On the Analysis of Drag Coefficient with Different Background Waves under Extreme Winds

The drag coefficient Cd is a key non-dimensional parameter for air-sea momentum flux in hurricane, as well as a wind input in wave models. Previous researchers have discussed the variability of Cd with varying wind under extreme wind forcing in both laboratory and field studies. It has been observed that Cd will saturate in laboratory studies or even decrease in field measurements with increasing wind forcing under extreme winds. Further research involving other wave parameters (for example, wave age and wave steepness) showed that Cd could also be a function of sea state, instead of solely being a function of wind speed, which is used in parameterization in most current wave models. However, due to the lack of field data under extreme wind conditions, the study of Cd on sea state of the open ocean is relatively rare. Also, the misalignment of the swells from wind forcing, which is very common in most hurricanes, could interfere with the Cd's measurement, causing this coefficient to decrease. Furthermore, none of the previous laboratory studies have discussed the drag coefficient dependence on different wave fields, especially under extreme wind forcing. Thus, to fill this gap, in our research we subjected JONSWAP spectrum waves generated from wave paddles with different significant wave height under wind forcings from a mild sea breeze to a Category 4 hurricane intensity in the SUrge STructural AIr-Sea Interaction (SUSTAIN) facility located at the University of Miami and measured both winds and waves. Our analysis showed that Cd from the wind-only condition (no paddle waves) was smaller than those cases in other experiments with mechanical paddle waves. The steeper the mechanical waves were, the higher Cd was, even under extreme wind forcing. We also observed drag saturation during all our experiments and it is the result of intense wave breaking as well as flow separation. After wind forcing was imposed, our results showed that Cd increased uniformly with increasing wave steepness including paddle waves and wind-waves, and finally clustered together under extreme wind forcings. This trend matched with results from previous researchers in field studies. Finally, we propose that future generation wave models should consider using wave steepness in the parameterization of wind input.