TOWER-BASED MEASUREMENTS OF NORMALIZED RADAR CROSS-SECTION FROM LAKE-ONTARIO - EVIDENCE OF WIND STRESS DEPENDENCE

We report here the dependence of the normalized radar cross-section (NRCS) on incidence angle, azimuth angle, wind speed, wind stress, and atmospheric stratification for Ku band microwave backscatter from a lake. The measurements were made in autumn 1987 on Lake Ontario, using a rotatin microwave system mounted on a research tower operated by the Canada Centre for Inland Waters. The results show that at intermediate incidence angles the NRCS on the lake generally increases faster with wind speed than it does on the ocean. We attribute this to the larger atmospheric drag coefficients which exist on the lake compared with the ocean, and we show that the results are more consistent with a dependence of the NRCS on wind stress than on wind speed near the surface. We find a stratification dependence of the NRCS similar to that previously reported at C band and show that at 40 degrees and 60 degrees incidence angles this dependence can be removed by parameterizing the NRCS in terms of either the friction velocity or neutral wind speed. At a 20 degrees incidence angle the stratification dependence is not removed by this procedure.