A New Sensor Platform for Investigating Turbulence in Stratified Coastal Environments

Damien Bouffard Environmental Engineering Institute, École Polytechnique Fédérale de Lausanne, Lausanne, Switzerland

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Ulrich Lemmin Environmental Engineering Institute, École Polytechnique Fédérale de Lausanne, Lausanne, Switzerland

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Abstract

Characterizing and quantifying vertical exchange processes is essential for understanding physical and biological dynamics in stratified lakes and oceans. Unfortunately, the role of mixing is still poorly understood because of the challenges of conducting field research on small-scale turbulence, especially in the vicinity of a thermocline. This study presents a new moored sensor platform that was designed to investigate small-scale turbulence structures in time and space. The objective is to determine all terms of the turbulent energy equation separately and simultaneously. The platform is equipped with a microstructure package for measuring shear, temperature, and temperature gradients, as well as with a vertical array of high-precision thermistor probes and acoustic Doppler velocimeters. The platform can be moved vertically in the water column using a bottom-resting winch that is connected to a shore station by a 1800-m-long communication cable. This cable allows real-time data access and control of the winch, and thus optimization of the measurement strategy. A field study in Lake Geneva, located between Switzerland and France, shows that this system is ideally suited for the analysis of the dynamics of baroclinic motions, such as internal Kelvin waves. First results indicate a clear relationship between low Richardson number and elevated dissipation, and suggest a mean flux Richardson number Rf = 0.14 ± 0.4. However, this measurement campaign was not as conclusive for the reason of the variability of Rf.

Current affiliation: Queen's University, Kingston, Ontario, Canada.

Corresponding author address: Damien Bouffard, Dept. of Civil Engineering, Queen's University, 58 University Ave., Kingston ON K7L 3N6, Canada. E-mail: damien.bouffard@a3.epfl.ch

Abstract

Characterizing and quantifying vertical exchange processes is essential for understanding physical and biological dynamics in stratified lakes and oceans. Unfortunately, the role of mixing is still poorly understood because of the challenges of conducting field research on small-scale turbulence, especially in the vicinity of a thermocline. This study presents a new moored sensor platform that was designed to investigate small-scale turbulence structures in time and space. The objective is to determine all terms of the turbulent energy equation separately and simultaneously. The platform is equipped with a microstructure package for measuring shear, temperature, and temperature gradients, as well as with a vertical array of high-precision thermistor probes and acoustic Doppler velocimeters. The platform can be moved vertically in the water column using a bottom-resting winch that is connected to a shore station by a 1800-m-long communication cable. This cable allows real-time data access and control of the winch, and thus optimization of the measurement strategy. A field study in Lake Geneva, located between Switzerland and France, shows that this system is ideally suited for the analysis of the dynamics of baroclinic motions, such as internal Kelvin waves. First results indicate a clear relationship between low Richardson number and elevated dissipation, and suggest a mean flux Richardson number Rf = 0.14 ± 0.4. However, this measurement campaign was not as conclusive for the reason of the variability of Rf.

Current affiliation: Queen's University, Kingston, Ontario, Canada.

Corresponding author address: Damien Bouffard, Dept. of Civil Engineering, Queen's University, 58 University Ave., Kingston ON K7L 3N6, Canada. E-mail: damien.bouffard@a3.epfl.ch
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