ASA 125th Meeting Ottawa 1993 May

4pAO1. Breaking waves and bubble plumes.

Peter H. Dahl

Andrew T. Jessup

Appl. Phys. Lab., College of Ocean and Fishery Sci., Univ. of Washington, Seattle, WA 98105

A large part of our knowledge of oceanic bubbles originates from acoustic measurements, for example, boundary layer stability [Thorpe, Philos. Trans. R. Soc. London Ser. A 304, 155--210 (1982)] and Langmuir structure [Zedel and Farmer, J. Geophys. Res. 96 (C5), 8889--8900 (1991)] are known to influence bubble field properties. What remains elusive are details of the transient properties of the bubble field, such as the spatial and temporal scale of bubble plumes due to breaking waves. During January 1992 acoustic measurements of bubble plumes from breaking waves were made from the R/P FLIP operating 400 nmi. Off the coast of California. Acoustic measurements were made from two sonars, a four-beam system (20--50 kHz) and a single-beam system (240 kHz), placed at the end of a 12-m subsurface boom attached to FLIP's hull at 28.5-m depth. The subsurface boom was specifically designed for this application, allowing the sonars to look up at the ocean surface from a stabilized platform. To document breaking wave activity over the acoustic system, a video camera was mounted on FLIP's aft boom and trained upon the acoustic footprint on the ocean surface. Surface waves, CTD, current, and meteorological measurements were also made. Space-time images of acoustic volume scattering clearly show bubble plume growth and decay. Details of the spatial and temporal scales of transient bubble plume and breaking wave event combinations are presented. In one case characterized by U[sub 10]=10 ms and H[sub 1/3]=2.3 m, newly generated bubble plumes were observed to reach a maximum depth of approximately 4 m in about 60 s after wave breaking occurred. [Work supported by ONR Code 124.]