SONAR OBSERVATIONS OF LANGMUIR CIRCULATION AND ESTIMATION OF DISPERSION OF FLOATING PARTICLES

Observations are described of Langmuir Circulation obtained using upwa rd-pointing bottom-mounted sonars, and a methodology to use the data t o estimate the dispersion of floating particles is suggested. Observat ions of linear bands of acoustic scatterers separated by 2-20 m and de tected using side-scan sonar in Loch Ness, Scotland, and in the southe rn North Sea are ascribed to subsurface bubbles in the convergence zon es produced by Langmuir circulation. Data from the two observation sit es are compared. The sonar is able to monitor the variability of the p atterns over many hours. When the currents are sufficiently small, as in Loch Ness, individual bubble clouds produced by breaking waves rema in in the beam long enough for their speed to be resolved, and the rat e of convergence into the bands can be estimated. It increases linearl y with wind speed. The acoustic data and direct measurements using cur rent meters are used to derive estimates of the response time of bubbl e bands to changes in wind, and their mean separation, length, and per sistence time. The bands in Loch Ness are shorter, but persist longer, than those in similar wind conditions in the relatively shallow and w ell-mixed North Sea, It is suggested that these differences may be asc ribed to the presence of turbulence generated by the shear stress of t he strong tidal currents on the seabed in the North Sea, a factor abse nt in Loch Ness. Models are devised to simulate the dispersion of plum es of floating particles released from a fixed position in a field of Langmuir circulation advected by tidal currents, using the sonar data. The estimates of diffusivities show an increase with wind speed, but are sensitive to the choice of some underdetermined parameters. The re sulting estimates of lateral dispersion of floating particles overlap the range of those of Faller and Auer.