Impact of sea spray on the surface boundary layer

Properly taking into account the effects of sea-spray on the overall air-sea momentum and enthalpy fluxes in hurricane prediction models is still a subject of research. Numerical experiments using theoretical and weather prediction models indicate that the ratio of transfer coefficient of heat to transfer coefficient of momentum must not be too far away from unit in order to simulate hurricanes of realistic intensity. On the other side, within the range in which reliable direct flux measurements exist, the observed value of this ratio over the sea is 0.70 at low winds speeds, and decreases significantly below 0.70 with increasing wind speed. One possibility for the discrepancy between numerical results and observations is the failure of the traditional bulk transfer coefficients to take into account the feedback effect of sea-spray. Winds of tropical cyclones generate large re-entrant sea-spray droplets which tend to increase the sea-air enthalpy transfer and, thus, have positive feedback to the intensification of hurricanes. On the other hand, sea-spray is generated at the expense of the momentum, which as a result causes the turbulent momentum flux to decrease.

This presentation highlights findings from a study in which the impact of sea-spray on the heat and momentum fluxes under equilibrium conditions associated with winds of tropical cyclones is investigated using a 1-D coupled sea-spray and surface boundary layer model. This spray model is capable of simulating the microphysical aspects of evaporation of saline water droplets of various sizes and their dynamic and thermal interaction with the turbulence mixing that is simulated by the Mellor-Yamada 1.5-order closure scheme. There is full coupling among the spray-droplet microphysics, turbulence mixing, and droplet transport. Sea-spray droplet generation is described by a state-of-the-art parameterization which predicts the size spectrum of sea-spray produced by the ocean in terms of wind speed, surface stress, and wave properties. Results from a series of simulations reveal salient characteristics of the way in which evaporating droplets of various sizes modify the turbulence mixing near the surface, which in turn affects further droplet evaporation. Based on these results, a physical framework is proposed to account for the effects of sea spray in the parameterizations schemes of surface momentum and heat fluxes that are commonly used in operational hurricane prediction models.