UBC Faculty Research and Publications

Measuring the Dissipation Rate of Turbulent Kinetic Energy in Strongly Stratified, Low-Energy Environments : A Case Study From the Arctic Ocean Scheifele, Benjamin; Waterman, Stephanie; Merckelbach, Lucas; Carpenter, Jeffrey R.

Abstract

We compare estimates of the turbulent dissipation rate, πœ–, obtained independently from coincident measurements of shear and temperature microstructure in the southeastern Beaufort Sea, a strongly stratified, low-energy environment. The measurements were collected over 10 days in 2015 by an ocean glider equipped with microstructure instrumentation; they yield 28,575 shear-derived and 21,577 temperature-derived πœ– estimates. We find agreement within a factor of 2 from the two types of estimates when πœ– exceeds 3 Γ— 10βˆ’ΒΉΒΉ W/kg, a threshold we identify as the noise floor of the shear-derived estimates. However, the temperature-derived estimates suggest that the dissipation rate is lower than this threshold in 58% of our observations. Further, the noise floor of the shear measurements artificially skews the statistical distribution of πœ– below 10βˆ’ΒΉβ° W/kg, that is, in 70% of our observations. The shear measurements overestimate portions of the geometric mean vertical profile of πœ– by more than an order of magnitude and underestimate the overall variability of πœ– by at least 2 orders of magnitude. We further discuss uncertainties that arise in both temperature- and shear-derived πœ– estimates in strongly stratified, weakly turbulent conditions, and we demonstrate how turbulence spectra are systematically modified by stratification under these conditions. Using evidence from the temperature-gradient spectral shapes and from the observed πœ– distributions, we suggest that the temperature-derived dissipation rates are reliable to values as small as 2 Γ— 10βˆ’ΒΉΒ² W/kg, making them preferable for characterizing the turbulent dissipation rates in the weakly turbulent environment of this study. The data may be downloaded at doi:10.14288/1.0368671.

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