Stanislav A. Ermakov

Manifestations of the Indian Ocean tsunami of 2004 in satellite nadir‐viewing radar backscatter variations

[1]xa0The paper reports on the first experimental evidence for space-observed manifestation of the open ocean tsunami in the microwave radar backscatter (in C- and Ku-bands; electromagnetic wave lengths 6cm and 2 cm respectively). Significant (a few dB) variations of the radar cross section synchronous with the sea level anomaly were found in the geophysical data record of the altimetry satellite Jason-1 for the track which crossed the head wave of the catastrophic tsunami of 26 December 2004. The simultaneous analysis of the available complementary data provided by the satellite three-channel radiometer enabled us to exclude meteorological factors as possible causes of the observed signal modulation. A possible physical mechanism of modulation of short wind waves due to transformation of the thin boundary layer in the air by a tsunami wave is discussed. The results open new possibilities of monitoring tsunamis from space.

On the role of wind direction in ERS SAR signatures of internal waves on the Iberian shelf

Results of an investigation of ERS SAR signatures of internal waves (IWs) on the Western Iberian shelf are shown. The mode of signature is defined by a parameter that evaluates the IW intensity profiles in relation to the mean backscatter of the image background. The signature mode parameter is found to be correlated with the angle between the IW propagation direction and the wind velocity. A simple model which takes into account modulation of short-scale surface waves by relative wind velocity to the surface current generated by the IWs is considered and was found to be in good qualitative agreement with the experimental results.

Tank study of radar backscattering from strongly nonlinear water waves

Wave tank experiments were performed to study features of microwave radar backscattering by strongly nonlinear gravity-capillary waves (GCWs). Backscattering by centimeter and partly decimeter GCW was found to be related to the microbreaking of GCWs and the generation of parasitic capillary ripples. The scattering of meter-scale GCWs was attributed to the effects of strong breaking, i.e., to the overturning of wave crests and the excitation of quasilinear (free) capillary ripples, while the scatterers for intermediate decimeter GCWs were both free and parasitic ripples.

Wave tank study of phase velocities and damping of gravity-capillary wind waves in the presence of surface films

The wave number-frequency spectra of gravity-capillary waves were measured using two optical spectrum analysers and an artificial gradient illuminator. It was found that phase velocities of centimetre-millimetre (cm-mm) scale waves on clean water do not obey the dispersion relation of free surface waves, but they increase with fetch, while frequencies of dominant decimetre (dm)-scale wind waves decrease with fetch. This observation implies that the wind wave spectrum contains nonlinear cm-mm-scale harmonics bound to the dominant waves and propagating with the phase velocities of the dominant waves. The relation between bound and free waves can be estimated from measurements of phase velocity. Wind ripple damping was found to be maximum at wavelengths around 5–7 mm. The latter effect agrees with results of our field experiments using artificial slicks and can be explained qualitatively by a nonlinear “cascade” damping mechanism, when the damping of dm-scale dominant waves leads to strong damping of their cm-mm-scale nonlinear harmonics including damping of “parasitic” capillary ripples.

Spreading of oil films on the sea surface: radar/optical observations and physical mechanisms

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.

Remote Sensing of Organic Films on the Water Surface Using Dual Co-Polarized Ship-Based X-/C-/S-Band Radar and TerraSAR-X

Microwave radar is a well-established tool for all-weather monitoring of film slicks which appear in radar imagery of the surface of water bodies as areas of reduced backscatter due to suppression of short wind waves. Information about slicks obtained with single-band/one-polarized radar seems to be insufficient for film characterization; hence, new capabilities of multi-polarization radars for monitoring of film slicks have been actively discussed in the literature. In this paper the results of new experiments on remote sensing of film slicks using dual co-polarized radars— a satellite TerraSAR-X and a ship-based X-/C-/S-band radar—are presented. Radar backscattering is assumed to contain Bragg and non-Bragg components (BC and NBC, respectively). BC is due to backscattering from resonant cm-scale wind waves, while NBC is supposed to be associated with wave breaking. Each of the components can be eliminated from the total radar backscatter measured at two co-polarizations, and contrasts of Bragg and non-Bragg components in slicks can be analyzed separately. New data on a damping ratio (contrast) characterizing reduction of radar returns in slicks are obtained for the two components of radar backscatter in various radar bands. The contrast values for Bragg and non-Bragg components are comparable to each other and demonstrate similar dependence on radar wave number; BC and NBC contrasts grow monotonically for the cases of upwind and downwind observations and weakly decrease with wave number for the cross-wind direction. Reduction of BC in slicks can be explained by enhanced viscous damping of cm-scale Bragg waves due to an elastic film. Physical mechanisms of NBC reduction in slicks are discussed. It is hypothesized that strong breaking (e.g., white-capping) weakly contributes to the NBC contrast because of “cleaning” of the water surface due to turbulent surfactant mixing associated with wave crest overturning. An effective mechanism of NBC reduction due to film can be associated with modification of micro-breaking wave features, such as parasitic ripples, bulge, and toe, in slicks.

Experimental study of dual polarized radar return from the sea surface

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.

Particularities of radar backscattering associated with wave breaking on the sea surface

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.

Remote sensing of evolution of oil spills on the water surface

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.

Damping of surface waves due to oil emulsions in application to ocean remote sensing

Applications of different radar and optical methods for detection of oil pollutions based on the effect of damping of short wind waves by surface films have been extensively studied last decades. The main problem here is poor knowledge of physical characteristics of oil films, in particular, emulsified oil layers (EOL). The latter are ranged up to 70% of all pollutants. Physical characteristics of EOL which are responsible for wave damping and respectively for possibilities of their remote sensing depend on conditions of emulsification processes, e.g., mixing due to wave breaking, on percentage of water in the oil, etc. and are not well studied by now. In this paper results of laboratory studies of damping of gravity-capillary waves due to EOL on water are presented and compared to oil layers (OL). A laboratory method used previously for monomolecular films and OL, and based on measuring the damping coefficient and wavelength of parametrically generated standing waves has been applied for determination of EOL characteristics. Investigations of characteristics of crude oil, oil emulsions and crude OL and EOL have been carried out in a wide range of surface wave frequencies (from 10 to 25 Hz) and OL and EOL film thickness (from hundredths of millimeter to a few millimeters. The selected frequency range corresponds to Bragg waves for microwave, X- to Ka-band radars typically used for ocean remote sensing. An effect of enhanced wave damping due to EOL compared to non emulsified crude OL is revealed.