Underway sensing of radiometric properties utilizing a novel sub-hull installation on R/V SONNE

Understanding light interaction with biogeochemical constituents in the marine environment has improved in the last few years due to small, fast, automate and affordable optical tools. Ocean color observations are dependent on available sunlight, wind speed influencing surface reflected glint, shading, sensor accuracy, sensitivity as well as spatial and spectral resolution influencing our abilities in sensing oceanic processes. In this study we present a new method useful in collecting radiometric information, namely upwelling radiance from a platform submerged in a novel hydrographic sub-hull of a research vessel. The information analyzed here was obtained during a recent field campaign in the Northwestern European shelf seas aboard the new German research vessel SONNE. A comparison of the measurements from this approach with a free falling hyperspectral profiler was conducted to determine the degree of uncertainty that results from ship shadow. We observed intensity deviations in a range of about 33% compared to profiling measurements which can be attributed to instrument shading and environmental perturbations. A form-fitting algorithm was adapted to receive corresponding depths with identical spectral shape indicating an equivalent light path of 22 m for the sub-hull installation. In-situ radiance reflectance was then calculated utilizing hyperspectral downward solar irradiance measurements. During an east to west transect in the North Sea we successfully applied the above method while cruising at a maximum speed up of 12 knots, resolving the mixing zone of CDOM (colored dissolved organic matter) dominant Baltic waters towards the open North Sea.