Observations of turbulence and mean flow in the low‐energy hypolimnetic boundary layer of a large lake

Abstract

Near‐bed measurements are reported for both mean flow and turbulence structure in the deep hypolimnetic waters of Lake Michigan (55 m depth) during stratified and unstratified periods to determine validity and restrictions of the expected law‐of‐the‐wall (LOW) behavior. Near‐bed currents were weak (U50 = 3, 16 cm s−1 for mean, maximum currents respectively at 50 cm elevation), dominated by subinertial energy across all seasons, and showed little seasonal variation in spite of the strong seasonality to wind forcing. Velocity structure for wave‐free conditions showed strong log‐linear trends within 1 mab, with over 98% of the 2152 velocity profiles producing significant log‐linear fits within the bottom meter and a strictly logarithmic velocity profile extending to only 66 cmab on average (Cd 50 = 0.0052; zo = 0.0015 m). Stratification was dynamically unimportant to mean flow and turbulence, but fitted log‐linear length scales suggest that deviations from strictly logarithmic velocity structure may be explained by flow unsteadiness. Turbulent quantities measured within 1 m of the bed including dissipation, turbulent kinetic energy, and turbulent length scales followed LOW expectations in the mean, but individual estimates deviated by several orders of magnitude. The observed deviations from LOW turbulent structure were found to be correlated with the log‐linear length scales fit to mean velocity profiles and were consistent with the effects of flow unsteadiness.

Ice‐Free Radiative Convection Drives Spring Mixing in a Large Lake

“…This 2‐order enhancement agrees well with other convective measurements in lakes (ice‐covered: Bouffard et al, ; surface‐cooled: Jonas, Stips, et al, ), where measured dissipation rates have been observed to increase from 10 −10 to 10 −8 W/kg within the convective mixed layer. Some turbulence enhancement is seen near the bed in the current study, where fewer temperature inversions were observed; this is most likely associated with the compact bottom boundary layer turbulence as described in Cannon and Troy ().…”

“…As radiative heating increases water column temperatures throughout the day, patches of warm water grow progressively larger near the surface (Figure 5a). Some turbulence enhancement is seen near the bed in the current study, where fewer temperature inversions were observed; this is most likely associated with the compact bottom boundary layer turbulence as described in Cannon and Troy (2018). Enhanced dissipation values follow the deepening convective mixed layer throughout the day, with full water column enhancement observed by early afternoon.…”

“…A 15‐day (5–20 April 2018) mooring deployment and microstructure campaign was used to obtain high‐resolution measurements of mean flow, stratification, and turbulence in Lake Michigan at a 55‐m deep site located near Milwaukee, Wisconsin, USA (43 ° 04 ' 27 ″ N,87 ° 45 ' 14 ″ W; Figure ), a location for which we have previously reported both hydrodynamic (Cannon & Troy, ; Troy et al, ) and biogeochemical conditions (Mosley & Bootsma, ; Shen et al, ). An instrumented tripod equipped with two acoustic current profilers and a thermistor string was deployed throughout the experiment, while vertical profiles of velocity shear and temperature were collected during three separate profiling days (5, 12, and 20 April).…”

Benthic Flow and Mixing in a Shallow Shoal Grass (Halodule wrightii) Fringe

“…Normalized velocity scales demonstrated some Reynolds number dependence (not shown), with estimates that were several orders of magnitude larger (S70: 0.13 ± 0.07; S40: 0.17 ± 0.09; S0: 1.57 ± 1.18) at the lowest flow speeds (< 5 mm/s). Similar current speed dependencies have been observed for the drag coefficient and friction factor in weakly energetic boundary layers, where increases in the apparent drag are often attributed to viscous effects [43] and/or unsteadiness [44]. In the current study, a lack of high quality (i.e., wave-free) data at low flow speeds precluded robust analyses of Normalized velocity scales (u 2 s / U NS 2 ) were used to investigate the variability of measured turbulence characteristics with current velocities at each study site (Figure 6).…”

“…Although current speeds were not resolved throughout the canopy, velocity profiles at S70 showed evidence of a potential canopy shear layer near the bottom of the measurement volume, where velocities sharply decreased at an elevation consistent with the penetration length scale (h − δ e ≈ 1 cm above the bottom). Conversely, the vertical structure of velocity profiles at S40, where the canopy shear layer penetrated to the bed, more closely matched that observed at S0, and log-fit roughness heights (z o , e.g., [44]) at both sites (not shown; z o ≈ 2 mm) suggested that the near-bed structure of velocity profiles was set by sediment characteristics rather than canopy-induced shear.…”