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°) 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°–45°. The drag reduction may occur even if the form drag of the wave train is positive. Plain Language Summary: This study uses computer simulations to investigate turbulent wind blowing over ocean surface waves that are misaligned (propagate in different directions) from the wind. The study focuses on frictional wind forcing (wind stress) imparted on the ocean surface, because the wind stress is an important boundary condition for ocean, atmospheric, and surface wave models. Surface waves can be misaligned from wind when the wind field rapidly changes such as during tropical storms or in shallow water where wave directions are modified by varying water depth. When waves propagate slower than wind in the wind direction, the waves increase the wind stress compared to flat surface (no wave) conditions. However, this study finds that the wind stress is reduced, that is, the ocean surface becomes smoother, when the misalignment angle between wind and waves exceeds about 22.5°–45°. Key Points: Large eddy simulations are preformed to study wind over misaligned surface wavesDrag coefficient is reduced due to misaligned surface waves for all wave ages under wind‐driven wave conditionsDrag reduction can occur even when wave form drag is positive [ABSTRACT FROM AUTHOR]