Effects of surfactant on the growth of wind waves — Simultaneous observations with optical and microwave sensors

The growth of wind waves was investigated in a circulating tank over slick and “clean” surfaces. The slick surface was produced through the aging of seawater, and the “clean” surface was obtained by overflowing. Water-surface slopes and microwave backscattering were simultaneously measured with an optical slope gauge and a continuous-wave radar, respectively. Dependencies of the mean-square slope and the radar backscattered power on the wind-friction velocity over these two surfaces were found to be different. A delayed growth of ripples over the slick surface was identified from optical and microwave measurements. The suppression of ripples by surface slicks was most significant at low winds up to the wind-friction velocity of 15 cm s⁻¹. A reduction of −18 dB in the mean-square slope is found at the wind-friction velocity of 11 cm s⁻¹. The exponent of power-law dependence of radar cross section on wind-friction velocity over the slick surface was much larger than that over the “clean” surface. The radar backscattering from the slick surface was reduced by −40 dB at light winds and saturated at high winds with the wind-friction velocities smaller than 11 cm s⁻¹ and larger than 24 cm s⁻¹, respectively.