Breaking wave contribution to low grazing angle radar backscatter from the ocean surface

Abstract

The anomaly of radar sea spikes, defined here as the non‐Bragg scattering events with backscattering cross‐section of horizontal polarization exceeding that of the vertical polarization, has been associated with steep wave features possibly going through wave‐breaking process, with or without whitecap manifestation. This property is exploited for using a dual polarized radar as a remote sensing breaking wave detector. Field data collected in the ocean covering wind speeds from 7 to 15 m/s, grazing angles from 1.4 to 5.5°, and with different levels of background swell influence are analyzed to quantify the radar cross‐section and Doppler velocity from sea surfaces with and without wave breaking. Key results of the breaking effects are increasing significantly the Doppler velocity of both polarizations (about 50% faster), enhancing the horizontally polarized backscattering cross‐section drastically (with 10–15 dB increase), and producing relatively small change in the vertically polarized cross‐section (about 1–2 dB increase). The presence of swell (in the same direction of wind waves) reduces both the radar backscatter and the impact of breaking waves on radar return. By inference, the swell presence decreases the ocean surface roughness and breaking activity. These results are consistent with earlier in‐situ surface wave measurements and our expectation of swell modification of breaking process due to interaction of short waves and the orbital velocity of long swell.