Theoretical modeling of the effect of ocean spray on vertical momentum transport under high-wind conditions

In-depth understanding and accurate modeling of ocean spray under high-wind conditions is essential for improving intensity forecast of hurricanes or severe storms. The spray influences the intensity and structure of a hurricane both mechanically and thermodynamically. Mechanical mixing of spray with the near-surface airflow has a strong effect on the momentum exchange dynamics between the atmosphere and the ocean mixing layer. Current research primarily focused on the pure mechanical influence of sea spray on the level of turbulence and wind speed in the near-surface atmospheric layer, assuming that a mechanical (mixing) effect of the spray on the averaged thermodynamic characteristics of the atmospheric surface layer near the sea surface is much stronger than the thermodynamic influence of the spray on the airflow.

In this study, two mathematical models are proposed to detail the influence of ocean spray on vertical momentum transport under high-wind conditions of a hurricane or severe storm. The first model is based on a turbulent kinetic energy (TKE) equation and accounts for the so-called lubrication effect due to the reduction of turbulence intensity. The second model is based on the Monin-Obukhov similarity (MOS) theory and uses available experimental data. It is demonstrated that the flow acceleration is negligible for wind speeds below a certain critical value due to the fact that the spray volume concentration is low for such velocities. For values larger than the critical speed, the spray concentration rapidly increases, which results in significant flow acceleration. Both models produce qualitatively similar results for all turbulent flow parameters considered in the paper. It was found that the MOS-based model tends to predict noticeably stronger lubrication effect than the TKE-based model, especially for slower wind speeds. The results of model calculations are in very good agreement with available experimental data for the spray production values near the upper bound. It is also shown that neither the value of turbulent Schmidt number in the TKE-based model nor the choice of a profile stability function in MOS-based model affect the spray-laden flow dynamics significantly.

This work is supported by a grant from the National Oceanic and Atmospheric Administration, Educational Partnership Program under the cooperative agreement NA06OAR4810187 and the National Science Foundation under Award No. HRD-1036563.