Aerosol transport in idealized wind-wave systems

This report deals with aerosol transport in turbulent wind over water waves. For simple terrain onshore, there appears to be consensus that aerosol transport is well described by simple models. However, for complex terrain or geometry, such as urban environments, advanced computational fluid dynamics models are needed. Although the wind flow over water is reminiscent of the wind over flat terrain onshore, there are some important differences, such as altered heat fluxes and the presence of wind generated waves. In this report we explore three idealized flow regimes that may be encountered offshore. In the first one, the wind is not capable of creating waves. The result is a flat surface. The second regime is when the winds are sufficiently strong to generate water waves. These so-called wind waves are slow compared to the local wind, and their bulk effect is to provide aerodynamic drag. The third regime occurs when distantly generated waves propagate into regions of calmer winds. These so called swell waves have fast propagation speeds compared to the local wind. We compare aerosol transport for these three cases by means of computational fluid dynamics (CFD) on a laboratory scale. In order to capture the effect of deposition on the water surface, we solved the turbulent motion in the air as well as the wave propagation in the water, and a particle method is used to track a large number of aerosols of different sizes. In line with existing literature, we find that the wind sea behaves as a rough surface. For the dispersion of aerosols, this has two major consequences. Firstly, cross-stream transport is enhanced. This leads to a wider plume for all particle sizes studied. Secondly, and arguably most important, there is a downward transport mechanism present, which leads to deposition of larger aerosols at the surface. Consequently, for wind seas there is reduced air concentrations of larger sized aerosols compared to the flat surface case. The plume width and the plume arrival time of the swell regime and the flat surface regime are almost indiscernible. However, since swell waves feed momentum to the air, there is an upward transport mechanism, which is most effective for larger aerosols. The most important consequence of this is that the plume is transported downstream almost as an isolated blob, thus counteracting the near surface clustering present in the flat surface case. This leads to higher concentrations downstream of the source. For small aerosols, the plume shape and arrival time is fairly similar for the three regimes. This suggests that simple operational dispersion models, such as Gaussian models, may be used for transport of neutral gases over waves. However, the results point to some differences in the concentration distribution between the three cases. For larger aerosols, this report shows clear evidence of altered dynamics both for wind seas and swell seas compared to the flat boundary layer flow. These effects are currently not captured by simple dispersion models but may prove to be important. The current results are obtained at a laboratory scale, and further research is warranted to investigate the effects on relevant atmospheric scales.