Scanning Doppler LiDAR Measurements Probing Wind Turbulence and Sea Spray Aerosol Concentration in the Marine Atmospheric Boundary Layer

The interaction between sea spray aerosols (SSA) and wind velocity turbulence is a critical aspect of air-sea interactions, affecting energy and mass transfer between the ocean and the atmosphere, thus the kinetic energy available in the marine atmospheric boundary layer (MABL). Simultaneous and co-located measurements of wind velocity and SSA concentration are performed with a scanning Doppler LiDAR in a coastal region. The horizontal streamwise velocity is probed by pointing the LiDAR laser beam in the mean wind direction, while the SSA concentration is probed through the backscatter coefficient of the LiDAR signal. Focusing on stormy conditions with a large amount of sea spray, a significant negative correlation is observed between turbulent fluctuations of the streamwise velocity and the SSA concentration, which on average leads to a substantial reduction in streamwise momentum and shear for the area affected by SSA. Furthermore, the vertical profile of the SSA concentration seems to consist of two layers: one closer to the sea surface with a roughly exponential reduction with increasing height, and a layer aloft where the vertical variation of SSA concentration is consistent with classical laws predicting scalar concentration in a turbulent boundary layer, such as the power law or the Prandtl logarithmic law. The analysis of the various terms contributing to the Reynolds-averaged advection-diffusion equation, which governs the turbulent transport of passive scalars in a turbulent boundary layer, suggests that the top layer is dominated by the vertical scalar turbulent flux, which is well-modeled through the gradient-diffusion hypothesis. In contrast, the lower layer is characterized by a negligible vertical gradient of the vertical scalar turbulent flux; thus, diffusion promoted by the wave-induced velocity field seems to be the dominating physical process for SSA transport. An analytical model is proposed to predict the vertical profile of SSA concentration covering both layers.