The dependence of <i>X</i> band microwave sea return on atmospheric stability and sea state

Abstract

The dependence of microwave sea retun at an incidence angle in the Bragg regime on wind speed, atmospheric stability, dominant wave slope, and dominant wave frequency has been measured. The measurements were made from a tower in the Gulf of Mexico with a CW, X band microwave system operating at a 45° incidence angle. Measured microwave parameters were relative cross sections and ocean wave‐radar modulation transfer functions obtained with antennas directed into wind and waves. The data reveal that cross sections increase with decreasing z / L ( z being height and L being the Obukhov length) in unstable atmospheric conditions. The rate of increase is much higher than that of surface stress calculated on the basis of present boundary layer models. Furthermore, these data indicate that in near‐neutrally stratified atmospheric conditions, X band cross sections at constant and relatively low wind speeds increase with long‐wave slope, while modulation transfer functions decrease with long‐wave slope. No clear dependence of either parameter on wave slope was observed in highly unstable conditions. The implications of these findings for other microwave frequencies and for scatterometry and microwave imagery of the sea are briefly discussed.