Remote measurements of damping and phase velocities of short wind waves in film slicks

Field experiments involving artificial slicks of different surface-active substances were carried out from an oceanographic platform on the Black Sea in September, 1997. Simultaneous measurements were made of the wavenumber-frequency spectrum of centimetre(cm)/millimetre(mm)-scale wind waves and of radar backscatter, using optical spectrum analysers and a coherent Ka-band radar operating at incidence angles about 60/spl deg/. Phase velocities of wind waves were retrieved from measurements of the Doppler shifts of the radar and optical analyser signals. It was found that the suppression of cm-mm-scale wind waves in slicks is accompanied by an increase in the average microwave Doppler shifts and, consequently, in the mean phase velocities of the short wind waves. The increase of phase velocity in slicks can be explained by the influence of bound harmonics of the decimetric waves which are assumed to dominate the spectrum of cm-mm-scale waves in slicks. Another reason for the increase in Doppler shifts is the strong modulation of short wind waves in slicks due to long wind waves.