Measurements of turbulence in the wave boundary layer using high frequency acoustics

Abstract

The properties of the turbulent boundary layer at the air–sea interface exhibits departures from classical wall bounded shear flows as a result of the free surface characterized by surface waves. As surface waves grow in the presence of wind, they become unstable and break, which leads to the injection of turbulence and bubbles into the water column. While an understanding of these physical processes is critical to fields of oceanography and mid-/high-frequency ocean acoustics, there remains much to be learned due in part to the difficulty of obtaining turbulence measurements at the air–sea interface as well as signal processing issues associated with separating turbulent water motions from wave orbital motions. One maturing solution to this measurement problem is the use of O(1) MHz pulse to pulse coherent Doppler sonars that allow direct wave number measurements in the inertial subrange of the turbulence. Recent field measurements of near-surface turbulence have been conducted by our group with this sonar technology both offshore Pt. Conception California and Hawaii. Measurement results will be discussed, including observations regarding the relationship between the intermittancy of the turbulent dissipation to the overlying surface wave field. [Work supported by ONR and NSF.]