Occurrence frequency of air flow separations over wind waves was measured to investigate relationship between the air flow separation and a drag coefficient. The occurrence frequencies converged to a value of 10% with windsea Reynolds number, RB, ranging between 200 and 1000. In that condition, the drag coefficients also have a constant value. Under moderate wind speed and short fetch condition, the air flow separation controls momentum transfer from air side to water side without wave breaking. As a result, despite growth of wind speed, wave breaking probability and wave steepness are still kept constant. This process is considered as a self-regulation mechanism of wind waves without wave breaking.
