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1. Signals of Nonturbulent Motions Caused by Stable Stratification in Near-Surface Sonic Anemometer Data

   https://doi.org/10.1175/JAS-D-24-0070.1  

   Pan, Ying; Wray, Samuel C; Patton, Edward G (June 2025, Journal of the Atmospheric Sciences)

   Bertello, Peter (Ed.)

   Abstract Understanding the interactions between turbulent and nonturbulent motions has been a persistent challenge faced by the community studying stably stratified turbulent flows. For flows with high Reynolds number, high Rossby number, and stable stratifications, nonturbulent motions involve physical mechanisms acting against instability development. Because turbulent motions are generated through an energy cascade via instability development, the presence of nonturbulent motions is expected to modify the energy distribution across scales compared to that of solely turbulent motions. The objective of this work is to identify in field data statistical signals of nonturbulent motions caused by stable stratification. The need to resolve energy-containing motions in both space and time requires high-frequency time series of velocity fluctuations collected using arrays of sonic anemometers. The analysis is performed using data from the Canopy Horizontal Array Turbulence Study (CHATS), during which a total of 31 sonic anemometers were deployed on a horizontal array and on a 30-m tower. Compared to other field campaigns which were also equipped with arrays of sonic anemometers, CHATS took an important advantage of already published nighttime canopy-scale waves derived from aerosol backscatter lidar images. After precluding complexities caused by nonstationarity and horizontal heterogeneity, signals of nonturbulent motions caused by stable stratification are identified from spatial autocorrelations of time-block-averaged velocity fluctuations. These signals are interpreted using existing understanding of turbulent canopy flows and two-dimensional Kelvin–Helmholtz instability development. The associated estimates of critical wavelengths and buoyancy periods agree well with the overall properties of nighttime canopy-scale waves derived from lidar images. Significance StatementThis work investigates statistical signals of nonturbulent motions caused by stable stratification in sonic anemometer measurements of near-surface atmospheric flows. The detected signals of nonturbulent motions agree with theoretical predictions of the impacts of stable stratification on turbulent canopy flows. This agreement suggests potential advantages for understanding stably stratified near-surface flows using canopy-resolving simulations. The automatic, objective, statistical detection procedures, as well as the intermediate products of the periods of statistically stationary, horizontally homogeneous, approximately two-dimensional mean flows, are useful for improving the understanding of canopy flows for various stability conditions. 

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2. Wind-Tunnel Experiments of Turbulent Wind Fields over a Two-dimensional (2D) Steep Hill: Effects of the Stable Boundary Layer

   https://doi.org/10.1007/s10546-023-00820-2  

   Zhang, Wei; Markfort, Corey D.; Porté-Agel, Fernando (September 2023, Boundary-Layer Meteorology)

   Flow separation caused by steep topography remains a significant obstacle in accurately predicting turbulent boundary-layer flows over complex terrain, despite the utilization of sophisticated numerical models. The addition of atmospheric thermal stability, in conjunction with steep topography, further complicates the determination of disrupted turbulent wind patterns. The turbulent separated flows over a two-dimensional (2D) steep hill under thermal stratification has not been extensively addressed in previous experimental studies. Such measurements are crucial for enhancing our comprehension of flow physics and validating numerical models. We measured the turbulent wind flows over a 2D steep hill immersed in a stable boundary layer (of the bulk Richardson Number = 0.256) in a thermally-stratified boundary-layer wind tunnel. The flow separation, re-circulation zone and flow reattachment were characterized by the planar particle image velocimetry technique. Vertical profiles of mean air temperature and its fluctuations are also quantified at representative locations above the 2D steep hill and in the near wake region. Results indicate that the separated shear layer, initiated near the crest of the 2D steep hill, dominates the physical process leading to high turbulence levels and the turbulent kinetic energy production in the wake region for both stable and neutral thermal stability. Although the stable boundary layer does not dramatically change the turbulent flow pattern around the hill, the mean separation bubble is elongated by 13%, and its vertical extent is decreased by approximately 20%. Furthermore, the reduced turbulence intensities and turbulent kinetic energy of the near wake flow are attributed to the relatively low turbulence intensity and low momentum of the stable boundary layer due to buoyancy damping, compared to the neutral boundary layer. Additionally, a distinct low-temperature region—a cold pool—is extended beyond the separation bubble, reflecting the significant sheltering effect of the 2D steep hill on the downwind flow and temperature field. 

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3. Homogeneous turbulence in a random-jet-stirred tank

   https://doi.org/10.1007/s00348-023-03721-9  

   Bang, Joo Young; Pujara, Nimish (December 2023, Experiments in Fluids)

   We report an investigation into random-jet-stirred homogeneous turbulence generated in a vertical octagonal prism-shaped tank where there are jet arrays on four of the eight vertical faces. We show that the turbulence is homogeneous at all scales in the central region of the tank that span multiple integral scales in all directions. The jet forcing from four sides in the horizontal direction guarantees isotropy in horizontal planes but leads to more energy in the horizontal fluctuations com- pared with the vertical fluctuations. This anisotropy between the horizontal and vertical fluctuations decreases at smaller scales, so that the inertial and dissipation range statistics show isotropic behavior. Using four jet arrays allows us to achieve higher turbulence intensity and Reynolds number with a shorter jet merging distance compared to previous facilities with two-facing arrays. By changing the array-to-array distance, the parameters of the algorithm that drives random-jet stirring, and attachments to the exits of each jet, we show that we are able to vary the turbulence scales and Reynolds number. We provide scaling relations for the turbulent fluctuation velocity, integral scale, and dissipation rate, and we show how these scales of motion are primarily determined by the properties of individual jets and the diffusion of their momentum with distance from the nozzles. Finally, we examine the signatures of individual jets in the turbulent velocity spectra and report the conditions under which individual jet flows, not fully mixed with the background turbulence, produce a spectral peak and the corresponding frequency associated with the jet forcing timescale. 

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4. Investigating Fire–Atmosphere Interaction in a Forest Canopy Using Wavelets

   https://doi.org/10.1007/s10546-024-00862-0  

   Desai, Ajinkya; Guilloteau, Clément; Heilman, Warren E; Charney, Joseph J; Skowronski, Nicholas S; Clark, Kenneth L; Gallagher, Michael R; Foufoula-Georgiou, Efi; Banerjee, Tirtha (May 2024, Boundary-Layer Meteorology)

   Abstract Wildland fire–atmosphere interaction generates complex turbulence patterns, organized across multiple scales, which inform fire-spread behaviour, firebrand transport, and smoke dispersion. Here, we utilize wavelet-based techniques to explore the characteristic temporal scales associated with coherent patterns in the measured temperature and the turbulent fluxes during a prescribed wind-driven (heading) surface fire beneath a forest canopy. We use temperature and velocity measurements from tower-mounted sonic anemometers at multiple heights. Patterns in the wavelet-based energy density of the measured temperature plotted on a time–frequency plane indicate the presence of fire-modulated ramp–cliff structures in the low-to-mid-frequency band (0.01–0.33 Hz), with mean ramp durations approximately 20% shorter and ramp slopes that are an order of magnitude higher compared to no-fire conditions. We then investigate heat- and momentum-flux events near the canopy top through a cross-wavelet coherence analysis. Briefly before the fire-front arrives at the tower base, momentum-flux events are relatively suppressed and turbulent fluxes are chiefly thermally-driven near the canopy top, owing to the tilting of the flame in the direction of the wind. Fire-induced heat-flux events comprising warm updrafts and cool downdrafts are coherent down to periods of a second, whereas ambient heat-flux events operate mainly at higher periods (above 17 s). Later, when the strongest temperature fluctuations are recorded near the surface, fire-induced heat-flux events occur intermittently at shorter scales and cool sweeps start being seen for periods ranging from 8 to 35 s near the canopy top, suggesting a diminishing influence of the flame and increasing background atmospheric variability thereat. The improved understanding of the characteristic time scales associated with fire-induced turbulence features, as the fire-front evolves, will help develop more reliable fire behaviour and scalar transport models. 

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5. Impacts of Terrain Slope and Surface Roughness Variations on Turbulence Generation in the Nighttime Stable Boundary Layer

   https://doi.org/10.1029/2024JD041815  

   Sun, Jielun; Bhimireddy, Sudheer_R; Kristovich, David_A_R; Wang, Junming; Hiscox, April_L; Mahrt, Larry; Petty, Grant_W (March 2025, Journal of Geophysical Research: Atmospheres)

   Abstract Terrain slopes with and without upslope large surface roughness impact downstream shear‐generated turbulence differently in the nighttime stable boundary layer (SBL). These differences can be identified through variations in the relationship between turbulence and wind speed at a given height, known as the HOckey STick (HOST) transition, as compared to the HOST relationship over flat terrain. The transport of cold surface air from elevated uniform terrain reduces downstream air temperature not much air stratification. As terrain slope rises, the increasing cold and heavy air enhances downstream hydrostatic imbalance, resulting in increasing turbulence for a given wind speed. That is, the rate of turbulence increase with wind speed from downslope flow is independent of terrain slope. Upslope large surface roughness elements enhance vertical turbulent mixing, elevating cold surface air from the terrain. Horizontal transport of this elevated, cold, turbulent air layer reduces the downstream upper warm air temperature. Benefiting from the progressive reduction of downstream stable stratification with increasing height in the SBL, wind shear can effectively generate strong turbulence. In addition to the turbulence enhancement from the cold downslope flow, the rate of turbulence increase with wind speed is elevated. This study demonstrates key physical mechanisms for turbulence generation captured by the HOST relationship. It also highlights the influence of terrain features on these mechanisms through deviations from the HOST relationship over flat terrain. 

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