FY 1983

L-band radar backscatter dependence upon surface wind stress: A summary of new SEASAT-1 and aircraft observations

Thompson, T.W., D.E. Weissman, and F.I. González

J. Geophys. Res., 88(C3), 1727–1735, doi: 10.1029/JC088iC03p01727 (1983)

Radar backscatter from the ocean depends on surface wind stress for a wide range of radar wavelengths. Here, we have reviewed the wind-radar relationships for L band radar wavelengths near 25 cm and 20° angle of incidence and HH polarization using a number of aircraft and SEASAT-1 SAR observations. At this wavelength, the L band backscatter coefficient (σ°) from the ocean appears to depend upon surface wind speed (||) and direction (φ) in the following manner: σ° = K ||^a [1 + b cos(2φ)]. The wind-speed coefficient a is 0.5 ± 0.1 for a wide range of wind speeds. The wind-direction coefficient b is near zero (i.e., b = 0.05 ± 0.05) for lower winds and stable marine boundary layers. However, one unusual aircraft observation suggests that b = 0.20 ± 0.05 for moderate wind speeds and an unstable marine boundary layer. We have examined a variety of aircraft and SEASAT-1 observations. SEASAT synthetic aperture radar (SAR) observations in conjunction with simultaneous SEASAT scatterometer measurements provide an excellent measurement of the wind-speed coefficient a since a variety of wind speeds can be observed in a few minutes. Aircraft observations provide an excellent measurement of wind-direction coefficient b since a number of directions can be examined in about l hour with special flight patterns designed for multiple look directions. Thus, both aircraft and spacecraft observations were studied in this work. We have interpreted these aircraft and spacecraft results in terms of existing theoretical models for radar scattering from the ocean. We believe that modulation of echo power by the magnitude of the wind occurs through both large-scale slope effects as well as through growth of the short gravity wave-height spectrum. Our data is near an angle of incidence of 20° where a two scale scattering model must be used.