Characterization and Quantification of Air-sea Momentum Flux by Direct Observations in the Coastal Ocean

The aerodynamic drag coefficient is used widely in weather and climate models to estimate the transfer of momentum across the air-sea interface. While decades of research have led to improvements to drag coefficient parameterizations, even the most robust are developed from open ocean measurements where conditions are spatially uniform. Complex coastal processes, such as wave shoaling and breaking, varying wind-swell incidence angles, rapid bathymetric changes, and shore-side topographic features, all contrast open ocean homogeneity, meaning that existing drag coefficient parameterizations are not effective near shore. Therefore, there is an urgent need to identify and systematically quantify coastal processes and features that impact the momentum flux. Our research, part of the DUring Nearshore Events eXperiment (DUNEX), took place between August 2021 and January 2023 at the Army Corp Field Research Facility in Duck, North Carolina. We deployed an array of four sonic anemometers on a beach tower 14-26 m above mean sea level and a fifth anemometer on the pierhead 500 m offshore. Each anemometer had a unique flux footprint, meaning the fluxes were sourced at intervals across and beyond the surf zone. Results show that the drag coefficient decreases with distance from shore. When compared to predictions from the Coupled Ocean-Atmosphere Response Experiment (COARE) algorithm, we find the drag coefficient at 14.4 m is significantly higher than predicted, while measurements at 18.4 m and 22.4 m are approximately equal to predictions. At 26.4 m and the pierhead, the values are lower than predicted. Our data are being used to develop a nearshore momentum flux parameterization, aiming to enhance understanding of the dynamic variability in the nearshore drag coefficient and ultimately improve the accuracy of models for predicting nearshore processes.