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

Abstract

Abstract This manuscript presents several improvements to methods for despiking and measuring turbulent dissipation values with acoustic Doppler velocimeters (ADVs). This includes an improved inertial subrange fitting algorithm relevant for all experimental conditions as well as other modifications designed to address failures of existing methods in the presence of large infragravity (IG) frequency bores and other intermittent, nonlinear processes. We provide a modified despiking algorithm, wavenumber spectrum calculation algorithm, and inertial subrange fitting algorithm that together produce reliable dissipation measurements in the presence of IG frequency bores, representing turbulence over a 30 min 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 subrange 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. Significance Statement Turbulent mixing is a process where the random movement of water can lead to water with different properties irreversibly mixing. This process is important to understand in estuaries because the extent of mixing of freshwater and saltwater inside an estuary alters its overall circulation and thus affects ecosystem health and the distribution of pollution or larvae in an estuary, among other things. Existing approaches to measuring turbulent dissipation, an important parameter for evaluating turbulent mixing, make assumptions that fail in the presence of certain processes, such as long-period, breaking waves in shallow estuaries. We evaluate and improve data analysis techniques to account for such processes and accurately measure turbulent dissipation in shallow estuaries. Some of our improvements are also relevant to a broad array of coastal and oceanic conditions.