Abstract
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 90o) 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 22.5-45o. The drag reduction may occur even if the
form drag of the wave train is positive.