Air-sea momentum flux measurements in the surf zone over breaking waves

The momentum exchange in the nearshore is very complex and still poorly understood. A unique high frequency air-sea momentum system, designed specifically for this experiment, was deployed in the surf zone on Sandy Hook, NJ for three days in June 2002. The system is comprised of four high-frequency salt-water wave staffs, a high-frequency Campbell Scientific 3-D sonic anemometer, a Sontek ADV, an Analog Device accelerometer, a Campbell Scientific, Inc. CR23X datalogger, a rechargeable battery pack and an onshore laptop computer. Measurements of significant wave height, associated peak period and phase speed, and mean wind speed and direction were collected at a frequency of 10 Hz, over 23 half-hour sampling intervals during the three day period. The data are post-processed to directly calculate the friction velocity, shear stress, stress direction, wave age and drag coefficient over the surf zone. Direct measurements of the turbulent fluctuations above the sea surface are obtained, allowing for the investigation of momentum fluxes across the air-sea interface and the affects of local wind stress on shoaling and breaking waves. Prior to processing the data, the Campbell Scientific CSAT3 sonic anemometer was post-calibrated under both laminar and turbulent wind conditions in the Davidson Laboratory high-speed towing tank to determine possible measurement corruption due to the anemometer head and probe supports. Unwanted distortion effects are found within a direction band between 160--200° relative to the head of the anemometer, limiting the use of turbulent wind data to ranges between 0--160° and 200--360°. A method to separate the turbulent air and wave stress contributions from the total stress is developed and applied to the field data. The data demonstrates a decrease in the neutral drag coefficient with increasing co-linear wind speeds, counter to data collected over deepwater developing seas. Strong wave-wind angle, atmospheric stability and wave breaking in the surf zone appear to be the dominant factors determining stress direction.