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° incidence angle and vertical polarization. Average normalized radar cross-section σ o values from all fetches follow a common trend for σ o as a function of u * so the fetch dependence is negligible. An empirical power-law model yields a high correlation between σ * 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 25cms -1 . Using this criteria, the data are well represented by a two-section power-law relationship between σ o and u * .