A coupled LES and observational approach to improve measurements of ocean turbulence from gliders

Abstract

Microstructure-equipped autonomous ocean gliders are proving to be an adequate platform for measuring turbulence with the advantage of extended duration and fine resolution measurements. However, it is still unclear how the movement of the glider platform impacts the flow around the attached sensors. The incident along-glider water velocity is not known but essential for the calculation of dissipation rates as errors introduce a significant bias. Steady-state or dynamical flight modules that incorporate all significant forces are required to compute the set of parameters that subsequently can be used for calculating turbulent dissipation rates. In our investigations, we provide more evidence for this method with the help of computational fluid dynamics (CFD) simulations with the large eddy simulation (LES) approach of the flow around
the glider and its sensors so that the bias in the velocity flow measurements can be estimated.
Through velocity measurements, velocity shear and therefore turbulent dissipation rates can be determined which are calculated for various ambient dissipation rates eam ranging from 10􀀀11m2s􀀀3 to 10􀀀6m2s􀀀3, thus covering the known range of possible and measurable turbulent motion around the world. This enables our study to investigate
the current limits of turbulence measurement on the glider platform and advises caution for the application of glider-based turbulence measurements in low-energetic (i.e. Arctic Ocean) or featureless abyssal plains of the oceans. For high levels of turbulence with the ambient dissipation rate greater than 10􀀀9m2s􀀀3 our results of the flow simulation indicate only small effects of the self-induced strain rate with a ratio less than 1.5 comparing the measured dissipation rate emeas in the simulation with the ambient dissipation rate.
These results attest the glider platforms suitability for turbulence measurements in most areas around the world but also indicate an underestimation of measured flow speeds around the probe tips of the microstructure probe by up to 15%, which should be taken into account for the calculation of turbulent dissipation rates.

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Identifiers

ORCID for Alberto Naveira Garabato: orcid.org/0000-0001-6071-605X

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