Elevated Momentum Flux in the Surfzone during a Storm Journal of the Atmospheric Sciences

Drag coefficient parameterizations, which are largely developed from homogenous deep ocean data, are ineffective nearshore where conditions are nonuniform. This is problematic because operational forecast accuracy depends upon reliable quantification of air–sea momentum transfer. This is especially important for storms which threaten coastal life and property. To help fill this knowledge gap, direct flux measurements were collected from the beach and pierhead in Duck, North Carolina, as part of the During Nearshore Event Experiment (DUNEX). The footprint analysis shows these fluxes were sourced in the surfzone and offshore, representing very different conditions. During a weeklong storm, wind speeds and significant wave heights were 20 m s−1 and 4 m, leading to a broad, vigorous surfzone. The drag coefficient in the surfzone was twice the offshore value, explained by increased roughness due to wind stress and bathymetric changes. The Charnock parameter is well predicted by wave age, but it is expected this is site-specific due to unique bathymetry. A horizontal wind speed gradient was observed and attributed to the high surfzone roughness. The wavelengths of the turbulent eddies in the surfzone were smaller than offshore or predicted by universal scaling. This research offers novel insights that can contribute to a crucial collective effort to develop robust coastal flux models, leading to improved forecasting.