Intermittency in Estuarine Turbulence: A Framework toward Limiting Bias in Microstructure Measurements

Abstract

Intermittent turbulence behavior has a number of implications for field sampling, namely, that if undersampled, it can result in over- or underestimates of turbulent kinetic energy (TKE) dissipation rates. Sampling thresholds and common distributions have previously been defined for oceanic environments, but estuaries remain relatively underrepresented. Utilizing vertical microstructure profilers is a robust way to directly measure TKE dissipation rates; however, microstructure sensors are delicate and conducting a limited number of profiles in a burst is desirable. In this work, a statistical framework is used to characterize intermittency in a partially mixed estuary. In particular, a multiple comparison test is proposed to evaluate the number of profiles required to sufficiently represent TKE dissipation averages. The technique is tested on a microstructure dataset from the Damariscotta River in Maine, which covers seasonal and fortnightly time scales. The Damariscotta River features a variety of bathymetric and channel complexities, which provide the opportunity to examine intermittency as it relates to different processes. Small-scale intermittency is prominent during stratified conditions in shallow locations as well as near channel-shoal morphology, channel bends, and constrictions.

Offshore Spreading of a Supercritical Plume Under Upwelling Wind Forcing: A Case Study of the Winyah Bay Outflow

“…In general, VMP and MicroCTD profiles showed a good agreement. While the ε magnitude at specific depths varied between consecutive profiles due to high intermittency of turbulence in a highly stratified coastal environment (e.g., Huguenard et al, 2019), the shape of profiles remained remarkably consistent for each station. Since uprising profiles of MicroCTD provided better nearsurface resolution (within the plume), only those are shown in Figure 9B.…”

Observations of Near-Surface Mixing Behind a Headland

“…Nonetheless, this work does indicate that near-surface mixing should be expected downstream of a headland near channel-shoal bathymetry in a macrotidal, low inflow system, typical of many Maine estuaries. For example, former research has identified the tendency for enhanced turbulence to occur near headlands [53,54] and near-surface mixing has been observed in other estuaries with eddies near channel-shoal bathymetry [22,30,55]. Those referenced studies observed near-surface mixing in headland-induced eddies were in the Damariscotta River, Maine, another macrotidal, low-inflow estuary.…”

“…The profiler was deployed downward and achieved a terminal velocity of 0.85 m/s after 1.5 m. The instrument was deployed in bursts of 4 casts at each station and averaged in order to limit bias from small scale intermittency. Averaging together four casts has been shown to be sufficient in similar regions with complex morphology [30]. Vertical casts did not reach the bottom, to avoid collision with the sea-bed and damage to the probes.…”

Synoptic observations of sediment transport and exchange mechanisms in the turbid Ems estuary: the EDoM campaign

“…Data were collected in a series of vertical casts, with approximately 5 casts performed every 15 minutes. The 5 casts were averaged and bootstrapped 6,000 times to provide a statistically significant measurement of TKE with depth (Efron & Gong, 1983;Huguenard et al, 2019;Ross et al 2019 and references therein). Turbulence and flocculation properties were also measured at SB_KNO in August 2018 using an MSS-90-S Microstructure Profiler sampling at 1024 Hz.…”