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
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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