PATTERNS OF BUBBLE CLOUDS ORGANIZED BY LANGMUIR CIRCULATION

A commonly observed property of near-surface bubble distributions is t heir collective organization into long rows aligned with the wind unde r the influence of Langmuir circulation. Time series observations with sonars having fixed orientation reveal the temporal evolution of bubb le distributions as they drift through the sonar measurement path. Her e this concept is extended to provide a time sequence, at 37-s interva ls, of two-dimensional images generated by horizontally rotating sonar s. 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 binar y representation of the convergence zone patterns from which their ori entation, length, spacing and other properties can be extracted. Altho ugh there is some angular spreading, most convergence lines are aligne d within 20 degrees of the wind. The spacing between convergence lines reveals a wide range of scales, but the mean spacing increases slight ly with wind speed. Measurement of downwind length reveals the presenc e of numerous short bubble clouds, possibly associated directly with w ave breaking; however, there is a general trend toward a length that i ncreases with wind speed. A dominant characteristic at higher wind spe eds is the formation of Y junctions in which three linear bubble cloud s are joined together. Each branch of a Y junction was observed to be approximately 50 m. The junctions preferentially point downwind with t he angle between the two side branches being approximately 30 degrees. 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 an d the formation of U-shaped vortex tubes.