Direct Eddy Covariance Flux Measurements of Wind Stress Under Tropical Cyclones and Hurricanes

While the surface turbulence heat flux fuels the hurricane intensification, the momentum flux or wind stress transfers the kinetic energy from the storm to the ocean to regulate the ocean mixing and stratification which in turn affect the Sea Surface Temperature and heat flux. The balance between the surface enthalpy flux and drag plays a critical role in tropical cyclone (TC) and hurricane intensification. Due to the lack of direct observations inside the TCs and hurricanes, studies largely based on numerical models, lab experiments, air-deployed dropsondes, and indirectly from momentum budget analysis, have suggested a large deviation of wind stress and drag coefficients at high wind speed of > 20 m/s in TC and hurricane conditions. Over the past three hurricane seasons, the Saildrone Uncrewed Surface Vehicles (USVs) have demonstrated their capability to intercept the TCs and hurricanes in extreme environments and provide real-time 1-minute and 5-minute averaged data to hurricane forecast centers. This study utilizes the high-resolution 20-Hz data made available once the Saildrone USVs returned after the hurricane season to investigate direct eddy covariance (EC) measurements of wind stress for a better understanding of the drag coefficients under TC and hurricanes. Particular attention will be paid to the variations in different wind and wave conditions.