Philipp A. Lange

On the Reduction of the Radar Backscatter by Oceanic Surface Films: Scatterometer Measurements and Their Theoretical Interpretation

Abstract During the two SIR-C/X-SAR missions in 1994, surface film experiments were performed in the North Sea with a 5-frequency/multipolarization scatterometer flown on a helicopter, in order to investigate the reduction of the radar backscatter in the presence of quasibiogenic and anthropogenic sea surface films, particularly, at different wind speeds. Under all wind conditions encountered in this study, the measured damping ratio (i.e., the ratio of the radar backscatter from a slick-free and a slick-covered water surface) increases with increasing Bragg wavenumber. It is shown that not only Marangoni damping theory, but also wind-induced effects, primarily the energy input by the wind into the wave spectrum, also have to be taken into account. The reductions measured at low to moderate wind speeds (3.5–4 m/s and 5 m/s) are qualitatively explained by means of a comparison of the different source terms of the action balance equation. For the case of high wind speed (12 m/s) a theoretical model for the damping ratios is developed. Using this model, the experimental data can well be reproduced, and the absence of the Marangoni damping maximum at intermediate Bragg wavenumbers (approximately 100 rad/m) can be interpreted. Furthermore, the model can explain the similarities between the radar backscatter reductions measured over quasibiogenic and anthropogenic surface films under high wind conditions.

Classification of sea slicks by multifrequency radar techniques: New chemical insights and their geophysical implications

Sea slick experiments with an airborne five-frequency radar scatterometer were performed in the presence of surface active substances that represent different fractions of biogenic slicks (fats, amines, sugar derivatives, and fatty acids). Measurements at water temperatures of 282.2 K (9.0°C) and 290.6 K (17.4°C) showed that temperature effects appear to play a secondary role for slick-induced water wave damping, at least in the temperature range encountered during the present experiments. Different procedures of slick generation, with and without application of a spreading solvent, indicated that the wave-damping effect in the short-gravity/capillary wave range, and thus the modification of backscattered radar signals, is not only dependent on the chemical structure but also on the arrangement and distribution (morphology; formation of domains) of the surface-active compounds. Thus far this aspect, which appears to be of particular importance for biogenic sea slicks, has been completely ignored. External infrared reflection-absorption spectroscopy laboratory measurements with infrared radiation in the wavelength range between 3.3 μm and 7 μm enabled us allowed to form a link between some important elements of the morphological structure of the monolayers and their viscoelastic characteristics, which are closely related to the wave-damping effect of surface active compounds and to the compounds influence on remote sensing signals. Furthermore, the IRAS measurements supplied detailed insight into the relaxation process that occurs during the generation of a sea slick and on a slick-covered undulating water surface. In particular, strong hydration/dehydration effects appear to play an important role.

Drift Response of Monomolecular Slicks to Wave and Wind Action

Abstract The drift responses of monomolecular slicks to wind and wave action have been investigated in a laboratory wind-wave tunnel and in the field. The wind-wave tunnel was fitted with a mechanical wave generator which produced uniform long-crested gravity waves. The field studies were performed off the island of Sylt (German Bight) in the North Sea. Laboratory results for the slick drift response to deep water gravity waves only were in excellent agreement with the Stokes irrotational mass transport drift velocity theory. The purely wind-induced slick drift was in good agreement with the results obtained by previous investigators using a variety of other floats. A number of laboratory and field investigations have been compared which show that the surface drift to wind speed ratio lies between 2.6–5.5%. A difference between the average ratios for the collected laboratory and field results indicates that the pure gravity wave drift component probably comprises 25–30% of the total surface drift. A quali...

Natural and man-made sea slicks in the North Sea investigated by a helicopter-borne 5-frequency radar scatterometer

Abstract In April 1994 natural sea slicks and five man-made slicks, which were spread as a pure substance and from two different spreading solvents, n-hexane and ethanol, respectively, were overflown by a helicopter carrying a five-frequency multi-polarization radar scatterometer, the so-called Heliscat. For the first time, natural sea slicks were successfully simulated by spreading hexadecanoic acid methyl ester (palmitic acid methyl ester (PME)), representing the fatty acid fraction of biogenic slicks, from the spreading solvent ethanol. It is shown that the different slick generation procedures (pure substance, spreading solvents n-hexane or ethanol) give rise to different influences on backscattered radar signals: a PME slick spread from ethanol induces slightly stronger suppressions of the radar backscattering than a PME slick spread from n-hexane; the wave damping maximum of oleyl alcohol (OLA) spread as a pure substance is shifted to higher wave numbers as compared to OLA slicks spread from solvent...

Wind‐wave tank measurements of bound and freely propagating short gravity‐capillary waves

Measurements of the surface elevation and slope and of the backscattered radar power at X and Ka band were carried out in a wind-wave tank with mechanically generated gravity waves as well as with wind waves on slick-free and slick-covered water surfaces. The measured radar Doppler shifts show that on a slick-free water surface, bound gravity-capillary (X and Ka band Bragg) waves are generated at the crests of steep gravity waves with frequencies between 3 and 5 Hz. However, steep gravity waves with a frequency of 2Hz do not generate bound Ka band Bragg waves, and the Ka band backscattering from these waves is associated with wave breaking. In the wind speed range from 1.5 to 10m/s, bound gravity-capillary waves contribute to the X and Ka band backscatter from slick-free water surfaces. The fraction of bound to freely propagating Bragg waves depends on, among other things, radar frequency, wind speed, wave amplitude of the dominant water wave, and slick coverage. In particular, the strong damping of the gravity waves by the slick at wind speeds of approximately 8 m/s leads to the disappearance of the bound Bragg waves and therefore to a reduction of the X and Ka band Doppler shifts to values corresponding to freely propagating Bragg waves. It is concluded that the study is pertinent to the understanding of former results of radar backscattering measurements in the presence of oceanic surface films.

Comparison between an amplitude‐measuring wire and a slope‐measuring laser water wave gauge

Capillary waves produced in a laboratory wind wave tunnel have been measured using a wire resistance‐type gauge (measuring wave amplitude) and a laser gauge (measuring wave slope). Comparison of power spectra of the gauges shows good agreement to 80 Hz, which is the upper frequency limit of the wire gauge. The upper frequency limit of the laser gauge depends upon laser beam diameter and is about 300 Hz.

Relaxation effects in monolayers and their contribution to water wave damping: II. The Marangoni phenomenon and gravity wave attenuation

Abstract The Marangoni wave theory is reviewed and extended to the short gravity water wave regime. This theoretical approach is verified by wave damping measurements in the Hamburg wind-wave tunnel performed in the presence of monomolecular hexadecanoic acid methyl ester, (Z)-9-octadecen-1-ol, and hexadecyltrimethylammonium bromide surface films. A good agreement of the theoretical with the experimental results is obtained, when performing the calculations with the help of dilational modulus and phase angle values, which were obtained from dynamic surface potential measurements over a wavy water surface (H. Huhnerfuss, P. A. Lange, and W. Walter, J. Colloid Interface Sci. 108, 430, 1985). Dilational modulus values determined by classical Langmuir-trough measurements are not suitable for determining wave damping ratios and give rise to too large values.

Relaxation effects in monolayers and their contribution to water wave damping: I. Wave-induced phase shifts

Abstract The newly developed experimental approach of measuring surface potential variations over propagating water waves cover with monolayers allows direct determination of phase angles θ between the hydrodynamic surface area variation and the surface film concentration (surface tension, surface potential) variation depending on the chemical structure and the relaxation behavior of the film molecules. These phase shift data and independently measured laboratory surface viscosity values determined by means of the canal method are used for calculating dilational modulus E values which turn out to be lower than E values obtained by classical Langmuir-trough measurements. As shown in Part II of this work (H. Huhnerfuss, P. A. Lange, and W. Walter, J. Colloid Interface Sci. 108, 442, 1985) these E values obtained under dynamic conditions are much more suitable for describing quantitatively the wave damping effects of monolayers.

The effect of artificial rain on wave spectra and multi-polarisation X band radar backscatter

We have carried out wind-wave tank measurements using wave-height and wave-slope gauges and a coherent 9.8 GHz (X band) scatterometer, when the water surface was agitated by heavy rain (160 mm h- 1 to 300 mm h -1 ) and by wind (2 ms- 1 to 12 ms -1 ). The upwind-looking scatterometer was operating at co- (VV- and HH-) and cross- (HV-) polarisation at a steep incidence angle of 28°. In the presence of rain, the power spectral density of the wind-wave spectra is enhanced at frequencies above about 5 Hz and it is reduced at lower frequencies. This is the net effect of surface roughness production by the rain-induced splash products and of wave damping by the rain-induced turbulence. We measured isotropic (rain-dominated) wave spectra at low wind speeds and anisotropic (wind-dominated) wave spectra at high wind speeds, with a transition wind speed that increases with rain rate. The radar backscattering at co-polarisation at low wind speeds is mainly caused by rain-induced ring waves, whereas at cross-polarisation, at all wind speeds, other rain-induced splash products, like crowns, stalks, and cavities, are the dominant scatterers. We have found a rain-induced increase of the radar backscatter at co-polarisation at wind speeds of up to 9 ms- 1 and at cross-polarisation at all wind speeds. At cross-polarisation the radar backscatter slightly depends on rain rate. Using our results an analysis of spaceborne synthetic aperture radar (SAR) images of tropical rain cells was performed.

Wind wave tank measurements of wave damping and radar cross sections in the presence of monomolecular surface films

Measurements of the damping of small gravity and gravity-capillary water surface waves covered with monomolecular organic films of different viscoelastic properties were performed in the wind wave tank facility of the University of Hamburg. The wind speed dependence of the radar cross sections for X and Ka band was measured with upwind looking microwave antennas. It is shown that Marangoni damping theory, which describes the damping of water surface waves by viscoelastic surface films, is not the only damping mechanism in wind wave tank experiments where the wind sea is not fully developed. The other source terms of the action balance equation, i.e., the energy input into the water waves from the wind, the nonlinear wave-wave interaction, and the dissipation by wave breaking, are affected differently by the various substances. It is hypothesized that this difference is caused by the different viscoelastic properties of the substances, i.e., by the different intermolecular interactions of the film molecules. A slight dip in the wind dependence of the radar cross section at Ka band at wind speeds of 8-9 m/s was measured, which corresponds to comparable reductions of the mean squared wave height and wave slope. Polarization ratios (i.e., the ratios of the radar backscatter at vertical and horizontal polarization) higher than those predicted by simple Bragg scattering theory for X band at low wind speeds and different incidence angles are explained within a (three-scale) composite-surface model. At higher wind speeds, where the polarization ratio decreases rapidly, breaking by wedges and spilling breakers is hypothesized to become more dominant. The dependence of the polarization ratio on the coverage of the water surface with a slick is explained qualitatively by means of the composite-surface model. Finally, it is stated that wind wave tank measurements in the presence of monomolecular surface films are useful for the verification of theories concerning radar backscattering, wave damping, and wind-wave and wave-wave interactions.