Measuring Turbulent Dissipation with Acoustic Doppler Velocimeters in the Presence of Large, Intermittent, Infragravity Frequency Bores

Abstract

This manuscript presents several improvements to methods for despiking and measuring turbulent dissipation values with Acoustic Doppler Velocitmeters (ADVs). This includes an improved inertial sub-range fitting algorithm relevant for all experimental conditions as well as other modifications designed to address failures of existing methods in the presence of large infra-gravity (IG) frequency bores and other intermittent, nonlinear processes. We provide a modified despiking algorithm, wavenumber spectrum calculation algorithm, and inertial sub-range fitting algorithm that together produce reliable dissipation measurements in the presence of IG frequency bores, representing turbulence over a 30 minute interval. We use a semi-idealized model to show that our spectrum calculation approach works substantially better than existing wave correction equations that rely on Gaussian based velocity distributions. We also find that our inertial sub-range fitting algorithm provides more robust results than existing approaches that rely on identifying a single best fit and that this improvement is independent of environmental conditions. Finally, we perform a detailed error analysis to assist in future use of these algorithms and identify areas that need careful consideration. This error analysis uses error distribution widths to find, with 95% confidence, an average systematic uncertainty of ±15.2% and statistical uncertainty of ±7.8% for our final dissipation measurements. In addition, we find that small changes to ADV despiking approaches can lead to large uncertainties in turbulent dissipation and that further work is needed to ensure more reliable despiking algorithms.