Modeling the Impact of Surfactants on Sea Spray Generation: A Possible Change in Regime at the Air-Sea Interface Under Major Tropical Cyclones?

Tropical cyclone intensity forecasts often fail to predict rapid intensification of storms, leaving coastal communities devastated. The air-sea interface is a critical, and often overlooked, factor in tropical cyclone dynamics that plays a significant role in rapid intensification. Many physical processes at the air-sea interface are poorly understood or parameterized, particularly under high wind speeds. Microscale processes at the air-sea interface, including sea spray and spume generation, are challenging to study under high winds, yet are an important aspect in tropical cyclone intensity. Surface active agents (surfactants), produced by marine microorganisms or applied as dispersants, are also present at the air-sea interface, and add yet another variable to the already complex physical processes occurring under high winds. Surfactants alter sea surface tension and form films on the sea surface under low wind speeds; Under high winds, surfactants modify spray and spume generation due to their effect on surface tension, which influences heat, momentum, and gas fluxes between the ocean and atmosphere. Laboratory results from the University of Miami under tropical cyclone conditions confirmed that surfactants indeed influence of spray generation mechanisms, showing distinct changes in the spray and spume generating structures. While multiple estimates of the sea spray generation function (SSGF) exist, scarce experimental data under high winds inhibits development of a confident SSGF for tropical cyclones.

To better understand spray generation under extreme conditions, a computational fluid dynamics model was developed that simulates spray generation under all five tropical cyclone categories. The model employs ANSYS Fluent’s Volume of Fluid to Discrete Phase Model, which converts water parcels to Lagrangian particles, representative of sea spray and spume. Utilizing a 528-core supercomputer and Fluent’s dynamic mesh adaption that refines high curvature areas, the model can resolve spray with radii starting from 100-mm. The numerical results, validated with Category 1 data from the laboratory experiment, revealed an increase in sea spray generation function under categories 1, 2, and 3 when surfactants were present in the model. Under Category 4 and 5 conditions, little to no impact of surfactants on spray generation was found. This might be explained by a change in regime under major (Cat. 4 and 5) tropical cyclones, which lacks a dependence on surface tension and other factors. Our numerical results as well as field and laboratory observations by other researchers, support a possible regime change at the air-sea interface during extreme tropical cyclone conditions. These findings reinforce the need for an improved understanding and parameterization of microscale physics at the air-sea interface in existing tropical cyclone forecast models to improve the intensity prediction accuracy.