Modeling and Analysis of the Ocean Spray Effects on Hurricane Dynamics

The progress in forecasting of tropical cyclone intensity remains limited due to the lack of understanding of the mechanical and thermodynamic effects of ocean spray on hurricanes. Current approaches that are used to study the ocean the spray influence on hurricane dynamics employ semi-empirical relationships between variables and parameters, which are often based on simplistic extrapolations of data obtained from low-wind-speed observations or, in some cases, from laboratory experiments. Such semi-empirical approaches have a serious methodological deficiency—they lack rigorous physical footing.

In this work, we have followed a different approach. Starting from fundamental principles of the theory of turbulent multiphase flows, we have developed a two-temperature non-equilibrium variable density model to describe dynamics and structure of a turbulent marine atmospheric boundary layer laden with evaporating spray under high-wind conditions of a hurricane. The model is formulated as a boundary value problem for a system of conservation equations of mass, momentum and thermal and turbulent kinetic energies for both gas and liquid phases. The model consistently describes a two-way coupling between mechanical and thermodynamic influences of the ocean spray. The description of the turbulent airflow laden with evaporating spray uses an Eulerian multi-fluid-type approach that considers spray as a continuous medium interpenetrating and interacting with the gas phase.

It has been found that evaporating spray significantly redistributes the total heat flux between latent and sensible heat fluxes even when the spray concentration is relatively low. In addition to redistributing the total heat flux, the ocean spray alters its value—the effect is especially strong for large values of spray concentration. We have found that the governing equations admit semi-self-similar solutions. In particular, the vertical distributions of latent and sensible heat fluxes can be scaled to a reference flux distributions under stretching transformations for a wide range of flow parameters (e.g. friction velocity, sea surface temperature, conditions at the upper edge of a turbulent boundary layer). Such a self-similar behavior enables fast calculation of the vertical heat fluxes modified by spray, which is very important for the development of a computationally efficient spray parameterization used in numerical weather prediction models.