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Field observations of radar backscatter modulation and radar Doppler shifts in slicks

Modulation of radar backscatter due to long wind waves for X- and Ka-band radars has been studied in field experiments with artificial organic slicks. A new effect, an increase of the radar backscatter modulation in slicks, has been revealed. The modulation transfer function (MTF) was analysed for non-slick and slick zones, and it is shown that the MTF magnitude in slicks can be several times larger than in non-slick areas. The phase of MTF is also changed in slicks. It is shown that the Doppler shift of the radar signal spectrum, averaged over the periods of long wind waves, is usually larger than the averaged Doppler shift of the current spectrum, the difference between the two is determined by the MTF and increases noticeably in slicks. The Doppler shifts can increase or decrease in slicks, depending on radar wavelength and surface film characteristics. The behaviour of the Doppler shift and the MTF is assumed to be determined by the relation between the free and bound components of the short wind-wave spectrum.