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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]

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]