Abstract
The role of wave breaking in microwave backscattering from the sea surface is a problem of great importance for the development of theories and methods on ocean remote sensing, in particular for oil spill remote sensing. Recently it has been shown that microwave radar return is determined by both Bragg and non-Bragg (non-polarized) scattering mechanisms and some evidence has been given that the latter is associated with wave breaking, in particular, with strong breaking such as spilling or plunging. However, our understanding of mechanisms of the action of strong wave breaking on small-scale wind waves (ripples) and thus on the radar return is still insufficient. In this paper an effect of suppression of radar backscattering after strong wave breaking has been revealed experimentally and has been attributed to the wind ripple suppression due to turbulence generated by strong wave breaking. The experiments were carried out in a wind wave tank where a frequency modulated wave train of intense meter-decimeter-scale surface waves was generated by a mechanical wave maker. The wave train was compressed according to the gravity wave dispersion relation (“dispersive focusing”) into a short-wave packet at a given distance from the wave maker. Strong wave breaking with wave crest overturning (spilling) occurred for one or two highest waves in the packet. Short decimeter-centimeter-scale wind waves were generated at gentle winds, simultaneously with the long breaking waves. A Ka-band scatterometer was used to study microwave backscattering from the surface waves in the tank. The scatterometer looking at the area of wave breaking was mounted over the tank at a height of about 1 m above the mean water level, the incidence angle of the microwave radiation was about 50 degrees. It has been obtained that the radar return in the presence of short wind waves is characterized by the radar Doppler spectrum with a peak roughly centered in the vicinity of Bragg wave frequencies. The radar return was strongly enhanced in a wide frequency range of the radar Doppler spectrum when a packet of long breaking waves arrived at the area irradiated by the radar. After the passage of breaking waves, the radar return strongly dropped and then slowly recovered to the initial level. Measurements of velocities in the upper water layer have confirmed that the attenuation of radar backscattering after wave breaking is due to suppression of short wind waves by turbulence generated in the breaking zone. A physical analysis of the effect has been presented.
Highlights
At the present time a lot of studies are focusing on a better understanding of microwave scattering from the sea surface
The increased interest in the problem is associated with new features of microwave backscattering revealed in field experiments and related to breaking waves, in particular in experiments on microwave radar probing of oil slicks
It was obtained when analyzing radar Doppler spectra that the velocities of microwave scatterers can differ from the intrinsic velocities of linear free Gravity–Capillary Waves (GCW) with Bragg wavelengths and the scatterers can be associated with the nonlinear structures moving with the phase velocities of carrying cm-dm-scale waves [15,18,20]
Summary
At the present time a lot of studies are focusing on a better understanding of microwave scattering from the sea surface. The increased interest in the problem is associated with new features of microwave backscattering revealed in field experiments and related to breaking waves, in particular in experiments on microwave radar probing of oil slicks It has been demonstrated, amongst others, that the widely used two-scale Bragg theory [1,2] is unable to describe some features of microwave scattering when interpreting observations with co-polarized radar both for clean water surface and for oil slicks (see, e.g., [3,4,5]). Micro breaking occurs for centimeter-decimeter (cm-dm)-scale waves and is associated with generation of small-scale structures—parasitic ripples, toes and bulges near the wave crests [8,9,10] Both types of wave breaking are supposed to be responsible for particularities of microwave scattering from the sea surface, their relative role is still not well understood, in particular, when interpreting radar observations of oil slicks [11,12,13].
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