Influence of Surfactants on Sea Spray Generation During Tropical Cyclones

Abstract:

Despite significant improvement in computational and observational capabilities, predicting intensity of tropical cyclones remains a challenge. One reason is that key physics, including microscale processes at the air-sea interface, are poorly understood and parameterized in existing models. Under tropical cyclones, the air-sea interface becomes a multiphase environment involving bubbles, foam, and spray. The presence of surface-active materials (surfactants) alters these microscale processes in an unknown way that may affect tropical cyclone intensity. The current understanding of the relationship between surfactants, wind speed, and sea spray generation remains limited. Here we show that surfactants significantly affect the generation of sea spray, which provides some of the fuel for tropical cyclones and their intensification. Using a numerical model confirmed with a laboratory experiment, which predicts spray radii distribution starting from a 100- μm radius, we show that surfactants increase spray generation by 20-34%. Specifically, we use a Volume of Fluid to Discrete Phase Model developed by ANSYS Fluent, which converts water parcels to Lagrangian particles to represent sea spray and spume. Fluent’s Evaporation-Condensation model allows us to model sea spray evaporation. Laboratory experiments conducted at the University of Miami show that surfactants alter sea spray generation by forming branch-like rather than finger-like structures, which fragment into sea spray or spume of various sizes. We anticipate that surfactants affect heat, energy, and momentum exchange through altered size distribution and concentration of sea spray, with consequences for tropical cyclone intensification or decline, particularly in areas of algal blooms and near coral reefs, as well as in areas affected by oil spills and dispersants. The presence of bio-surfactants, as well as anthropogenic surfactants, can be identified from advanced synthetic aperture radar techniques.