On Microwave Scattering by Breaking Waves
Microwave radars, when viewing breaking waves at low grazing angles with high spatial resolution, observe sharp transient bursts of backscatter having large cross sections (of order 1 m2 or more), and a characteristic polarization dependence, with horizontal polarization giving sharper (briefer) returns down to about 1° grazing angle, below which horizontal and vertical returns look much the same. X-band returns have disclosed a modulation with periods of 10–20 ms. The transient nature of breakers, along with the several structurally different forms they may take, makes it difficult to establish a deterministic scattering model having any real credibility from observation alone. However, there is one theoretical model of a breaking wave, the “entraining plume model of a spilling breaker” (Longuet-Higgins and Turner, 1974), that contains structural and dynamic features which can be used as a basis for microwave scattering calculations. This chapter seeks to examine some of the characteristics of the radar return from structures suggested by the plume model. It is shown that several of these plumes, emerging from the wave crest at different times, could produce radar returns with the right cross section (a few square meters) and the right modulation period (10–20 ms). Moreover, the model explains certain observed shifts in the envelope and phase of the modulation produced by small (~10%) changes in radar frequency. In order to bring in polarization, near-field effects of the wave face just ahead of the advancing plume are considered. The results suggest that close to the crest, where wave slopes are steep, vertical and horizontal returns would be roughly the same. As the plume moves down the wave face encountering decreasing wave slopes, the horizontal return drops off sharply, while the vertical return peaks broadly over most of the wave. face. This is consistent with the observation of sharp horizontal and broad vertical returns, and with the tendency of the two polarizations to look alike at extreme grazing angles, where only the wave peaks remain visible. While neither rigorous nor definitive, these calculations suggest that accelerating plumes, or something like them, might provide a self-consistent explanation for many of the properties of microwave scattering by breaking waves.
KeywordsBreaking Wave Wave Crest Grazing Angle Radar Cross Section Wave Face
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- Hansen, J. P., I. D. Olin, and B. L. Lewis (1981): High resolution radar backscatter from the sea. Abstracts of the Open Symposium on Remote Sensing, URSI XXth General Assembly, Washington, D.C., 30.Google Scholar
- Kalmykov, A. I., A. S. Kurekin, Y. A. Lementa, I. Y. Ostrovskiy, and V. V. Pustovoytenko (1976): Scattering of microwave radiation by breaking sea waves. Gor’kiy Radiofiz. 19, 1315–1321 (translation).Google Scholar
- Kerr, D. E. (ed.) (1951): Propagation of Short Radio Waves, McGraw-Hill, New York, 461.Google Scholar
- Long, M. W. (1975): Radar Reflectivity of Land and Sea, Heath, Boston.Google Scholar
- Saxton, J. A., and J. A. Lane (1952): Electrical properties of sea water. Wireless Eng. Oct., 269–275.Google Scholar