A laboratory study of wave steepness effects on sea spray generation

Sea spray generation is vital for air-sea interactions, however, it has been showing large differences of four orders of magnitude under the same wind speed. It is well known that wind speed can affect sea spray generation, and other physical processes should also be further considered. Surface wave states are widely believed to have significant impacts on sea spray generation, however, they have not yet been specifically and quantitatively verified. This study investigated the effects of surface waves on sea spray generation through a series of controlled laboratory experiments in a large wind-wave tank. Background mechanical waves with different wave heights and periods at a constant high wind speed (U₁₀ = 22 m/s) were synchronically introduced to generate sprays, and sprays with radii ranging from 1.5 to 25 μm were recorded by an optical particle counter. The experimental results indicated that different wave steepness has dramatic effects on sea spray generation function (SSGF) under the same wind speed, which can be close to two orders of magnitude. Then, a power law as a function of wind and wave steepness $u_{*}^{1/3}S{C}_D^{-1/2}$was proposed to describe the sea spray production rate (F).

Plain Language Summary

Sea sprays generated by wave breaking exist at the air-sea interface and have significant impacts on the air-sea mass, momentum and heat exchanges, especially under extreme ocean conditions. In the past 30 years, sea spray generation function (SSGF) controlled by wind speed has been widely adopted, although surface wave effects on sea spray generation have received high attention. The impacts of surface waves have rarely been considered due to sea spray observation difficulties at high sea states, and previous SSGFs have shown large differences of four orders of magnitude. Focusing on this tremendous difference, we conducted a series of wind-wave tank experiments under different wave conditions at a high wind speed to investigate the impact of surface waves on the SSGF. Our results showed that wave steepness is an important factor modulating the SSGF, which provides quantitative evidence for better understandings on sea spray generation and hence their influences on air-sea momentum and heat fluxes.