Bubble Clouds and Langmuir Circulation: Observations and Models

Abstract

Concurrent measurements of the rate of dissipation of turbulent kinetic energy and the void fraction and size distribution of near-surface bubbles are described. Relatively high dissipation rates and void fractions are found in bubble bands produced by Langmuir circulation. The mean dissipation rates observed in the bands are close to those at which the dynamics of algae is significantly affected. The data are used to test basic assumptions underpinning models of subsurface bubbles and associated air?sea gas transfer. A simple model is used to examine the qualitative effect of Langmuir circulation on the vertical diffusion of bubbles and the representation of Langmuir circulation in models of gas transfer. The circulation is particularly effective in vertical bubble transfer when bubbles are injected by breaking waves to depths at which they are carried downward by the circulation against their tendency to rise. The estimated value of the ratio r of the eddy diffusivity of particles (resembling bubbles) Kp to the eddy viscosity Kz depends on depth z and on the form selected for Kz. The effects of nonoverlapping or superimposed Langmuir cells of different size may be very different. Multiple nonoverlapping cells of similar scales with Kz independent of depth can result in concentration profiles that resemble those of a law-of-the-wall Kz. It is demonstrated that model prediction of bubble distributions and of gas transfer (which is related to bubble submergence time) is sensitive to Kz and to the size distribution of Langmuir circulation cells.