Patterns of Bubble Clouds organized by Langmuir Circulation

Abstract

A commonly observed property of near-surface bubble distributions is their collective organization into long rows aligned with the wind under the influence of Langmuir circulation. Time series observations with sonars having fixed orientation reveal the temporal evolution of bubble distributions as they drift through the sonar measurement path, Here this concept is extended to provide a time sequence, at 37-s intervals, of two-dimensional images generated by horizontally rotating sonars. Observations obtained during a storm in the Strait of Georgia show individual Langmuir convergence zones as they evolve above the freely drifting sonar. The resulting images are processed to generate a binary representation of the convergence zone patterns from which their orientation, length, spacing, and other properties can be extracted. Although there is some angular spreading, most convergence lines are aligned within 20° of the wind. The spacing between convergence lines reveals a wide range of scales, but the mean spacing increases slightly with wind speed. Measurement of downwind length reveals the presence of numerous short bubble clouds, possibly associated directly with wave breaking; however, there is a general trend toward a length that increases with wind speed. A dominant characteristic at higher wind speeds is the formation of Y junctions in which three linear bubble clouds are joined together. Each branch of a Y junction was observed to be approximately 50 m. The junctions preferentially point downwind with the angle between the two side branches being approximately 30°. Although the junctions deform with time, they can be readily tracked through successive images The existence of convergence zone junctions suggests the reconnection of counterrotating longitudinal vortices and the formation of U-shaped vortex tubes.