The dependence of X band microwave sea return on atmospheric stability and sea state

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.

[1]  William J. Plant,et al.  Parametric dependence of ocean wave-radar modulation transfer functions , 1983 .

[2]  J. Mitchell,et al.  Aircraft measurements of the microwave scattering signature of the ocean , 1977, IEEE Journal of Oceanic Engineering.

[3]  John C. Daley,et al.  Variation of the NRCS of the sea with increasing roughness , 1971 .

[4]  D. Ross,et al.  On the relationship of radar backscatter to wind speed and fetch , 1978 .

[5]  W. C. Keller,et al.  Doppler Spectra in Microwave Scattering from Wind Waves , 1971 .

[6]  J. Wright,et al.  Modulation of short gravity‐capillary waves by longer‐scale periodic flows—A higher‐order theory , 1979 .

[7]  William J. Plant,et al.  Ocean wave‐radar modulation transfer functions from the West Coast Experiment , 1980 .

[8]  J. Wright,et al.  Microwave scattering and the straining of wind‐generated waves , 1975 .

[9]  Correlation of radar backscattering cross sections with ocean wave height and wind velocity , 1971 .

[10]  Heinrich Hühnerfuss,et al.  The damping of ocean surface waves by a monomolecular film measured by wave staffs and microwave radars , 1981 .

[11]  Heinrich Hühnerfuss,et al.  Measurements at 13.9 GHz of the radar backscattering cross section of the North Sea covered with an artificial surface film , 1978 .

[12]  J. Wright.  A new model for sea clutter , 1968 .

[13]  L. McGoldrick,et al.  On the rippling of small waves: a harmonic nonlinear nearly resonant interaction , 1972, Journal of Fluid Mechanics.

[14]  W. Large,et al.  Open Ocean Momentum Flux Measurements in Moderate to Strong Winds , 1981 .

[15]  G. Valenzuela,et al.  Nonlinear energy transfer in gravity–capillary wave spectra, with applications , 1972, Journal of Fluid Mechanics.

[16]  W. L. Grantham,et al.  SASS measurements of the Ku -band radar signature of the ocean , 1982 .

[17]  Heinrich Hühnerfuss,et al.  The modification of X and L band radar signals by monomolecular sea slicks , 1983 .

[18]  T. W. Thompson,et al.  L band radar backscatter dependence upon surface wind stress: A Summary of new SEASAT‐1 and aircraft observations , 1983 .

[19]  Klaus Hasselmann,et al.  High-frequency radar observations of sea waves travelling in opposition to the wind , 1978 .