Parametrization of the wind stress (drag) over the ocean is an essential issue in numerical analysis of the ocean–atmosphere interactions for climate modelling, satellite observations of air–sea fluxes, and other purposes. While wind stress over the ocean has been the subject of study for 50 years, the parametrization of the ocean surface momentum flux by a drag coefficient is still an uncertain process. There are many uncertainties not only in the way of proper parametrization, but in understanding the physical processes of the generation of stress by the wind system over the complicated nature of the ocean surface.

The drag coefficient has traditionally been treated as a function of the mean wind speed at a certain level, say at 10 m. Alternatively the coefficient can be represented by an aerodynamic roughness parameter. However, the spread of the observed values indicates that the question is not so simple. In 1955 Henry Charnock proposed a disarmingly simple expression for aerodynamic roughness, which was expressed in terms of the air friction velocity and the acceleration of gravity, independent of the state of ocean waves. While this expression has been widely used, there are important deviations and these were studied as a function of the wave age, a parameter representing the state of growth of wind waves relative to the local wind speed. Alarmingly, the trend of the observational values of aerodynamic roughness could be interpreted as opposite to the theoretical prediction according to the experiments considered.