Vertical Momentum Transport in a Spray-stratified Marine Atmospheric Boundary Layer Under High-wind Conditions of a Hurricane

One of the factors affecting tropical cyclone dynamics is the exchange of momentum, heat and moister between air and sea surface. Recently it has been shown that ocean spray plays a very important role in this air-sea interaction. In this work we primarily focus on the role of spray in air-sea momentum exchange under high-wind conditions of a tropical cyclone. An accurate description of air-sea momentum exchange is essential for accurate forecasting of hurricane track and intensity, therefore for making correct risk assessments.

In this study we develop several mathematical models for the spray-stratified atmospheric boundary layer. The first model is based on a turbulent kinetic energy (TKE) equation, the second model is based on the Monin-Obukhov similarity (MOS) theory and uses available experimental data and the third model is based on higher-order E-epsilon turbulence closure. The E-epsilon turbulence closure uses full prognostic equations for turbulent kinetic energy and dissipation rate with no mixing length assumption, and accounts for turbulent energy transport.

Our mathematical model accounts for several mechanisms through which the ocean spray affecting the turbulent airflow: it suppresses turbulent mixing due to stratification, injects momentum into the flow and redistributes horizontal flow momentum, and introduces inertia effect into the system. The spray stratification and momentum injection accelerate the airflow, while the spray-induced flow momentum redistribution reduces the wind speed. It is shown that full consideration of all these factors is important since none of them significantly larger than the others for all possible conditions. It is demonstrated that spray acceleration effect dominates over a wide range of airflow speeds causing the drag coefficient reduction. The results of calculations agree very well with available experimental data. An effort to develop a proper spray parameterization based on this theoretical consideration and the results of numerical experiments is in progress.