The dependence of ocean backscatter at Ku-band on oceanic and atmospheric parameters

The Jet Propulsion Laboratory NUSCAT K_u-band scatterometer successfully acquired ocean backscatter data over a wide variety of oceanic and atmospheric conditions during the Surface Wave Dynamics Experiment (SWADE). Ten flights resulted in 30 h of data collection were conducted on the NASA Ames C130 aircraft. The SWADE experimental area was deployed with several buoys providing appropriate in situ measurements to correlate with the radar backscatter for incidence angles from 10 to 60° at both horizontal and vertical polarizations. The NUSCAT-SWADE backscatter data base was utilized in conjunction with buoy measurements to investigate ocean backscatter signatures over the range of conditions encountered during SWADE. To account for modulations and fluctuations in incidence angles, a backscatter analysis independent of a priori geophysical model functions was developed, tested, and implemented. Results for backscatter azimuth modulations in terms of upwind, downwind, and crosswind radar returns are compared to airborne RADSCAT results and to SASS-I and II geophysical model functions versus neutral wind speed (U_N). NUSCAT-SWADE results are closest overall to SASS-II values, fit best with SASS-I at 10° incidence angle, and are significantly higher than RADSCAT. Effects of friction velocity (u*), wave age (c_o/u*), and significant wave height (H_1/3) on ocean backscatter are studied. The backscatter shows a good positive sensitivity to u*, an inverse dependence on c_o/u*, and no systematic trend with H_1/3 excluding cases of large swells. Coefficients of empirical relations between backscatter and neutral wind speed, friction velocity, and wave age are derived for 10-40° incidence angles at both horizontal and vertical polarizations. Covariance studies of backscatter with the derived relations to winds show an overall deviation factor in the order of 1 dB for ocean signatures including uncertainties in surface conditions