Modulation of sea surface radar cross section by surface stress: Wind speed and temperature effects across the Gulf Stream

During the past several years, many radars (Moskowitz, 1973; Larson et al., 1976) have observed the distinct and interesting features associated with the Gulf Stream and its boundaries. Some of these Gulf Stream radar features are small scale, with dimensions comparable to and slightly greater than long gravity waves. Other features are larger, with dimensions much greater than the length of long gravity waves. This study describes radar cross-section variations within the Gulf Stream and just outside, seen with a ‘scatterometer’ type measurement. The significant features of these radar cross-section data were that the Gulf Stream always had a higher cross section per unit area (interpreted here as a greater roughness) than the water on the continental shelf. Also, a steep gradient in cross section often was seen at the expected location of the western boundary. There also were longer scale (10–20 km) gradual fluctuations within the stream. These roughness variations are correlated with the surface shear stress that the local wind imposes on the sea. By using the available surface truth information regarding the wind speed and direction, an assumed Gulf Stream velocity profile and high-resolution ocean surface temperature data obtained by the very high resolution radiometer on board a NOAA polar-orbiting satellite, profiles of sea surface stress were calculated for comparison with radar cross-section data. This study demonstrates that the measured radar cross-section variations are correlated with computed surface stress and that both surface-air temperatures and surface winds are important factors in determining surface stress and radar cross section.

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