Air-water momentum flux observations over shoaling waves

Abstract

This paper deals with simultaneous momentum flux observations from four towers placed at different depths along a shore-normal line at the west end of Lake Ontario, Canada. The towers were at nominal depths of 12, 8, 4, and 2 m, in order to study the influence of shoaling waves on the air-water momentum flux, namely the effect of wave steepness and celerity on the aerodynamic roughness of the water surface. Analysis of the data have shown that the momentum fluxes cannot be explained by mean downwind speed alone (U10), in contrast to open ocean experiments in which the wave maturity is generally near full development. Surface roughness (z0), normalized on wave height (η), was parameterized by both the inverse wave age (U10/cp) and the root-mean-square wave slope (θ), with excellent correlation in each case (r2 = 0.79). Multiple regression using both the inverse wave age and the root-mean-square slope lead to the best results: z0/η = 2.26(U10/cp)1.82θ3.83; R2 = 0.90. These findings are presented as further proof, to other recent observations, that surface roughness is dependent on sea state.