Comparison of Reynolds Stress Estimates Derived from Standard and Fast-Ping ADCPs

Abstract

A field experiment was conducted to directly compare the effects of different sampling modes on Reynolds stress estimates calculated from acoustic Doppler current profilers (ADCPs). Two 1.2-MHz ADCPs were deployed concurrently over a fortnightly cycle: one collected single-ping measurements using mode 1 and a second ADCP employed the fast-ping rate mode 12 with subping-averaged data recorded at the same sample rate as the first ADCP. While mode 12 clearly has a lower noise floor for the estimate of mean velocities, it has been an open question whether the averaging of subpings leads to a biased estimate of turbulence quantities, due to the temporal averaging inherent in this approach. Using the variance method, Reynolds stresses were estimated from the two ADCP datasets and compared with stresses computed directly from the velocity records obtained with a pair of fast sampling acoustic Doppler velocimeters (ADVs) collocated with the ADCPs. Mode-12 stresses were more accurate than mode 1 in comparison to ADV-derived stresses, and mode 12 exhibited much lower measurement uncertainty than mode 1. Mode 1 appears to overestimate stresses by 20% in this study. The lower noise floor associated with mode 12 suggests that the variance method may be used with mode 12 to resolve smaller stresses than would be possible with mode 1.