Field observations of radar backscatter from the ocean surface under low wind speed conditions

Unique simultaneous field observations of wind speed and images of vertically polarized radar backscatter from the ocean surface at low grazing angles during low wind and weakly stable stratification show interesting similarities and an important difference, depending on whether the average wind is falling or rising. When the wind speed begins to fall, there is a delay before the area-averaged radar backscatter begins to fall. When the wind speed begins to rise, the backscatter rises immediately. This observation points to hysteresis in the relationship between radar backscatter and wind speed such that more wind is necessary to generate radar backscatter of a given intensity than is necessary to maintain it. This likely arises from differences between the generation of wind-driven gravity capillary waves and the maintenance of gravity capillary waves and/or microbores. Also, saturation was observed in the radar backscatter at wind speeds between 2.1 and 2.8 m s(-1) for both rising and falling winds. This saturation may be related to the energetics of the centimeter-scale gravity capillary waves and/or microbores that cause the backscatter. During rising winds the backscatter may saturate because fine-scale features of the ocean surface reach a maximum amplitude well before wind-derived energy is transferred upscale to larger waves, whose tilting effect would ultimately produce the familiar increase in radar backscatter with increasing wind. During falling winds the backscatter may saturate because of residual momentum in the surface wave field. Finally, for both rising and falling winds, radar backscatter and wind speed are, in general, correlated above a wind speed of 1.7 m s(-1), while below this value they are not. This observation of the critical wind speed necessary to generate or maintain wind-related backscatter is consistent with results from a recent field observation and with existing laboratory-based observations.