Autonomous surface platforms equipped with pulse-coherent high-resolution (HR) ADCPs are a promising tool for measuring turbulence and estimating turbulent dissipation rates, ε(z), close to the air-sea interface. However, surface gravity waves generate significant bias in ε(z) if not sufficiently separated from the turbulent signal. In a previous study, the authors developed a method of isolating wave orbital velocities from the data using empirical orthogonal functions (EOFs). Low-mode EOFs had characteristics of surface gravity waves, while higher-mode EOFs had characteristics of turbulence. After filtering empirical wave profiles constructed from the low-mode EOFs from the data, resultant ε(z) were in close agreement with law-of-the-wall scaling during quiescent conditions. In this study, we further validate the EOF-filtering technique by comparing EOFs of the HR ADCP data with those computed from synthetic wave data which does not contain turbulence. As expected, low-mode EOFs of the synthetic data are in strong agreement with those of the real data, while high-mode EOFs reflect only noise due to the absence of turbulence. Wave profiles constructed from the low-mode EOFs are then used to quantify the potential for bias in ε(z) if wave velocities are not sufficiently filtered from the data.

The five-beam Nortek Signature series acoustic Doppler current profilers (ADCPs) include a vertical beam that can be operated as an echosounder. These echosounders record an echo intensity level as a function of range in hundredths of a decibel. While these recorded levels provide valuable qualitative information about scattering from the water column, without calibration the units’ recorded echo intensities cannot be linked quantitatively to scattering processes. In this report we summarize calibration results for six Nortek Signature1000 units. The echosounders were calibrated in the field while deployed on 4th generation Surface Wave Instrument Floats with Tracking (SWIFTs) by suspending 38.1-mm tungsten carbide spheres with 6% cobalt binder below. Here, we summarize the equations used to process Nortek Signature series echosounder data, general calibration procedures for echosounders, the methodology used to calibrate the six units, the results of the calibrations, and uncertainties and recommendations for future work. In addition, we present post-processed, calibrated echosounder data from a deployment of the SWIFTs equipped with the Signature1000s in Mobile Bay, Alabama.

High resolution profiles of vertical velocity obtained from two different surface-following autonomous platforms, Surface Wave Instrument Floats with Tracking (SWIFTs) and a Liquid Robotics SV3 Wave Glider, are used to compute dissipation rate profiles ε (z) between 0.5 and 5 m depth via the structure function method. The main contribution of this work is to update previous SWIFT methods (Thomson 2012) to account for bias due to surface gravity waves, which are ubiquitous in the near-surface region. We present a technique where the data are pre-filtered by removing profiles of wave orbital velocities obtained via empirical orthogonal function (EOF) analysis of the data prior to computing the structure function. Our analysis builds on previous work to remove wave bias in which analytic modifications are made to the structure function model (Scannell et al. 2017). However, we find the analytic approach less able to resolve the strong vertical gradients in ε (z) near the surface. The strength of the EOF filtering technique is that it does not require any assumptions about the structure of non-turbulent shear, and does not add any additional degrees of freedom in the least-squares fit to the model of the structure function. In comparison to the analytic method, ε (z) estimates obtained via empirical filtering have substantially reduced noise and clearer dependence on near-surface wind speed.