A laboratory study of friction-velocity estimates from scatterometry: low and high regimes

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Abstract
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 Ka-band scatterometer aimed upwind at 30° incidence angle and vertical polarization. Average normalized radar cross-section σo values from all fetches follow a common trend for σo 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 σo and u∗.

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