Ocean glider flight in the presence of surface waves

Lucas M Merckelbach,Jeffrey R Carpenter

doi:10.1175/jtech-d-20-0206.1

Lucas M Merckelbach, Jeffrey R Carpenter

https://doi.org/10.1175/jtech-d-20-0206.1

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Abstract

AbstractAutonomous, buoyancy-driven ocean gliders are increasingly used as a platform for the measurement of turbulence microstructure. In the processing of such measurements, there is a sensitive (quartic) dependence of the turbulence dissipation rate, ϵ, on the speed of flow past the sensors, or alternatively, the speed of the glider through the ocean water column. The mechanics of glider flight is therefore examined by extending previous flight models to account for the effects of ocean surface waves. It is found that due to the relatively small buoyancy changes used to drive gliders, the surface wave-induced motion, superimposed onto the steady-state motion, follows to a good approximation the motion of the wave orbitals. Errors expected in measuring ϵ at the ocean near-surface due to wave-induced relative velocities are generally less than 10%. However, pressure perturbations associated with the wave motion can be significant when using the glider-measured pressure signal to infer the glider vertical velocity. This effect of surface waves is only present in the shallow water regime, and can also affect glider depth measurements. It arises from an incomplete cancellation of the wave-induced pressure perturbation with the hydrostatic component due to vertical glider displacements, whereas for deep-water waves this cancellation is complete.

Highlights

Autonomous, buoyancy-propelled underwater gliders have become an important component of modern ocean observation systems (Testor et al 2019)
We quantified the effects of a surface wave field on the flight characteristics of ocean gliders
Two primary effects were identified: (i) the orbital motion of the surface waves results in a flow past the glider that deviates from the prediction of standard flight models, and (ii) glider vertical positions, and vertical velocity estimates, can be altered by wave-induced pressure and depth perturbations to the measured pressure signal

Summary

Introduction

Autonomous, buoyancy-propelled underwater gliders have become an important component of modern ocean observation systems (Testor et al 2019). We use linear wave theory to model the interaction between surface waves and a moving submersed body (i.e., the glider), and quantify the extent to which the presence of surface waves leads to additional errors in the estimation of the dissipation rate of turbulent kinetic energy when using dynamic glider flight models. This allows for an understanding, and recommendations, for using gliders to study ocean turbulence at the near-surface (as in, e.g., diurnal warm and rain layers), as well as in shallow coastal seas

Relative glider velocities induced by surface waves

Findings

Surface wave effects in the estimation of vertical velocity

Conclusions