We present the results of laboratory and field measurements on the stability of
wind-driven water surfaces. The laboratory measurements show that when exposed
to an increasing wind starting from rest, surface current and wave generation is
accompanied by a variety of phenomena that occur over comparable space and time
scales. Of particular interest is the generation of small-scale, streamwise vortices, or
Langmuir circulations, the clear influence of the circulations on the structure of the
growing wave field, and the subsequent transition to turbulence of the surface flow.
Following recent work by Melville, Shear & Veron (1998) and Veron & Melville
(1999b), 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 remains elusive. Within
the range of parameters of the experiments, both the surface waves and the Langmuir
circulations first appear at constant Reynolds numbers of 370 ± 10 and 530 ± 20,
respectively, based on the surface velocity and the depth of the laminar shear layer.
The onset of the Langmuir circulations leads to a significant increase in the heat
transfer across the surface. The field measurements in a boat basin display the same
phenomena that are observed in the laboratory. The implications of the measurements
for air–sea fluxes, especially heat and gas transfer, and sea-surface temperature, are
discussed.
Generation of attached Langmuir circulations by a suspended macroalgal farm
