Velcro Measurement of Turbulence Kinetic Energy Dissipation Rate ε

Abstract

Turbulence in the ocean results from many different processes operating over a wide range of space scales and timescales, with spatial and temporal variability particularly extreme in coastal oceans. If the origins and effects of turbulent processes in the ocean are to be understood, it is essential to supplement the small number of accurate measurements of turbulent properties produced by current techniques. This paper documents a new acoustic large-eddy technique for estimating turbulent kinetic energy dissipation rate ε, using measurements of vertical velocity from a specialized acoustic Doppler current profiler. The method is calibrated against a subset of ε profiles taken with now-standard airfoil probe methods during a single cruise. Without further adjustment of the calibration constant, the large-eddy acoustic estimates successfully describe the remaining profiler data from the same cruise, as well as data obtained with a different microscale profiler on a subsequent cruise. Deciding when measured vertical velocity should be considered ?turbulent? is a general problem in stratified fluids, addressed here with an ad hoc criterion based on comparison of a large-eddy overturning timescale and the local Brunt?Väisälä period. This criterion is tuned by requiring that most large-eddy estimates made with data from a coastal inlet characterized by very low turbulence levels should be classified as nonturbulent. The resulting criterion then classifies an increasing percentage of large-eddy estimates as ?turbulent? as the microprofiler ground-truth data document increasing levels of turbulent dissipation. The new technique is used to demonstrate the complexity of the field of ε in energetic coastal tidal fronts, on spatial and temporal scales that cannot be resolved by standard microprofiler measurements.