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Although scaling laws and coherent structures have been well-studied in non-neutral boundary layer flows over a long period of time, a potential connection between them is still worth exploring. This work explores the structure inclination angle under non-neutral stratification conditions by way of a connection between turbulent scaling and Monin–Obukhov similarity theory. A model for theoretical structure inclination angles is built by extending the refined "turbulent eddy" model of H.-J. Huang [Phys. Rev. Fluids 4, 114702 (2019)] under stratified stability conditions. The variations in the inclination angle values with the stability parameter from the current model are found to be in good agreement with results from the literature, laboratory results, and atmospheric surface layer observations. In addition, the results show that increasing positive/negative buoyancy leads to increasing/decreasing inclination angles and steeper/moderate angles at higher wall-normal heights. This variation is parameterized and can be used to improve existing near-wall models. [ABSTRACT FROM AUTHOR]

Three sites with different surface roughness were selected to explore the turbulent/synoptic separation and self-similar wall-attached coherent structures in the atmospheric surface layer. At each site, the facility permits synchronous multi-point measurements of three-dimensional wind velocity and temperature at different heights, as well as synchronous measurements via the global positioning system among the three sites. A filter based on the linear coherent spectrum between two sites (separated by 500 m) is adopted to separate turbulent and synoptic signals. After the separation, the two-point correlations of the filtered turbulent streamwise velocity component reveal that increasing surface roughness leads to less coherence in both the wall-normal and streamwise directions. The present results with unstable stratification and different surface roughness also demonstrate good agreement with the self-similar range of the wall-attached turbulence reported in Baars et al. (J Fluid Mech 823:R2, 2017). The aspect ratio of coherent structures (defined as the ratio of streamwise wavelength to the wall-normal offset) for the streamwise and spanwise velocity components and temperature increases with increasing surface roughness. [ABSTRACT FROM AUTHOR]

A field experiment was carried out on a dry flat bed of Qingtu Lake in Minqin, China to measure the velocity and temperature in the atmospheric surface layer at high Reynolds number. The facility, the Qingtu Lake Observation Array, permits synchronous multi-point measurements of three-dimensional wind velocity and temperature at different wall-normal and spanwise positions. Data were selected to analyze the characteristics of the large-scale coherent structures under different stratification stability conditions. The results confirm the previous findings that the positive buoyancy can be considered as a wall-normal force to have a "lifting" effect increasing the size of large-scale structures and further illustrate that negative buoyancy in the stable condition has an opposite effect leading to small structures with less wall-normal coherence. The stability dependence of structure inclination angles for all three velocity components is built by a parametric model indicating a log-linear trend under near-neutral and unstable conditions. In addition, the unique measurement array at this facility also permits us to study the three-dimensional form of these features and the associated temperature fluctuations. By computing the space average for momentum and heat flux, we found that the threshold stability parameter z / L = − 0.1 , where z = 2.5 m and L is the Obukhov length scale, can act as a role to determine the dominant flow by shear-driven and buoyancy-driven, respectively. Last, the streamwise length scales for temperature component are investigated where a smaller length scale corresponds to a stronger buoyancy condition, but with larger values at higher heights. [ABSTRACT FROM AUTHOR]

Accurate knowledge of the contacts between surface roughness and the resultant wind speed are important for climatic models, wind power meteorology, agriculture and erosion hazards especially on sand saltation in arid and semi‐arid environments, where vegetation cover is scarce. In this study, synchronous measurements of three‐dimensional wind speed below 5 m are carried out in three different surface roughness conditions in Minqin, China, and the difference in the turbulence statistics and the structure of the very large‐scale motions (VLSMs) were revealed. The results show that the slope of the mean wind profile (MVP), the turbulent kinetic energy (TKE) and Reynolds stress increase with the surface roughness. The roughness seems to suppress the ejection events and the surface roughness will not only weaken the energy of the VLSMs, but also reduce the scale values of VLSMs near the wall. These influences may cause some changes regarding the dust transportation in streamwise and vertical directions during the sand and dust storm (SDS). That is, the decrease of the mean velocity near the ground will reduce the dust transportation in the streamwise direction and influence of the roughness on the ejection and sweep events will change the dust transportation in the vertical direction. Furthermore, the increase of roughness will weaken the scale and energy of VLSMs, which will lead to the decrease of the capacity of dust transportation. © 2019 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The quadrant and octant analysis methods are used to investigate the characteristic of the atmospheric surface layer (ASL) turbulent flow based on the field observation data obtained from the Qingtu Lake Observation Array (QLOA). The results indicate that a stronger thermal convection intensity can lead to a larger intensity ratio of Reynolds stress from each quadrant to the total Reynolds stress. In the near-neutral ASL, the time ratios of Reynolds stress from each quadrant against the total Reynolds stress at different heights and friction velocities are approximately constant, i.e., D Q 1 ≈0.19, D Q 2 ≈0.296, D Q 3 ≈0.2 and D Q 4 ≈0.314. And the turbulent kinetic energy (TKE) ratios of each quadrant against the total TKE (streamwise or vertical) for the ejection events and sweep events are pretty similar at different wall-normal heights. The flow characteristics tend to be isotropic with the increasing of the vertical distance. Moreover, a threshold value is also applied to separate the most important events from the less significant ones. Last, the contribution of Octant 2 and Octant 8 is dominant to the Reynolds stress, and the intensity ratios of Octant 2 and Octant 8 to the total Reynolds stress will be increased with the thermal convection intensity. The abundant dynamics behaviors revealed by the quadrant and octant analysis could benefit us a wider understanding of the ASL coherent structures. • The time ratio of Reynolds stress at different heights are approximately constant. • The flow characteristics tend to be isotropic with the increase of the vertical distance. • TKE ratio for the ejection events and sweep events are near similar at different heights. • The intensity ratios of Octant 2 and Octant 8 increase with the thermal convection intensity. [ABSTRACT FROM AUTHOR]