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Abstract

The water sided mass boundary layer at the air-sea interface is a sublayer of the viscous boundary layer and acts as the bottle neck for air-sea gas transport. The dominant transport mechanism of the mass boundary layer is molecular diffusion. For low to moderate wind speeds, the main mechanisms occurring in the mass boundary layer are the upwelling of horseshoe vortices, the development of along wind streaks, and the so called microscale breaking, a breaking of the wave crests without bubble entrainment. This thesis investigates the development of these mechanisms in dependence of fetch and wind speed at the Aeolotron wind wave facility, Heidelberg. The collocated and synchronized visualization techniques boundary layer imaging, active thermography, and wave slope imaging enable a unique set of data by providing insight into the turbulent processes taking place in the mass boundary layer and their interaction with wind waves. Three different fetch dependent regimes could be identified that each contribute differently to air-sea heat exchange: a laminar regime with no wind waves, an overshoot regime for dominant gravity wavelengths smaller than 13 cm, and a declining regime for larger wavelengths. Additionally, a further step has been taken in understanding the cause of the microscale breaking mechanism. The initial disturbance is an accumulation of the mass boundary layer thickness in the trough of the parasitic capillary waves with its spontaneous release initiating the microscale breaking.