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 with 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-degrees incidence angle and vertical polarization. Average normalized radar cross-section sigma0 values from all fetches follow a common trend for sigma0 as a function of u*, so the fetch dependence is negligible. An empirical power-law model yields a high correlation between sigma0 and u*, but because systematic anomalies arise, we re-examine a turbulence 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 and u*.