The Character of the Clutter Observed during the Passage of the Cold Front

Observations of the sea clutter with the Doppler radar were carried out at Chosi in November, 1985. The distribution of Doppler velocities undergoes a change during the passage of a cold front, and it is mainly explained by the change of the wind direction. But some part of it is attributable to the orbital velocity of the swells. Then it is properly supposed that the Doppler velocities commonly observed depend firstly on the surface wind, and secondly on the height and period of the swells. The velocities of the white-crested waves of the sea surface were measured with a video camera about 500m apart from the radar site. It is found that both the velocities of the white-crested waves and radar's scatterers on the sea surface nearly coincide. On the crest of the white-crested waves, where the sea water is mixed together with the air, the sea surface rises steeply. Its shape seems to be effective in scattering the microwave.