Perturbations in the marine surface layer wind field which
are induced by the underlying gravity waves are critical to the exchanges of momentum and energy between wind and waves. These perturbations are difficult to identify due to the presence of turbulent fluctuations in the air flow which are typically several times larger than wave-induced perturbations. The structure of the wave-induced flow has been reported for laboratory flows; however, measurement and data processing difficulties have precluded similar analyses in the field. The current research has involved the measurement of a data set well suited to identification of
wave-induced perturbations, as well as the development of processing
techniques (phase-averaging) which extract these quantities in the presence of wind turbulence and complicated wave fields.
Data collected using the R/P FLIP during the Marine Boundary Layers Experiment (MBL) include the instantaneous wave height directly beneath the air flow sensors (twelve levels of cup and vane anemometers, five levels of three-component ultrasonic anemometers, and three levels of pressure fluctuations between 2 and 20 meters). The wave-height measurement is necessary for direct calculation of wave-induced fields.
A technique for directly extracting phase-averages from field data based on zero-crossings of the wave height signal will be presented. In addition to phase-averaging, the method can be extended to separate perturbations according to the wavenumber of the generating surface wave as well, which allows the answering of questions such as whether the wave-induced flow is similar above long and short waves. The technique is applied to the MBL data set. The results reveal the structure of the wave-induced wind field, including perturbation magnitudes, their rates of decay with height above the surface, and the height dependence of the phase angle between the perturbations and the wave. The importance of coordinate frame (fixed heights above the mean ocean surface versus constant heights above the wave surface) will be discussed for both time-averaged and phase-averaged quantities.