Direct measurements of wind stress over the surf zone

Abstract

Field data of the wind stress over surf zone waves are presented from an open ocean beach on the East Australian Coast. Two ultrasonic anemometers were deployed at nominal heights of 5 and 10 m above the water surface in the intertidal and inner surf zones, with concurrent measurements of water levels and offshore wave parameters. Following a rigorous quality control procedure, the wind stress was determined by the eddy correlation technique. A constant stress layer was observed between 5 and 10 m elevation. Considering nearneutral conditions only, the wind drag coefficients were found to systematically change with the wind angle of approach relative to the shoreline and are much smaller for longshore wind than during onshore wind. The concept of an apparent wave steepness changing with wind direction is suggested to explain this behavior. The drag coefficients over the surf zone during onshore wind and near-neutral conditions were determined to be almost twice the values expected at the same wind speed and open ocean conditions. The observed Charnock coefficient was similarly an order of magnitude larger than open ocean values. A wave celerity of the order of that expected in the inner surf zone is required to explain the observed large roughness and drag coefficients using existing wave age-dependent parameterizations. This suggests that the slower wave celerity in the surf zone is an important contributor to the increased wind stress, in addition to the sawtooth wave shape.

Numerical Study on the Effect of Air–Sea–Land Interaction on the Atmospheric Boundary Layer in Coastal Area

“…The advection and SGS terms are calculated explicitly, while the viscous term is advanced implicitly. The fractional-step method [11] is employed to satisfy the divergence-free condition given by Equation (1). The Poisson equations are solved using an open-source math library called Portable, Extensible Toolkit for Scientific Computation (PETSc) [12].…”

“…Following Shabani et al [1], the value of α is set to 0.01 and 0.11 for open ocean and surf zone, respectively. To determine u * , we use the field measurement data of time-averaged wind speed U 1 at z = z 1 .…”

“…The parameterizations used by these tools are usually derived from open-ocean problems. As a result, many unique features and complex processes in littoral regions, such as the increase of surface roughness in a surf zone [1][2][3] and the abrupt change in sea-land terrain, have not been addressed.…”

Observations of Nearshore and Surf Zone Wind Stress

“…At the same time, quality controls were also implemented to filter out a limited number of data blocks collected during very low tide conditions in order to avoid such internal boundary layer effects, and to ensure that the assumption of constant stress layer is not violated. Full details of the experiment, instrumentation, and quality controls are discussed in Shabani (2013) and Shabani et al (2014).…”

“…A detailed review of existing studies of wind stress on the ocean surface has recently been provided in Shabani et al (2014), while a comprehensive discussion of the air-sea interaction topic and associated physical processes is given in Shabani (2013).…”