A Laboratory Study of Whitecap Lifetime

Under continued influence from the wind, waves grow until they become unstable and break. The breaking process entrains air, creating plumes of bubbles below the surface which rise and form whitecaps. These whitecaps are identified as either actively generated (stage A), or decaying (stage B). Stage A whitecaps appear along the crest of a wave as it breaks, stage B are the patches left on the surface following wave breaking. Collectively, below surface bubble plumes and whitecaps are referred to as foam and their extent is quantified as fractional coverage upon the ocean surface W. Wind speed U is believed the main driver of whitecap formation and empirical expressions of W are typically formulated as W(U). W normally grows with U’s third power but there is a great deal of uncertainty, largely due to significant variability in stage B lifetime.

W is used extensively to estimate marine boundary layer processes such as aerosol production, gas transfer, surface winds, spray generation, and energy dissipation. However, these processes are associated with stage A or stage B, but not necessarily both, consequently, the W(U) relationship can be unreliable for this purpose. Here, we present initial results from a laboratory experiment in which 3-D breaking waves were generated and observed using infrared and visible cameras. Through this research we seek to understand the factors that impact stage B lifetime whereby working towards a more robust W parameterization. This will help lead to greater accuracy when estimating air-sea interaction processes.