Computational and laboratory experiments on microphysics of the air-sea interface under hurricane conditions

Physics of the air-sea interface are among the factors determining intensity of tropical cyclones. We have conducted a series of laboratory experiments at Air-Sea Interaction Salt Water Tank (ASIST), which have been coordinated with numerical modeling using the Volume of Fluid Large Eddy Simulation (VOF LES) method. The microstructure of the air-water interface under hurricane force wind revealed formation of the “projectiles” and “sheets” of water fragmenting into large spray particles. This process resembled Kelvin-Helmholtz shear instability between fluids with large density difference. In addition, we conducted experiments on the effect of surfactants on the spray size distribution, which was traceable up to very high wind speeds. We extrapolate the results of numerical and lab experiments to tropical cyclone conditions. The two-phase environment developing near the air-sea interface suppresses short gravity-capillary waves and alters the aerodynamic properties of the sea surface. Implementation of the new parameterization of the drag coefficient in operational models (GFDL, HWRF) is expected to improve predictions of tropical cyclone intensity, storm surge, and the associated wave field.