Test deployment of the “turbulence mooring” By Ilker Fer, Geophysical Institute, University of Bergen The ocean surface is a complex boundary where air-sea fluxes of mass, momentum and energy take place. The processes in this dynamic interface are of crucial importance for ocean circulation and ecosystems in general. The coupling between surface gravity waves, winds and currents in the adjacent turbulent boundary layers influence the weather and are important for the structural behavior and fatigue of off-shore wind turbines in particular. The dependence of sea-surface drag on environmental conditions (e.g., wind speed, wave age, wave height, wave slope, and wind-wave alignment) and the characteristics of upper ocean turbulence in windy conditions (e.g., intermittent wave breaking, airflow separations, generation of sprays and bubbles) still remain unresolved. There is huge gap in our knowledge about air-sea interaction. Direct measurements are needed in the upper ocean in order to improve the understanding of the surface boundary layer of the ocean and its representation in numerical models in the form of validated parametrizations. In NORCOWE work package 5, a near-surface turbulence measurement system is designed and constructed in collaboration with Rockland Scientific International, Canada. Ilker Fer and Helge Bryhni (Geophysical Institute, University of Bergen) participated in the cruise of F.F. Johan Hjort to conduct a test deployment of this novel turbulence measurement system. Successful deployment return 5 days of continuous time series of turbulence in early April, at about 12 m from the sea surface in Vestfjorden, off Lofoten. Turbulence mooring (Figure) is an ocean near-surface turbulence measurement system designed to collect time series at a fixed level. The buoy is the upper element of a bottom- anchored mooring line, allowed to align with the current. The system allows for measurements using two independent methods sampling different parts of the turbulence spectrum:
1. Eddy correlation measurements of turbulent momentum flux and heat flux sampled in
the energy containing and near-inertial subrange.
2. Dissipation rate measurements in the dissipation subrange.
Records from the accelerometers and the 6-D motion sensor allow for applying necessary corrections for the platform motion. The entire system is powered by two rechargeable Lithium-Ion battery packs with an estimated operating time of 500 h. With a 25% duty cycle, this instrument can sample 20 GB of data for about 85 days.
This system was purchased under NORCOWE and will provide data for parts of the PhD work of Mostafa Bakhoday Paskyabi at Geophysical Instutiute, University of Bergen. The data collected during NORCOWE will be analyzed to describe the link between forcing and turbulent mixing in the surface boundary layer, describe the effect of surface waves, wave-induced oscillatory currents, and platform motion.
(Right) Design of the turbulence mooring. A swivel will allow it to rotate freely, pointing the sensors toward the flow. (Down, left) Ilker has just installed the turbulence sensors and is putting the caps on. (Down, right) Helge, together with two of the crew members, is recovering the turbulence mooring from a light boat.
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