Hieronymi, Martin (2011) Solar radiative transfer into the ocean: A study on underwater light fluctuations due to surface waves. (PhD/ Doctoral thesis), Christian-Albrechts-Universität, Kiel, Kiel, Germany, 103 pp.

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Abstract

The thesis is about the solar radiative transfer into the ocean. Particular emphasis is placed on underwater light fluctuations due to focusing surface waves. The study includes measurements at high sea and computational simulations of the light propa-gation in seawater and wave-induced radiative variability.
Surface waves of different sizes have an optical lensing effect; they cause focusing of light beams at various depths. The underwater propagation of light depends on the scattering and absorption properties of seawater. Strongest light fluctuations appear near the surface at water depths of 0.5 to 10 m. At 1 m depth, radiative enhancements with a factor of 40 compared to the mean light level can be achieved. These short-term extreme values refer to the downwelling irradiance. The reason for the most intense irradiance peaks are surface waves with lengths of 20 cm to 5 m. In theory, light flashes with a radiative intensification of the factor 1.5 can appear down to 80 m of water depth. The range of possible irradiance peaks is discussed with respect to all relevant ocean waves. Even 200 m long swell waves can originate small irradiance changes below the 90 m depth level.
In natural sea states waves of different sizes are superposed. Their respective lensing effect controls the subsurface light regime. The mechanisms of those interactions are analyzed. Local wind, which is primarily associated with ultra-gravity waves, strongly affects light fluctuations within the near-surface region down to 10 m depth. The most intense radiative peaks result from moderate wind conditions with velocities of 2 to 7 m s-1. Below 10 m depth, the temporal and spatial light variability is driven by super-posed fully developed gravity waves of the corresponding sea state. Comparable strong variations arise from 1.5 m high waves. Even in 100 m depth slight wave-induced light field variance was found.

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