National Oceanic and
Atmospheric Administration
United States Department of Commerce


 

FY 1981

Studies of the dependence of L-band backscatter on sea surface winds using the synthetic aperture radar

Weissman, D.E., W.E. Brown, Jr., T.W. Thompson, F.I. González, and W.L. Jones

In Oceanography from Space, J.F.R. Gower (ed.), Plenum Press, New York, 543–551 (1981)


The data from several sea surface studies with the L-band synthetic aperture radar, operated from both high altitude aircraft and the SEASAT-1 spacecraft, show a consistent wind speed dependence of the microwave scattering coefficient. The measurement technique involves special processing of the backscattered signal to provide: (1) larger than usual resolution cells (a few kilometers in area) to average over the small scale features produced by the surface gravity waves, and (2) an incidence angle of 20° from nadir. This approach observes the average scattering coefficient over this "scatterometer footprint". The sensitivity of these L-band measurements to the wind is less than that at the higher Ku-band frequencies but is still substantial. Not only do surface winds affect the scattering coefficient, but surface temperatures and strong current variations have also been observed to affect it. These results point to the surface friction velocity as the fundamental quantity that controls the surface roughness sensed by this oblique back-scattering technique. Radar data analysis has been combined with surface wind observations wherever possible, and/or wind velocity inferred by the Ku-band radar scatterometer to estimate the parameters of an empirical power-law dependence on the wind magnitude and the amplitude of a sinusoidal azimuthal variation. These measurements include a wide range of wind speed conditions: 3 m/s to 45 m/s, and the Gulf Stream current variation and a maximum range of 5°C temperature variation. Theoretical studies are in progress to interpret this behavior of the scattering coefficient in terms of the growth of the surface roughness (wave spectrum and slope) with the wind speed.




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