6TROPICAL Effect of polydisperse spray on hurricane and severe storm dynamics

Understanding various factors that influence hurricane and severe storm dynamics is important for accurate forecasting and effective mitigation of the consequences of these powerful events. Sea spray is one such factor that affects the vertical heat and momentum fluxes in the hurricane boundary layer. It has been shown that the intensity of a hurricane depends on the balance between these fluxes, as the energy of a hurricane is gained through a heat flux from the ocean and dissipated through momentum transport back to the ocean. However, the influence of spray on hurricane dynamics remains a subject that is not yet fully understood or quantified. This lack of understanding primarily stems from difficulties associated with mathematical modeling and physical measurement of a turbulent marine atmospheric boundary layer (MABL) laden with sea spray under high-wind conditions.

The current study investigates the effect of polydisperse sea spray on heat and momentum fluxes in a turbulent MABL under high-wind conditions of a hurricane. Sea spray influences MABL through two distinct mechanisms: thermodynamic and mechanical. The thermodynamic effect involves moisture and heat introduced or removed by spray droplets leading to changes in temperature, humidity, and vertical fluxes of sensible and latent heat within MABL. The mechanical effect is due to momentum exchange between air and droplets. It results in two competing phenomena: the deceleration of airflow caused by spray inertia and its acceleration due to turbulence suppression by the spray attributed to the air-droplet slip and the so-called gravity lubrication.

In the past, we have extensively studied the effects of a monodisperse spray on vertical fluxes in a MABL. However, the influence of spray polydispersity has not been adequately investigated. To address this gap, we have developed a Eulerian multi-fluid model of a spray-laden MABL that consistently accounts for droplet size distribution. This model adopts a modern theory of turbulent multiphase flows treating droplets of different sizes, air, and water vapor as separate interacting turbulent phases. Each phase is characterized by its own velocity, temperature, and turbulent kinetic energy (TKE) distributions and is described by the set of coupled conservation laws. The conservation laws for polydisperse spray are derived from the Boltzmann-type master kinetic equation enabling consistent quantification of the air-droplet momentum, heat, and TKE exchange in a turbulent flow. These conservation laws are complemented with a multi-fluid E-epsilon turbulent closure. We have analyzed various droplet size distribution spectra and correlation laws relating spray production to wind speed. The study reveals that the effect of sea spray on airflow and heat transport characteristics is strongly influenced by the shape of the droplet-size spectra and spray concentration.

The authors acknowledge support by grants from the National Science Foundation, U.S.A. under Awards No. 1832089 and 2302221