Dependence of drag coefficient on the directional spreading of ocean waves

Drag coefficient C 10 is one of the main characteristics used for calculation of surface stress based on mean wind speed U 10. Most of the dependences employ the sea drag as a function of this wind speed. It has been proposed, however, that for a given wind speed C 10 can depend on a number of other properties in the air-sea system. In the present paper, dependence of the drag coefficient on the directional spreading of surface waves is studied numerically. It is shown that such dependence can be significant. For a given wind speed, the sea drag can grow as much as 25% depending on the width of directional spectrum. The highest drag corresponds to the narrowest spectra, and for very narrow directional distribution it saturates. The largest impact of the sea-drag directional dependence is observed for the highest winds. Accounting for the directional spread of surface waves is therefore essential to improve parameterizations of C 10.