Reduction of drag coefficient due to misaligned wind-waves

Recent field observations suggest that the air-sea momentum flux (or the drag coefficient) is significantly reduced when the dominant wind-forced surface waves are misaligned from local wind. Such conditions may occur under rapidly changing strong winds (such as under tropical cyclones) or in coastal shallow waters where waves are refracted by bottom topography. A recent Large Eddy Simulation (LES) study also shows that the drag coefficient is reduced by a misaligned strongly forced wave train (with a small wave age of 1.37). In order to investigate more realistic field conditions, this study employs LES to examine the effect of a misaligned (up to 90 degrees) surface wave train over a wide range of wave age up to 10.95. For all wave ages examined, the drag coefficient is reduced compared to the flat surface condition when the misalignment angle exceeds around 22.5 degrees-45 degrees. The drag reduction may occur even if the form drag of the wave train is positive.