Effect of Wave Directions on Orientation and Magnitude of Surface Wind Stress under Typhoon Megi (2010)

The relationship between the wind-wave spectrum and surface wind stress T in tropical cyclones, especially for the misalignment |0| between the wind and T, was investigated using data from three Electromagnetic Autonomous Profiling Explorer (EM-APEX) floats deployed in Super Typhoon Megi (2010). The floats measured T by integrating downward momentum flux in the ocean and, in a recent development, directional spectra of surface waves. The wind was captured by aircraft surveys. At wind speeds from 25 to 40 m s21, the |0| increased with the increasing angle between the wind and dominant waves. The |0| was small near the eyewall, where wave energy concentrated in a narrow frequency band. At the location far away from the eyewall, where most spectra were bimodal in directions with similar frequencies, the stress direction might be similar to the high-frequency waves. The misalignment between the wind and propagating swell might affect the growth and directional spreading of wind waves under tropical cyclones. The resulting wave breaking might then release wave momentum into the ocean as most stress clockwise from the wind direction. C||, the downwind drag coefficient, increased with increasing inverse wave age of dominant waves. |C & BOTTOM;|, the magnitude of the crosswind drag coefficient, was significant when low-frequency waves deviated from the wind by more than 908. The wave directions are used in the inverse wave age for scaling drag coefficients. The new parameterization based on wave dynamics can be useful for improving the prediction of wind stress curl under storms.