| Microscale breaking wave are short gravity waves that break without air entrainment. Microscale breaking waves occur at low to moderate wind speeds (i.e. 4–12 m s−1) and are typically O(0.1–1)m in length. This work reports on a series of laboratory experiments investigating the flow field beneath microscale breaking waves. In this study, surface temperature measurements were made using infrared imagery and the two-dimensional velocity fields were simultaneously measured using digital particle image velocimetry (DPIV).; Microscale breaking was found to generate strong near-surface vortices that produced the wakes visible in the IR images. The fractional area coverage of the wakes generated by microscale breaking waves was closely correlated with the wave slope and the turbulent kinetic energy, demonstrating that as the wind speed increases, the waves become steeper and this leads to more microscale wave breaking, which enhances near-surface turbulence. At a wind speed of 4.5 m s−1, 11% of the waves were microscale breaking waves, whereas, 91% of the waves were microscale breaking waves at a wind speed of 11.0 m s−1. It was shown that on average, microscale breaking waves are significantly larger in amplitude and steeper than non-breaking waves and that the turbulent kinetic energy generated by a microscale breaking wave was up to a factor of two larger than that generated by a non-breaking wave. The coherent structures generated by wind wave were detected and their statistical properties were used as an input to the surface renewal model to predict air-water gas transfer velocities. The predicted transfer velocities increased by a factor of eight over the wind speed range 4.5 to 11.0 m s−1 and were comparable with measured values. Our laboratory results showed that at wind speeds greater than 7 m s −1, more than 80% of the waves were microscale breaking waves, that more than 86% of the near-surface turbulent kinetic energy and more than 90% of the air-water gas flux was due to microscale breaking waves. This led to the conclusion that microscale breaking waves may prove to be the dominant mechanism that controls the rates of air-sea gas and heat transfer. |
