Publication Date:
2003-03-25
Description:
Microscale breaking wind waves cover much of the surface of open waters exposed to moderate wind forcing. Recent studies indicate that understanding the nature and key features of the surface skin flows associated with these small waves is fundamental to explaining the dramatic enhancement of constituent exchange that occurs in their presence. We describe a laboratory study in which velocity measurements were made within a few hundred micrometres of the surface of microscale breaking wind waves without bubble entrainment, using flow visualization and particle image velocimetry (PIV) techniques for a range of wind speed and fetch conditions. Our measurements show that for each experiment, the mean surface drift directly induced by the wind on the upwind faces and crests of these waves is (0.23 ± 0.02 ua* in the trough increasing to (0.33 ± 0.07) ua* at the crest, where ua* is the wind friction velocity. About these mean values, there is substantial variability in the instantaneous surface velocity up to approximately ±0.17 ua* in the trough and ±0.37 ua* at the crest. This variability can be attributed primarily to the modulation of the wave field, with additional contributions arising from fluctuations in wind forcing and near-surface turbulence generated by shear in the drift layer or by the influence of transient microscale breaking. We observed that in a frame of reference travelling with a microscale breaking wave, the transport in the aqueous surface layer is rearward along its entire surface, except within and immediately upwind of the spilling region. Moreover, we found that transport of surface fluid rarely occurs forward over the crest and into the spilling region. This is in marked contrast with previously envisaged wind drift layer flow structures. Visualizations and PIV measurements demonstrate the important role of microscale wind-wave breaking in the direct transport of fluid from the surface to the highly turbulent domain below. Many hundreds of surface flow visualization images were carefully examined. These showed that at the toe of each microscale breaker spilling region, there is an intense and highly localized convergence of surface fluid, with convergence rates generally exceeding 100 s-1. By comparison, observations of surface convergence attributable to parasitic capillary activity are modest. The changes in mean surface drift along the upwind faces of the waves are equivalent to mean surface divergences of between 0.2 and 1.3 s-1. Flow visualizations of the surface layer along the upwind (windward) faces of these waves revealed the occurrence of locally intense flow divergence. However, maximum values of the divergence rate were observed to be only of order 10 s-1. Hence these divergence zones are much more diffuse than the convergence zones at the toes of spilling regions. Overall, our measurements strongly support the view that microscale breaking is likely to be the dominant process in the enhancement of sea surface exchange at moderate wind speeds, as has been suggested by a number of previous investigators. Using a simple model based on our observations, it is shown that microscale breaking is potentially a very effective process in the observed enhancement of constituent transfer for U10 ≥ 4 m s-1 (where U 10 is wind speed measured 10 m above the surface) and this mechanism exceeds by a wide margin the strength of other previously proposed mechanisms.
Print ISSN:
0022-1120
Electronic ISSN:
1469-7645
Topics:
Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
,
Physics
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