Detecting breaking ocean waves through microwave scattering

Remote sensors operating at electromagnetic wavelengths cannot significantly penetrate ocean water but are effective at observing surface phenomena (such as waves andwinds). Radar remote sensing can also be used to measure ocean currents and the presence of surfactants or slicks. These capabilities are presently being exploited to track the Gulf of Mexico oil spill, for example. For radars transmitting at microwave frequencies, breaking (or nearly breaking) waves generate much stronger returns than would be expected based on existing scattering models. For example, Figure 1 shows an image from a marine radar operating from shore at Newport (Oregon).1 Linear features running vertically are indicative of breaking ocean waves. Relating these bright returns to an accurate count of the number of breaking waves has a number of applications. For example, wave breaking is an important mechanism for air-sea interaction that influences the generation and strength of tropical storms and hurricanes. Identification of breakingwaves is also important for maritime operations, where wave breaking presents a primary danger. Finally, closer to shore, observations of wave breaking would allow a better understanding of the complex driving forces of the nearshore currents that erode beaches. Where waves are not breaking, the radar backscatter from the ocean surface is generally well described by the Bragg scattering mechanism and the two-scale model of slightly rough surfaces.2 However, the hydrodynamic conditions in areas of active wave breaking are significantly different from regions of nonbreaking waves. Hence, the conditions governing scattering of microwave radiation and the data captured in radar images also change. Inside the breaking-wave crest (or ‘wave roller’), air and water are mixed continuously, and a volume of entrained bubbles and ejected droplets propagates with the crest (see Figure 2). As a breaking wave passes, resurfacing bubbles and remnant foam are left in its wake. For microwave sensors, sea Figure 1. Marine-radar image from Newport (Oregon). Colors represent normalized backscattered intensity. UTM: Universal transverse Mercator coordinate system.