Wave Kinematics at High Sea States

Abstract

Measurements of currents close to the ocean surface and within the crests of large, steep waves have been acquired with an incoherent bistatic sonar mounted on the seafloor. The sonar uses a single narrow-beam transmitter/receiver and three fan-beam receivers set in a triangular configuration around the source. Acoustic pulses transmitted from the seafloor are scattered by bubble clouds and the sea surface to the four receivers and may be transformed into velocity components as a function of elevation. Individual estimates of the currents at, and close to, the surface are made with sufficient temporal resolution to identify kinematics in the crests of large waves. Observations acquired in the Danish sector of the North Sea are examined to evaluate both the potential merits and limitations of the measurement approach. At lower wind speeds, sidelobe scatter from the surface reaches the receiver simultaneously with the volume scattered signal arriving from a few meters beneath, contaminating the velocity measurement at this depth. At higher wind speeds, bubble clouds and increased roughness of the surface combine to suppress this effect, permitting reliable near-surface measurement. A numerical simulation has been implemented to explore some aspects of sonar performance including turbulent velocity fluctuations and bubble density gradients. Additional analysis is carried out to examine bubble suppression of sidelobe scatter. The observations lead to some conclusions regarding wave kinematics during a storm in which the wind speed reached ?17 m s?1. At the ocean surface, the downwind velocity in the crests of large waves substantially exceeds that predicted by the second-order Stokes model, but in the wave troughs the current is close to the nonlinear prediction.