[A-AS06] Advances in Tropical Cyclone Research: Past, Present, and Future

[AAS06-06] Modeling Contribution of Sea Spray in Air-Sea Enthalpy and Momentum Exchange Coefficients in Rapidly Intensifying Tropical Cyclones

★Invited Papers

*Alexander Soloviev¹, Breanna Vanderplow¹, Roger Lukas² (1.Nova Southeastern University, 2.University of Hawaii at Manoa)

Keywords:tropical cyclone, rapid intensification, sea spray, sea surface

Tropical storm intensity prediction remains a challenge in tropical meteorology. Some tropical storms undergo dramatic rapid intensification, such as Hurricane Maria 2017, which intensified to a Category 5 storm within 24 hours and destroyed Puerto Rico. The official forecast and all computer models were unable to predict its rapid intensification. Hurricane Dorian in 2019 was predicted as a tropical storm; unpredictably, it intensified into a Category 5 storm and devastated the Bahamas. The tropical cyclone forecast substantially depends on computer power, environmental information, and the physics incorporated in the model. Even though computer power increased significantly during the last 40 years, and the available environmental information is now much better, there has only been a little to no improvement in the tropical cyclone rapid intensification prediction. A likely explanation is that the physics of tropical cyclones, especially momentum and energy fluxes between the ocean and atmosphere and the role of the air-sea interface in this process, have not been properly incorporated in forecast models. The computational and laboratory experiments by Soloviev et al. (2017) suggest that there is an “aerodynamic drag well” around a 60 m/s wind speed, which could explain the process of rapid storm intensification under the assumption of a constant enthalpy exchange coefficient. The enthalpy exchange coefficient is influenced by sea spray. Questions regarding the appropriate method for the incorporation of sea spray in tropical cyclone models still exist (Peng and Richter 2019). Due to the spray feedback effect, sub-micrometer and micrometer scale sea spray particles, which have relatively large residence time, may contribute significantly contribute to the air-sea flux of enthalpy in tropical cyclones. The total mass of these small spray particles is also too small to influence the momentum exchange and drag coefficient. Larger sea spray particles (spume) are not subject to the substantial feedback effect due to a shorter residence time. In addition, spume has a much larger mass and can take a part of the airflow momentum to accelerate the spray particles to the speed of the airflow, which may noticeably increase the air-sea drag coefficient under major tropical cyclones. In order to study the effect of spume on the thermodynamics of tropical cyclones, we have implemented a Volume of Fluid to Discrete-Phase Model (VOF to DPM). This model re-meshes the areas with increased curvature, which are suspicious for the interface instability. The generated water particles that satisfy the condition of asphericity are converted into Lagrangian particles, which are involved in a two-way interaction with the airflow. Spray size distributions measured in air-sea interaction facilities are used for model verification. Due to dynamic remeshing, VOF to DPM resolves spray particles ranging in size from tens of micrometers to a few millimeters, which corresponds to spume. The VOF to DPM model also incorporates the effect of surfactants. The inclusion of surfactants increases the overall amount of spume particles under tropical cyclone conditions. Results of the numerical simulation show a dramatic increase of spume generation under major tropical cyclones. Though sub-micrometer and micrometer scale spray particles are not resolved in this simulation, they are less significant in the enthalpy and momentum exchange at the air-sea interface than spume. These results are expected to contribute to the parameterization and proper treatment of spray in forecasting models, including cases of rapid intensification.

References:

Peng, T. and D. Richter, 2019. Sea Spray and Its Feedback Effects: Assessing Bulk Algorithms of Air–Sea Heat Fluxes via Direct Numerical Simulations. Journal of Physical Oceanography 49, 1403-1421.

Soloviev, A. V., R. Lukas, M. A. Donelan, B. K. Haus, I. Ginis, 2017. Is the state of the air-sea interface a factor in rapid intensification and rapid decline of tropical cyclones? Journal of Geophysical Research: Oceans 122, 10174-10183.