Eddy covariance measurements of air-sea heat and momentum fluxes under tropical cyclones and hurricanes in the northwest Tropical Atlantic

Abstract

Tropical cyclones (TCs) and hurricanes are among the strongest Mesoscale Convection Systems originating from the tropical oceans and can cause significant loss of lives and properties when landing. Prediction of TCs, especially their rapid intensification, remains challenging for numerical forecasts. Theoretical and modeling studies have shown that the surface turbulence heat flux fuels hurricane intensification, while 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 (sum of sensible and latent heat flux) and drag plays a critical role in the 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. During the 2021-2023 hurricane seasons, a fleet of 5-12 Saildrone Uncrewed Surface Vehicles (USVs) have been deployed each year to intercept the TCs and hurricanes to make direct observations of the extreme air-sea interaction process. They provided real-time 1-minute averages of near-surface meteorology and ocean variables (5-minute for ocean currents) to hurricane forecast centers. This study utilizes the high-resolution 20-Hz data made available once the Saildrone USVs returned from their cruises 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. The directly observed drag coefficient, as well as the EC heat transfer coefficient (for sensible heat flux), will be compared to those used in the bulk flux algorithm (COARE) and in forecast models. Particular attention will be paid to the variations in different wind and wave conditions within the mesoscale system.