Air‐sea exchange of heat in the presence of wind waves and spray

Abstract

Waves extract a considerable part of the surface stress. While breaking, they eject spray into the atmosphere. Spray evaporates and influences a balance of heat and moisture above the waves. A one‐dimensional model of the stratified marine surface boundary layer (MSBL) accounts directly for the impact of waves on the momentum flux and the impact of sea spray on fluxes of heat and moisture. The model is viewed as a higher order parameterization of the MSBL compared to the bulk parameterization. The model is based on the balance equations of momentum, the turbulent kinetic energy and the dissipation rate, heat, and moisture. A general experimental knowledge is used to parameterize the jet droplet concentration above the sea. That is, the surface droplet concentration is proportional to the cube of the friction velocity of the air, and the fast decay of droplet concentration with elevation above waves is parameterized by exponential decay. The exchange coefficients for heat, moisture, and momentum are computed from the wind speed and the sea state. Consistency of the dynamical part of the model is checked against measurements of the drag coefficient. Consistency of the thermodynamical part is checked against measurements of the sensible heat flux for light to moderate winds. The impact of spray is then assessed for stronger winds. It is shown that for a wind speed of about 25 m s−1 and above the impact of sea spray on heat and moisture fluxes becomes significant. The magnitude and the sign of the spray mediated heat and moisture fluxes depend on stratification of the atmosphere. To settle the issue whether or not sea spray plays an important role in exchange of heat and moisture above the sea, simultaneous direct measurements of sensible heat and moisture flux under different stratification conditions at wind speeds of about 25 ms−1 are needed.