The role of wind waves and sea spray in air sea interaction

In this thesis, a number of processes occurring near the air-sea interface are modelled in order to investigate the role of waves and spray (water droplets ejected into the air as a consequence of breaking waves) in the exchange of momentum and heat between the atmosphere and the sea. Knowledge on this subject is particularly relevant for weather and climate models, in which this exchange forms an important boundary condition.

In the first part, numerical simulations of the turbulent air flow over waves are presented. In these simulations, the effect of viscosity just above the water surface is not neglected, contrary to what is usually done. Calculations show that this leads to a considerably faster growth of short waves, which is in agreement with existing measurements. Subsequently, an analytical model is developed, which gives an equally adequate description of the flow as advanced numerical models. As such, it is a useful tool for applied studies.

In the second part, the influence of spray on the heat balance over the sea is considered. A simplified model shows that during storms evaporating spray can cause a significant increase in the moisture flux and decrease in the heat flux, respectively. An exact quantification is difficult, since the amount of spray generated under given conditions is poorly known due to lack of measurements. When evaporating spray is taken into account in a weather prediction model, this can locally have a strong influence on the structure of the boundary layer, but large-scale effects are relatively small. Therefore, it is concluded that, for the time being, spray does not have to be considered in weather prediction.