Contribution of Sea Spray in Air-Sea Enthalpy and Momentum Exchange Coefficients in Rapidly Intensifying Tropical Cyclones

Abstract:

Hurricane Maria 2017 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; it intensified into a Category 5 storm and devastated the Bahamas. Prediction of rapid intensification remains a challenge in tropical meteorology. A likely explanation is that the physics of tropical cyclones, notably momentum and energy fluxes between the ocean and atmosphere and the role of the air-sea interface in this process, have not been properly understood and incorporated in forecast models. Computational and laboratory experiments suggest that there is an “aerodynamic drag well” around a wind speed of 60 m/s, which could explain the process of rapid storm intensification, though under the assumption of a constant enthalpy exchange coefficient. The enthalpy exchange coefficient is influenced by sea spray. To study the effect of spray on thermodynamics of tropical cyclones, we implemented a Volume of Fluid to Discrete Phase Model (VOF to DPM). This model remeshes the areas with increased curvature, which are susceptible to the interface instability. The generated water particles are converted into Lagrangian particles, which are involved in a two-way interaction with the airflow. Spray size distributions measured at the UM RSMAS SUrge STructure Atmosphere INteraction facility was used for the model verification. Due to dynamic re-meshing, VOF to DPM resolves spray particles ranging in size from tens of micrometers to a few millimetres (spume). Though sub-micrometer and micrometer scale spray particles are not resolved, they are less significant in the heat and momentum exchange at the air-sea interface than spume due to the spray feedback effect. These results are expected to contribute to a better treatment of spray in forecasting models, including cases of rapid intensification.