Breaking waves affecting microwave backscatter: 1. Detection and verification

This paper is the first of a two-part series conceming detection and characterization of wave breaking when using microwave techniques. The importance of wave breaking in both microwave remote sensing and air-sea interaction has led to this investigation utilizing a K,, band continuous wave Doppler scatterometer. Simultaneous microwave, video, and environmental measurements were made during the SAXON-CLT experiment off Chesapeake Bay in the fall of 1988. The scatterometer was pointed upwind at an incidence angle of 45 o and had an illuminated area that was small compared with the wavelength of the dominant surface waves. This first paper presents the schemes developed to detect individual breaking waves and verification of the method using video recordings. The most successful scheme is based on thresholds in both the radar cross section and the Doppler bandwidth. Microwave events consisting of a sea spike in the radar cross section accompanied by a large bandwidth were found to be associated with the steep forward face of waves in the process of breaking. The location of the illuminated area with respect to the phase of the breaking wave and the stage of breaking were found to influence the detectability of individual breaking waves. Approximately 70% of the sea spikes associated with waves that produced whitecaps were identified by the most successful detection scheme. The second paper examines how the degree of wave breaking, as measured by the microwave technique developed in this paper, depends on wind and wave conditions.