A note on minimum wind stress for wave inception

Many experiments have been conducted to evaluate the rate of energy transfer from wind to waves through the measurement of fetch-wise wave growth under a steady wind. There has been little systematic study, however, of local inception of waves by the wind. Earlier experiments on wave initiation were performed with conventional wave height gauges (Ursell, 1956) which are inaccurate for such purpose, while optical wave-slope gauges were used in recent experiments (Cox, 1958; Scott, 1974; Long & Huang, 1976) where the wind stress was not measured. Both macro- and microscopic structures of the disturbed water surface were measured in the present study along with the wind profiles. There is no minimum wind velocity found for the inception, but there is a critical wind-friction velocity for effective wave growth. The conditions of this effective growth are compared favorably with both resonance (Phillips, 1957) and viscous interaction (Miles, 1962) mechanisms.