Presenter: Alexander Soloviev (Nova Southeastern University)

Description:
The air-sea interface is still a major unknown in tropical cyclone dynamics. In addition to wave breaking, direct disruption of the air-sea interface due to the Kelvin-Helmholtz instability takes place under tropical cyclone conditions, which results in two processes– suppression of short gravity-capillary waves and generation of sea spray. The former process results in the reduction of the air-sea drag coefficient, while the latter process increases the drag coefficient in major tropical cyclones. Unifying these two processes produces a local minimum in the dependence of the drag coefficient on wind speed around 60 m/s (an aerodynamic drag well), which may explain rapid intensification of tropical cyclones (Soloviev et al. 2017). Richter’s et al. (2021) reanalysis of high wind drag coefficient inferred from dropsonde profiles in hurricanes has also revealed an aerodynamic drag well around 60 m/s. In this work, we study the dynamics and thermodynamics of sea spray in tropical cyclones with a multiphase Volume of Fluid to Discrete Phase Method (VOF to DPM) and ANSYS Fluent evaporation-condensation module. The method has been partially verified at the UM RSMAS Surge Structure Atmosphere Interaction facility. Numeric simulations with VOF to DPM showed a dramatic increase of spray generation with increasing intensity of tropical cyclones from category 1 to 5. A substantial part of the small particles suspends in the turbulent airflow and evaporates, thus contributing less to the total air-sea enthalpy flux to the tropical cyclone. This resembles the effect of negative feedback on the enthalpy flux. The main contribution to the momentum and enthalpy fluxes is from the reentrant spray (large particles – spume). The next step is unifying the sea spray and interfacial enthalpy exchange coefficients with the goal of advancing tropical cyclone intensity forecasting and climate predictions.