Statistics of Geometric Properties of Breaking Wind Waves Observed in Laboratory

Abstract

We present the results of a statistical analysis of the geometric properties of the breaking wind waves observed in the large Marseille-Luminy wind-wave facility. These investigations were performed within the framework of the LUMINY program aiming at better understanding the effects of wave breaking on gas transfer at the air-water interface [de Leeuw et al., 2001]. They were concerned with both wind-generated waves of wavelength increasing with fetch and paddle-generated waves amplified by wind. Breaking was detected in time series of the wave height derivative signals using a local geometrical criterion. We distinguished two different stages of development of the breaking process: the incipient breaking and fully-developed breaking. We first show that the asymmetric profile of the wind waves at the inception of breaking is invariant with scale and, therefore, is self-similar. The statistics of the geometric properties of the breaking wind waves whatever the development of the breaking process, as the average slope distribution along the crest, the mean slope and the relative jump height of the breaking region, do not vary with fetch and do not depend on the way the waves were initially generated, corroborating that the shape of breaking wind waves is self-similar. The results then suggest that wave breaking at the sea surface and its induced effects both on momentum and gas transfer may be characterized by only one wave parameter as the rate of breaking.