Air-Sea Momentum Flux in Tropical Cyclones

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The intensity of a tropical cyclone is significantly influenced by two competing physical processes at the air-sea interface. It strengthens by drawing thermal energy from the underlying warm ocean but weakens due to the drag of rough ocean surface. These processes change dramatically as the wind speed increases above 30 m/s. The air-sea momentum flux at hurricane winds is still poorly understood, which severely affects the capability of forecasting the path and intensity of tropical cyclones. Ocean surface waves undoubtedly play a central role, but sea spray plays an increasingly important role. This project addresses this challenging problem by analyzing field measurements (surface wave spectra, underwater ambient sound, wind, etc.) in three tropical cyclones and stress calculated using an ocean momentum budget. The analyses are guided by the state-of-the-art UWIN-CM (the University of Miami Unified Wave INterface Coupled Atmosphere-Wave-Ocean Model) simulations. Variations in wave stress and breaking rate as a function of wind speed and directional wave properties will be investigated by comparisons with the UWIN-CM simulations and by analyses of the modeled wave dissipation function. Knowledge acquired in this project will improve the understanding and forecasting of hurricanes and will directly benefit the public. In addition, poor understanding of hurricanes is partly due to the lack of reliable measuring techniques in the harsh hurricane condition. This project will develop the underwater Lagrangian and acoustic techniques, which are able to monitor the air-sea interface from the beneath. A user-interface designed movie will be created to display how scientists observe hurricanes using different techniques, such as satellites, aircrafts, and underwater instruments. The movie will be played to visiting children, in science talks, in local schools and on the Internet.By combining the unique field measurements and state-of-the-art UWIN-CM simulations, we will answer the following scientific questions: (1) How important are equilibrium-range waves in controlling the air-sea momentum flux in tropical cyclones? We hypothesize that for wind speeds higher than 30 m/s the stress on the ocean surface is larger than the equilibrium-range wave breaking stress. (2) How does the wave breaking rate vary with wind speed and the complex surface wave field? At moderate wind speeds the wave breaking rate increases with increasing speed. Does this continue at extreme high winds? (3) Can we detect acoustic signatures of sea spray at high winds? Measurements of sea spray in tropical cyclones are very rare. We will seek for the acoustic signatures of spray droplets impacting the ocean surface. (4) What are the processes controlling the air-sea momentum flux? We will use the coupled atmosphere-wave-ocean model to guide the interpretation of the field measurements, assessing its ability to predict and using internal diagnostics to assess the importance of various processes. Knowledge gained by answering these questions will improve the understanding and modeling of the air-sea momentum flux and improve the capability of forecasting tropical cyclones.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.