Defense Date

8-7-2020

Document Type

Thesis

Degree Type

Master of Science

Degree Name

Marine Science

First Advisor

Alexander Soloviev, Ph.D.

Second Advisor

Richard Dodge, Ph.D.

Third Advisor

William Perrie, Ph.D.

Fourth Advisor

Roger Lukas, Ph.D.

Keywords

sea spray, tropical cyclone, hurricane, air-sea interaction, spume, computational fluid dynamics, CFD, ANSYS Fluent, productivity, surfactants, surface-active materials

Abstract

Despite significant improvement in computational and observational capabilities, predicting intensity and intensification of major tropical cyclones remains a challenge. In 2017 Hurricane Maria intensified to a Category 5 storm within 24 hours, devastating Puerto Rico. In 2019 Hurricane Dorian, predicted to remain tropical storm, unexpectedly intensified into a Category 5 storm and destroyed the Bahamas. The official forecast and computer models were unable to predict rapid intensification of these storms. One possible reason for this is that key physics, including microscale processes at the air-sea interface, are poorly understood and parameterized in existing forecast 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.

A computational fluid dynamics (CFD) model was used to simulate spray radii distributions starting from a 100 micrometer radius as observed in laboratory experiments at the University of Miami Rosenstiel School of Marine and Atmospheric Sciences SUSTAIN facility. Results of the model were verified with laboratory experiments and demonstrate that surfactants increase spray generation by 34% under Category 1 tropical cyclone conditions (~40 m s-1 wind). In the model, we simulated Category 1 (4 Nm-2 wind stress), 3 (10 Nm-2 wind stress), and 5 (20 Nm-2 wind stress) conditions and found that surfactants increased spray generation by 20-34%.

The global distribution of bio-surfactants on the earth is virtually unknown at this point. Satellite oceanography may be a useful tool to identify the presence of surfactants in the ocean in relation to tropical cyclones. Color satellite imagery of chlorophyll concentration, which is a proxy for surfactants, may assist in identifying surfactant areas that tropical cyclones may pass over. Synthetic aperture radar imagery also may assist in tropical cyclone prediction in areas of oil spills, dispersants, or surfactant slicks.

We anticipate that bio-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.

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