Momentum Flux in the Surf Zone during a Landfalling Storm

Tropical storm forecast accuracy is limited, in part, by inadequate understanding of the air-sea momentum flux in extreme and high wind speed environments. Direct flux measurements in these conditions are therefore needed to further our understanding of the dynamic coupling between the ocean surface and lower atmosphere. This is especially true near shore where forecasts are used to evaluate risk in the critical hours before tropical storm landfall. In this presentation I will examine fluxes measured in Duck, North Carolina during a northeastern frontal passage which led to wind speeds of 20 m s^-1 and 4 m significant wave heights. Sonic anemometers deployed on the beach and pierhead (~500 m offshore) captured the flux in very different regimes. The drag coefficient in the surf zone was 3-5 times greater than observed offshore, and there was a wind speed gradient caused by slowing of the wind as it crossed the surf zone. The wavelengths of the dominant turbulent eddies which transport the energy in the surf zone were smaller than at the pierhead or predicted by universal scaling. It will be shown that turbulence in the surf zone appears to have similar properties to tropical storms which may help inform our comprehension of both. This research is expected to help improve our understanding of tropical storms though more lucid realization of air-sea fluxes near shore where open-ocean parameterizations are ineffective, potentially leading to more accurate models.