorescent lidar echo signals and spatial distribution of optically active components in waters of the Gorky reservoir at 40 x 10 km experimental area covering the floodplain, channel and estuaries of inflowing rivers was performed. The measurements were carried out during 4 days from 1 August to 4 August 2017 and from 20 September to 22 September 2018 at the stations and continuously along the motorboat track with the help of fluorescent lidar UFL-9 which allows to restore concentrations of chlorophyll a, colored organic matter and total suspended matter in the upper water layer with thickness of about 0.2 to 1.0 m for eutrophic waters. This paper presents maps of their spatial distribution, demonstrating different scales of inhomogeneities, average values and spatial variability of chlorophyll a, total organic carbon and total suspended matter as well as data on variance and spatial spectrum of fluctuations of the elastic backscattering signal. Regressions between mean values and coefficients of variation of suspension and chlorophyll a, and between coefficients of variation of suspension and energy of the elastic backscattering signal are established.
A shape of marine slicks is an important characteristic which can be used when solving a problem of detection and identification of oil spills on the sea surface. Slick shape and its spatial-temporal evolution depend on many environmental processes, such as wind speed, nonuniform marine currents, internal waves, etc. In the context of the problem of oil spill dynamics it is very important, particularly at initial stages of oil spill evolution, to describe correctly processes of oil film spreading. Until recently the most popular was the Fay’s model of film spreading which, however, could not correctly explain some obvious effects, e.g., asymmetry of film slicks in the downwind and crosswind directions. In this paper new results of field studies of spreading of surfactant films are presented. The experiments with spills of surfactants were conducted on the Gorky water reservoir using a methodology of contouring slicks with a GPS receiver mounted on a motor boat, and also aerial photography from UAV. The following results have been obtained. First, the effect of elongation of oil spills in the wind direction, revealed in our previous experiment, is confirmed. Quantitative data on growth rates of along- and cross-wind slick axes are obtained characterizing initial stages of the spreading process. Second, new effects have been revealed which are: a) saturation of the cross- and along-wind axes at some intermediate stages of slick evolution, and b) further decrease of the along wind slick axis and the slick square, and a tendency to a circular shape at late stages of the slick evolution. A physical model, explaining qualitatively the observed effects is developed.
Dual-polarized microwave radars are of particular interest nowadays as perspective tool of ocean remote sensing. According to conventional models the microwave radar backscattering at moderate and large incidence angles 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 is supposed to be independent on polarization. At present our understanding of physical origin of different components of radar return is still insufficient. In particular, an important problem of variations of Bragg and non Bragg components (BC and NBC, respectively) along the profile of a long surface wave remains poorly investigated. This paper is focused on data processing and analysis of results of field studies of BC and NBC variations over the long wave profile using dual co-polarized X-band radar. It is demonstrated that the intensities of Bragg and non Bragg components are non-uniformly distributed over the long wave profile: BC is not strongly modulated due to long surface waves and dominates near the long wave troughs. NBC is characterized by the appearance of strong spikes near the crests of intensive long waves and contributes significantly to the radar return in the spikes supposedly due to intensification of breaking of short, cm-dm-scale wind waves. It is shown that relation between BC and NBC changes in the presence of surfactants on the water surface because of different suppression of the two components in slicks.
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.
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.
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.
Slicks on the sea surface are characterized by attenuation of short wind waves and appeаr in radar imagery at moderate incidence angles as areas of reduced intensity. In the proximity of oil platforms, ship routes, fish farms, etc. marine slicks are often identified as oil spills or biogenic films. However, probability of false alarm when detecting film slicks is very high because of the occurrence of structures in radar images looking similar but not related to surface films (“lookalikes”). One of the most frequent "look-alikes" is wind depression areas (WDAs) where the wind excitation of short surface waves is reduced compared to the ambient background. Results of field observations of films slicks and WDA are described and differences in character of wind wave attenuation in different parts of the wind wave spectrum are revealed. Model calculations of wave damping degree (contrast) in film slick and in WDA are carried out and are shown to be in general agreement with experiment. Capabilities of dual-polarization and multi-band microwave radar for discrimination between film slicks and “look-alikes” are analyzed based on experiment and model results.
Marine slicks are one of the most common features on the sea surface and a significant part of the slicks is a result of accidental or deliberate oil spills. The shape of oil slicks is their important characteristic that can be used to identify the nature of slick signatures in radar or optical images of the sea surface and possibly to describe them quantitatively. Nowadays, however, there is a lack of systematic experiments with slicks, and the very physical mechanisms of slick spreading are still not well understood. This paper presents results of controlled experiments with spills of surfactants, and a possible physical mechanism of slick asymmetry is discussed. Experiments with artificial film slicks were carried out in different environmental conditions: from an Oceanographic Platform on the Black Sea, and from a vessel on the Gorky Water Reservoir. Slick shape and its evolution were studied using photographic methods, and satellite radar imagery. In the satellite experiments surfactants were poured on the surface at certain time intervals before the satellite overpass. It is obtained that film spreading is not axial symmetric, and the spills are stretched along the wind, a long-to-short slick axis ratio weakly depends on spreading time and grows with wind speed. A physical mechanism of slick deformation due to mean surface currents induced by wind waves is proposed. Namely, drift currents induced by oblique propagating surface waves increase in film slicks due to enhanced wave damping and these currents result in reduced spreading rate in the cross wind direction. Theoretical analysis of slick spreading accounting for the effect of surface waves is presented, and theoretical estimates are shown to be consistent with experiment.
Ship wakes can be clearly seen in satellite radar and optical images of the sea surface, and understanding of physical mechanisms responsible for the wake signatures is very important to develop methods of ship detection/identification. The wake surface signatures at small and intermediate stages are characterized by a smooth centerline area where surface waves are depressed due to the vessel turbulence and by a pair of rough bands at the sides of the centerline wake. At large wake ages two slick bands (a “railroad track” wake) appear instead of the rough bands, while the smooth centerline band is practically absent. In this paper results of field studies of the mean flow structure near the wake are presented. It is shown that two mean circulating currents (“rolls”) rotating in the opposite directions are formed at two sides of the median vertical plane of the wake. Near the water surface the rolls result in diverging horizontal flows, decreasing near the wake edges. Wind waves propagating against the diverging currents are amplified due to a wave straining mechanism thus increasing the surface roughness. Film sampling was carried out when crossing the wakes and analysis of films collected within the “railroad” slick bands and outside the bands has revealed enhanced surface wave damping, obviously due to accumulation of surfactants in the slick bands; the surfactant compression is explained by the action of the diverging currents. The diverging currents as part of the rolls and the surfactant transport to the water surface are supposed to be associated with air bubbles generated by ship propellers.
Results of field experiments on radar imaging of surfactant films using satellite SAR (TerraSAR-X) co-located with Xband
and Ka-band radar (scatterometer) measurements are described and analyzed. The experiments were performed using surfactant films with pre-measured physical parameters, the surface tension and the film elasticity, at low to moderate wind and at different radar incidence angles. Contrasts characterizing depression of radar backscatter in slicks have been obtained. Theoretical analysis of radar contrasts for low-to-moderate incidence angles has been carried out based on a hydrodynamic model of wind wave damping due to films and on a composite radar imaging model. The
hydrodynamic model takes into account wave damping due to viscoelastic films, wind wave generation and a phenomenological term describing nonlinear limitation of the wind wave spectrum. The radar model takes into account Bragg scattering and specular scattering, the latter is usually negligible compared to the Bragg effect at moderate incidence angles (larger than 30-35 degrees), but is obtained to give noticeable contribution to radar backscattering at smaller incidence angles particularly for slick areas when cm-scale ripples is strongly depressed by films. Theoretical calculations of radar contrasts in slicks are compared with experiment.
The range – time optical images [1, 2] of capillary waves using artificial diffuse illumination of water surface like
sky illumination was derived in water tank. The software for processing of wave’s images was developed. The
technique for recording splashes appearing for strong winds is developed using range – time surface images.
The developed method can be used for investigation of free and bounded capillary waves, wave’s breaking,
action of internal waves on surface waves [3,4] and scattering of radio and acoustic waves by rough surface in
laboratory and natural conditions. The study of gravity-capillary waves in the tank of the IAP RAS was carried
out employing the developed optical system with artificial diffuse illumination.
Field observations co-located and simultaneous with satellite radar imagery of biogenic slick bands on the sea
surface aimed to study relation between slicks and marine stream currents were carried out in the coastal zone of the
Black Sea. Measurements of the current velocities at different depths were performed using an acoustic Doppler current
profiler (ADCP) and surface floats. Samples of surfactant films inside/outside slick bands were collected from the water
surface with nets. The sampled films were reconstructed in laboratory conditions and measurements of the damping
coefficient of gravity-capillary waves and the surface tension were carried out using an original parametric wave method.
It is obtained that the banded slicks are characterized by enhanced concentration of surfactants due to their compression
by convergent current components. The slicks are revealed to be oriented along the stream currents and are located in the
zones of current shears. Small convergent transverse velocity components are observed near slick boundaries and are
responsible for slick formation in stream shear currents. Different examples of slicks formed by stream shear current are
described. Results of a case study of two streams of different directions merging and forming a banded slick in a shear
zone with convergent transverse current components are presented. Another case study is when a flow below a thermocline coming to the shore meets a bottom slope and a vertical current occurs resulting in horizontal divergence and convergence on the surface.
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