A LABORATORY STUDY OF FRICTION-VELOCITY ESTIMATES FROM SCATTEROMETRY - LOW AND HIGH REGIMES

Measurements from scatterometers pointing at wind-waves in three large wave-tanks are examined to study fetch effects and the correlation wi th wind friction-velocity u. Time-series measurements were made at 13 , 35, and 95 m with a K(a)-band scatterometer aimed upwind at 30-degre es incidence angle and vertical polarization. Average normalized radar cross-section sigma0 values from all fetches follow a common trend fo r sigma0 as a function of u, so the fetch dependence is negligible. A n empirical power-law model yields a high correlation between sigma0 a nd u, but because systematic anomalies arise, we re-examine a turbule nce approach that delineates low and high regimes with a transition at u of approximately 25 cm s-1. Using this criteria, the data are well represented by a two-section power-law relationship between sigma0 an d u.