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 ø 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. Results using microwave measurements are compared with previ-ous measurements of wave breaking and with analytical modeling by other authors. Previous research concerning the characteristics of microwave backscatter from breaking waves was recently summarized by Jessup et al. [1990]. The term "sea spike" generally refers to abrupt, large-amplitude excursions in the backscattered radar cross section. The association of sea spikes with steep and breaking waves was first observed in measurements at grazing incidence angles [Katzin, 1957; Long, 1974, 1983; Kalmykov and Pustovoytenko, 1976], some of which included the use of video recordings [Lewis and Olin, 1980] and range tracking [Ewell et al., 1984]. The correlation between sea spikes and breaking waves has also been noted in measurements at moderate incidence angles. Scattering mechanisms postulated for this measurement regime [Alpers et al., 1981] have been used to improve predictions of the mean radar cross section based on the composite surface model, especially for horizontal polarization [Lyzenga et al., 1983; Donelan and Pierson, 1987]. In the laboratory, polarizationindependent returns from breaking waves were measured by Duncan et al. [1974]; Kwoh and Lake [1984] found specular scattering from the turbulent wake region and the generation of capillary waves, as well as a nonspecular contribution attributed to wedge diffraction. Field measurements by Kwoh et al. [1988] also showed evidence of specular returns from breaking waves. Bra...
