A study of the sffect of ocean spray on vertical momentum transport under high-wind conditions of tropical cyclone

It is important to develop an accurate theoretical model of ocean spray in order to be able to understand tropical cyclone (TC) dynamics and, in turn, help TC intensity prediction. The spray effect on the intensity and structure of a hurricane is two-fold - thermodynamic and mechanical. The thermodynamic effect can be briefly described as follows: the ocean spray injected into the air could significantly affect sensible and latent heat fluxes in the near-surface layer, therefore the TC dynamics. Here we concentrate on mechanical aspects of ocean spray effect on the air-sea interaction. In this study, we propose a mathematical model for the ocean spray effect on the vertical momentum transport under a high wind environment associated with hurricanes or severe storms. It is shown that the ocean spray can accelerate the wind due to reduction of turbulence intensity which is caused by the ocean spray droplets elevation in the gravitational field. Our model contains a positive feedback between the flow velocity and the spray concentration since the spray concentration is a function of the flow velocity itself. It is demonstrated that the flow acceleration is negligible for the wind velocities below a certain critical value due to the fact that the spray volume concentration is low for such velocities. Spray volume concentration rapidly increases once the flow velocity exceeds this critical value causing a noticeable flow acceleration. The magnitude of the flow acceleration rapidly increases until the spray concentration reaches its maximum value. The obtained theoretical/numerical has been compared with the experimental results available in the literature, as well as some existing theoretical and numerical investigations which predict a significant (up to an order magnitude) flow acceleration caused by the ocean spray presence. The very strong spray-induced flow lubrication predicted by these models has been reexamined in the present work and is determined to be unjustified.

The sensitivity of ocean spray lubrication effect to the theoretical model has been investigated. Specifically we found that the lubrication effect is very sensitive to the influence of spray stratification on the turbulence mixing length. We account for this effect by employing full prognostic equations for turbulent kinetic energy and dissipation rate with no mixing length assumption. An effort to develop a proper spray parameterization based on the theoretical consideration and the results of numerical experiments is undertaken. Consequently, we will incorporate the spray parameterization in the realistic Weather Research and Forecast (WRF) numerical model in order to improve the accuracy of TC intensity prediction.