Laboratory Measurements of the Generation of Langmuir Circulations and Surface Waves

Abstract

We present the results of laboratory experiments on the stability of a wind-driven water surface to surface waves and Langmuir circulations. The laboratory measurements, which are made possible by a variety of modern quantitative flow visualization techniques, show that this classical wave-generation problem presents a variety of interesting phenomena that occur over comparable space and time scales. Of particular interest is the clear influence of the Langmuir circulations on the structure of the wave field. Following recent work by Melville, Shear & Veron (1998) and Veron & Melville (1999), we show that the waves that are initially generated by the wind are then strongly modulated by the Langmuir circulations that follow. Direct measurements of the modulated wave variables are qualitatively consistent with geometrical optics and wave action conservation, but quantitative comparison is elusive. Within the context of the Craik-Leibovich theory of Langmuir circulations, the scaling is clearly O(1), with the surface currents being comparable to the phase speed of the waves. We discuss the results in the context of the available theoretical models. It is a pleasure to dedicate this paper to Professor Leibovich on the occasion of his 60th birthday.