The Changed Sign of Swell Coherent Stress During Swell Conditions

Data measured by ultrasonic anemometer moored at a fixed platform located in South China Sea has been used to analyze turbulence within wave boundary layer. Compared to wind seas, it can be found a dominant area in power spectra, cospectra and Ogive curves under swell conditions from the measurements at 8 m height above sea surface, which is consistent with earlier studies. Our result also shows that the cospectra have both negative and positive regions, which represent the upward and downward momentum induced by swell waves. The wave coherent stress derived from the cospectra shows that it is more larger than the traditional method, implying earlier studies have greatly underestimated the stress of swell. Our study also found that the change of sign of swell coherent stress was related to the wave age, i.e. for c_p/u_* ≥ 60, swell induced the upward momentum, for c_p/u_* < 60, vice versa.