Estimation of friction velocity from the wind-wave spectrum at extremely high wind speeds

The equilibrium range of wind-waves at normal and extremely high wind speeds was investigated experimentally using a high-speed wind-wave tank together with field measurements at normal wind speeds. Water level fluctuations at normal and extremely high wind speeds were measured with resistance-type wave gauges, and the wind-wave spectrum and significant phase velocity were calculated. The equilibrium range constant was estimated from the wind-wave spectrum and showed the strong relationship with inverse wave age at normal and extremely high wind speeds. Using the strong relation between the equilibrium range constant and inverse wave age, a new method for estimating the wind speed at 10-m height (U₁₀) and friction velocity (u*) was proposed. The results suggest that U₁₀ and u* can be estimated from wave measurements alone at extremely high wind speeds in oceans under tropical cyclones.