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In this study we examined a data set of nearly two‐year collection and investigated the effects of low‐level jets (LLJ) on near‐surface turbulence, especially wind direction changes, in the nocturnal boundary layer. Typically, nocturnal boundary layer is thermally stratified and stable. When wind profiles exhibit low gradient (in the absence of LLJ), it is characterized by very weak turbulence and very large, abrupt, but intermittent wind direction changes (∆WD) in the layers near the surface. In contrast, presence of LLJs can cause dramatic changes through inducing wind velocity shears, enhancing vertical mixing, and weakening the thermal stratification underneath. Ultimately, bulk Richardson number (Rb) is reduced and weakly stable conditions prevail, leading to active turbulence, close coupling across the layers between the LLJ height and ground surface, relatively large vertical momentum and sensible heat fluxes, and suppressed ∆WD values. Rb can be a useful parameter in assessing turbulence strength and ∆WD as well. The dependence of ∆WD on Rb appears to be well defined under weakly stable conditions (0.0 < Rb ≤ 0.25) and ∆WD is generally confined to small values. However, the relationship between ΔWD and Rb breaks when Rb increases, especially Rb > 1.0 (very stable conditions), under which ΔWD varies across a very wide range and the potential for large ΔWD increases greatly. Our findings have provided important implications to the plume dispersion in the nocturnal boundary layers. Plain Language Summary: This paper investigated how low‐level jets affect the near‐surface turbulence, especially wind direction changes, in the nocturnal atmospheric boundary layers. Atmospheric stability parameter (e.g., Rb‐bulk Richardson number) can be the key in determining the wind direction changes in the nocturnal atmospheric boundary layer. Under weakly stable conditions (0.0 < Rb ≤ 0.25), in general, wind direction only changes within a small range (less than 10°) between two consecutive 10‐min intervals. In contrast, when Rb > 1.0 (very stable conditions), wind direction can vary across a very wide range and the probability for large wind direction changes also increases. Low‐level jets often enhance the turbulence near the surface, reduce the atmospheric stability and lead to small wind direction shifts. In absence of low‐level jets, nocturnal boundary layers are characterized by very weak turbulence and very large, abrupt, but intermittent wind direction changes (from a couple of 10° to 100°) in the layers near the surface. Our findings have provided important implications to the plume dispersion in the nocturnal boundary layers. Key Points: Bulk Richardson number (Rb) can be a useful parameter in assessing wind direction change (∆WD) near the surface in the nocturnal boundary layer∆WD is generally confined to small values under weakly stable conditions (0.0 < Rb ≤ 0.25) but varies across a very wide range with increasing potential for large ΔWD under very stable conditions (Rb > 1.0), which implies a complicated relationship between Rb and ΔWDPresence of the Low‐Level Jets (LLJ) enhances the turbulence activities, decreases the Rb values, leads to relatively large vertical momentum and sensible heat fluxes, and suppresses ∆WD values [ABSTRACT FROM AUTHOR]

Here, for non-breaking short surface waves, we have experimentally determined the value of the turbulent eddy viscosity ν T or its ratio ν T * ≡ ν T / ν , where ν is the water kinematic viscosity. The non-breaking wave-generated turbulent eddy viscosity ν T was found to depend on the ratio of the wave period, T, to the microscale Kolmogorov time scale, τ η . Our observations were consistent with ν T * = 1.46 · (T / τ η) − 2.6 when (T / τ η) < 0.9 . That implied that the ν T * ∝ ϵ − 1.3 , where ϵ is the background turbulent energy dissipation rate. The near-surface turbulent flow associated with non-breaking waves was characterized by a short inertial subrange. The background turbulence appears to modulate the amount of energy the non-breaking waves dissipate locally and, consequently, the wave's decay rate. Our results imply that the background turbulent flow acts as a lubricant, permitting waves to propagate further when traveling over a more energetic turbulent background flow. Our results have implications for the modeling of oceanic wave propagation or the air–sea exchange processes. [ABSTRACT FROM AUTHOR]

We describe a new field campaign over a steep, snowy 30 ∘ alpine slope, designed to investigate three recurrent issues in experimental studies of steep-slope katabatic winds. (1) Entrainment is known to be present in katabatic jets and has been estimated at the interface between the jet and the boundary layer above it. However, to our knowledge, the slope-normal velocity component has never been measured in the katabatic jet. (2) It is hard to accurately measure turbulence in the first tens of centimetres above the surface using standard sonic anemometry due to the filtering effect of the long instrument path. The present field experiment used a three-dimensional multi-hole pitot-type probe with a high sampling frequency (1250 Hz) that was positioned as close to the surface as 3 cm. It provides three-dimensional mean velocity and Reynolds stress tensor from which dissipation can be estimated, as well as spectra for the turbulent quantities. Energy spectra reveal a well-developed inertial range and capture the inertial scales and some of the dissipative scales. (3) Measuring turbulence on a mast usually provides information about mean and turbulent quantities at certain discrete heights because the sensors are sparsely located inside the jet. We present the first measurements of well-developed katabatic flows where the full wind-speed and temperature profiles acquired, from tethered balloon are available at the location of the measurement mast, which comprises three-dimensional anemometry and thermometry. [ABSTRACT FROM AUTHOR]

Here, for non-breaking short surface waves, we have experimentally determined the value of the turbulent eddy viscosity νT or its ratio νT*≡νT/ν, where ν is the water kinematic viscosity. The non-breaking wave-generated turbulent eddy viscosity νT was found to depend on the ratio of the wave period, T, to the microscale Kolmogorov time scale, τη. Our observations were consistent with νT*=1.46·(T/τη)−2.6 when (T/τη)<0.9. That implied that the νT*∝ϵ−1.3, where ϵ is the background turbulent energy dissipation rate. The near-surface turbulent flow associated with non-breaking waves was characterized by a short inertial subrange. The background turbulence appears to modulate the amount of energy the non-breaking waves dissipate locally and, consequently, the wave’s decay rate. Our results imply that the background turbulent flow acts as a lubricant, permitting waves to propagate further when traveling over a more energetic turbulent background flow. Our results have implications for the modeling of oceanic wave propagation or the air–sea exchange processes.

The Damariscotta River in midcoast Maine is a weakly stratified estuary characterized by several constrictions and channel bends that affect tidal asymmetry and material transport. Microstructure and current velocity measurements were collected at a cross‐channel transect in the northern reach of the estuary during spring and neap tidal conditions to study the effects of a channel bend immediately upstream of a constricted sill on intratidal dynamics. During the flood phase, a counterclockwise gyre is formed upstream of the headland, which enhances landward‐directed flow in the channel and is countered by seaward‐directed flow over the shallow shoal. Semidiurnal and quarter‐diurnal patterns of lateral advection and stress divergence emerge in the surface layer because of these secondary flows. Lateral advection effects dominate the dynamics in neap tide, and although they are stronger in spring tide, they are partially obscured by bottom friction forces that are proportional to the along‐channel velocity squared. A novel harmonic decomposition technique is introduced to determine the relative importance of advection and stress divergence on quarter‐diurnal velocity generation, and their implications to neap/spring variability in estuarine water quality are discussed. Plain Language Summary: The Damariscotta River in midcoast Maine is a weakly stratified estuary characterized by several constrictions and channel bends that affect tidal asymmetry and material transport. This article is an in‐depth study on how a headland near the Glidden Ledges constriction affects the flow dynamics of the northern estuary. Microstructure and current velocity measurements were collected at a cross‐channel transect in the northern reach of the estuary during spring and neap tidal conditions. During the flood phase, a counterclockwise gyre is formed upstream of the headland, which enhances landward‐directed flow in the channel and is countered by seaward‐directed flow over the shallow shoal. These flow dynamics affect tidal asymmetry through both lateral and vertical velocity gradients. They must also be considered alongside frictional forces between the estuary current and sediment along the bottom. This research helps to explain what makes the upper estuary so effective at retaining warm water and nutrients vital to the oyster aquaculture industry. Key Points: Constricted flow around a headland results in elevated horizontal and vertical velocity shear in the water column, especially at the surfaceGyres induce quarter‐diurnal patterns in velocityStress divergence, through bottom friction and other mechanisms, in addition to advection influence quarter‐diurnal overtides [ABSTRACT FROM AUTHOR]

An autonomous upwardly-moving microstructure profiler was used to collect measurements of the rate of dissipation of turbulent kinetic energy ( ε ) in the tropical Indian Ocean during a single diurnal cycle, from about 50 m depth to the sea surface. This dataset is one of only a few to resolve upper ocean ε over a diurnal cycle from below the active mixing layer up to the air–sea interface. Wind speed was weak with an average value of ~5 m s −1 and the wave field was swell-dominated. Within the wind and wave affected surface layer (WWSL), ε values were on the order of 10 −7 –10 −6 W kg −1 at a depth of 0.75 m and when averaged, were almost a factor of two above classical law of the wall theory, possibly indicative of an additional source of energy from the wave field. Below this depth, ε values were closer to wall layer scaling, suggesting that the work of the Reynolds stress on the wind-induced vertical shear was the major source of turbulence within this layer. No evidence of persistent elevated near-surface ε characteristic of wave-breaking conditions was found. Profiles collected during night-time displayed relatively constant ε values at depths between the WWSL and the base of the mixing layer, characteristic of mixing by convective overturning. Within the remnant layer, depth-averaged values of ε started decaying exponentially with an e-folding time of 47 min, about 30 min after the reversal of the total surface net heat flux from oceanic loss to gain. [ABSTRACT FROM AUTHOR]