Near-neutral Atmospheric Surface Layer Research Articles

Fluctuating characteristics of streamwise wind speed and total saltation mass flux in the near-neutral atmospheric surface layer

Eolian sand transport in the atmospheric surface layer (ASL) is a typical kind of gas–solid two-phase flow at a very high Reynolds number that fluctuates over a wide range of spatial and temporal scales. Based on the high-frequency time series of streamwise wind speed (u) and total saltation mass flux (q) measure at three different observation sites, the fluctuating characteristics of u and q in the near-neutral ASL have been analyzed. Our study suggests that the probability density function (PDF) of the normalized streamwise wind speed fluctuations at the height of approximately 0.5 m follows a standard normal distribution, and the PDF of the q presents a lognormal distribution in the near-neutral ASL during strong and steady wind-blown sand. The investigations on the premultiplied spectral of q and u show that, as the energy of very large-scale motions (VLSMs) increases, the influence of the VLSMs on q becomes more significant. In addition, the stochastic couplings between u and q have been analyzed by using space–time correlations and linear coherence spectrum. Our results suggest that the VLSMs have a significant effect on the low-frequency fluctuations of q in the near-neutral ASL.

Identification of the energy contributions associated with wall-attached eddies and very-large-scale motions in the near-neutral atmospheric surface layer through wind LiDAR measurements

Recent works on wall-bounded flows have corroborated the coexistence of wall-attached eddies, whose statistical features are predicted through Townsend's attached-eddy hypothesis (AEH), and very-large-scale motions (VLSMs). Furthermore, it has been shown that the presence of wall-attached eddies within the logarithmic layer is linked to the appearance of an inverse-power-law region in the streamwise velocity energy spectra, upon significant separation between outer and viscous scales. In this work, a near-neutral atmospheric surface layer is probed with wind light detection and ranging to investigate the contributions to the streamwise velocity energy associated with wall-attached eddies and VLSMs for a very-high-Reynolds-number boundary layer. Energy and linear coherence spectra (LCS) of the streamwise velocity are interrogated to identify the spectral boundaries associated with eddies of different typologies. Inspired by the AEH, an analytical model for the LCS associated with wall-attached eddies is formulated. The experimental results show that the identification of the wall-attached-eddy energy contribution through the analysis of the energy spectra leads to an underestimate of the associated spectral range, maximum height attained and turbulence intensity. This feature is due to the overlap of the energy associated with VLSMs obscuring the inverse-power-law region. The LCS analysis estimates wall-attached eddies with a streamwise/wall-normal ratio of about 14.3 attaining a height of about 30 % of the outer scale of turbulence.

Characteristics of sand transport fluctuation in near-neutral atmospheric surface layer

Characteristics of sand transport fluctuation in near-neutral atmospheric surface layer

PIV measurement of buffer and logarithmic layers with detached eddies which mimics the neutral atmospheric surface layer

Accurate comprehension of turbulence characteristics in the atmospheric surface layer (ASL) under near neutral conditions, which is a lower part of the atmospheric boundary layer and a very high-Re number flow, is critically required in view of the increasing and broadening use of numerical weather prediction models. The models need to estimate turbulence fluxes of momentum, heat and moisture in the ASL as boundary conditions. On the other hand, observations (Högström 1990, Drobinski et al. 2007) have revealed that the fluxes under near-neutral conditions are often inconsistent with Monin-Obukhof theory, which has been widely used in models. The observations were conducted over flat surfaces with homogeneous roughness, and thus the violation from the theory might not be due to the underlying surface conditions. Thus, aiming to investigate an origin of the violation from the theory, we have carried out a wind tunnel experiment on the logarithmic layer along a smooth flat wall with a larger-scale disturbance, which mimics the near-neutral atmospheric surface layer (Hattori et al. 2010). In the present study, we especially examine a PIV measurement with a long-distance microscope lens to discuss the interaction of turbulences structures between buffer and logarithmic layers, which must give a clue on Reynolds number effects

An investigation for influence of intense thermal convection events on wall turbulence in the near-neutral atmospheric surface layer

Based on the field observation data in the near-neutral atmospheric surface layer (ASL) at the Qingtu Lake Observation Array, a new experimental data processing of the second-order statistic distribution of the high Reynolds number wall turbulence was presented which considered the influence of the intense thermal convection events (ITCEs). Following the conventional data selection in the literature, i.e., |z/L|, it is known that the variation of the large- and/or the very-large-scale motions (LSMs and VLSMs) cannot be effectively performed only by this method, which motivates us to find other factors influencing these turbulent motions, e.g., the ITCEs. From the data analysis of the probability density distribution of vertical heat flux, it is found that although its mean value tends to zero, its variance is large rather than zero, which suggests to us some ITCEs exist in the natural motions, although it has less frequent occurrences. In order to characterize the effect of such ITCEs, an additional parameter ψ for scaling the ratio of the buoyancy force to the viscous force is proposed in the data selection progress. The results show that the greater the |ψ|, the greater the impact of the ITCEs on ASL wall turbulence. Furthermore, our investigation reveals that the ITCEs may be one of the reasons why the VLSMs exhibit the Top-Down mechanism.

Turbulence length scales in a low-roughness near-neutral atmospheric surface layer

This paper investigated the integral length scales of turbulence in a low-roughness atmospheric surface layer (ASL), characterised by very smooth terrain in the Utah desert during near-neutral conditions, and evaluated the Engineering Sciences Data Unit (ESDU) 85020 and 86010 predictions for the turbulence length scales in a low-roughness ASL. The correlation integral method was used to estimate the integral length scales of the velocity components with longitudinal, lateral and vertical separations from sonic measurements on a vertical tower and spanwise array in the Surface Layer Turbulence and Environmental Science Test (SLTEST) field experiment. It was found that the longitudinal integral length scales calculated using near-neutral SLTEST data followed a logarithmic relationship with height proportional to the mean velocity profile with approximately constant integral time scale, however the sizes of the longitudinal components of the energy-containing eddies in the low-roughness flat terrain were 2–3 times smaller than those previously measured during field experiments in open country terrains. The calculated length scales with longitudinal separations over the very smooth terrain characteristics of the salt flats at Dugway were not consistent with those predicted by ESDU 85020. In contrast, the scaling of the lateral and vertical components of the three-dimensional turbulence structure with respect to the longitudinal component in the low-roughness ASL were consistent with similarity theory predictions in ESDU 86010 that the scaling ratios are independent of terrain roughness. Furthermore, this confirms the large dependence of the longitudinal turbulence length scales on the upstream terrain roughness and highlights the large variation of turbulence length scales observed at different low-roughness sites in the literature.

Spatial Variation of Statistical and Spectral Properties of the Stream Wise and Wall-Normal Velocity Fluctuations in the Near-Neutral Atmospheric Surface Layer

Based on high-quality near-neutral atmospheric-surface-layer (ASL) data obtained from an observational site located in flat desert and existing experimental results for a friction Reynolds number Reτ 106, the change of the second-order statistics of the streamwise velocity component with height is consistent with the experimental results for Reτ < 106, while the second-order statistics of the wall-normal velocity component increase linearly with height. In combination with the experimental results for Reτ < 106, the variation of the slope of streamwise turbulence intensity with height and the variation of the slope and intercept of the wall-normal turbulence intensity with height are quantified to reveal that both the streamwise and wall-normal velocity variances follow a generalized logarithmic law, with the distribution of the streamwise and wall-normal velocity components satisfying sub- and super-Gaussian distributions. Spectral analysis shows that the variation of the wavenumber corresponding to the pre-multiplied streamwise and wall-normal spectra peaks with the ratio z/δ obeys the form $$ k_{x} \delta = a(\delta /z)^{b} $$ , where z is the height, kx is the streamwise wavenumber, and δ is the ASL thickness. For the pre-multiplied streamwise spectra, $$ b = 0.5 \pm 0.1 $$ and the value of a may have a Reynolds-number dependence, while for the pre-multiplied wall-normal spectra, $$ a = 2.2 \pm 0.09 $$ and $$ b = 1.0 \pm 0.1 $$ . The von Karman constant, the Townsend–Perry constant and other parameters also display a weak Reynolds-number dependence.

Representation of the Reynolds stress tensor through quadrant analysis for a near-neutral atmospheric surface layer flow

To disseminate the role of the eddy motions on the anisotropic states of the Reynolds stress tensor ($$\varvec{B}$$), we devise a novel methodology based on the quadrant analysis, where the distributions of the invariants of $$\varvec{B}$$ are studied on a $$u^{\prime }$$–$$w^{\prime }$$ quadrant plane, with $$u^{\prime }$$ and $$w^{\prime }$$ being the turbulent fluctuations in the streamwise and vertical velocities. We apply this methodology to a near-neutral atmospheric surface layer (ASL) flow, derived from a field experiment dataset having multi-level turbulence measurements. The results show that in a near-neutral ASL flow, the anisotropic states of $$\varvec{B}$$ are determined by the distribution of the streamwise and cross-stream velocity variances ($$\sigma _{u}^2$$ and $$\sigma _{v}^2$$) on the $$u^{\prime }$$–$$w^{\prime }$$ quadrant plane. By studying the contour maps of the invariants of $$\varvec{B}$$ on the $$u^{\prime }$$–$$w^{\prime }$$ quadrant plane, we discover three distinct zones with elliptical boundaries in the $$u^{\prime }$$–$$w^{\prime }$$ plane, across which the anisotropic states of the eddy motions evolve. We find that the eddy motions which occur We also notice that the distinction between these three zones in the $$u^{\prime }$$–$$w^{\prime }$$ plane is prominent at the lowest measurement level, but becomes progressively indistinguishable as the measurement height increases.

Applicability of Taylor’s Hypothesis for Estimating the Mean Streamwise Length Scale of Large-Scale Structures in the Near-Neutral Atmospheric Surface Layer

A field investigation of the mean streamwise length scale Lx of large-scale structures and the convection velocity is performed in a high-Reynolds-number (Reτ ~ 106) atmospheric surface layer (ASL). Based on selected high-quality synchronous data obtained at different streamwise positions, the length scale Lx and global convection velocity Uc are extracted in the logarithmic region of the near-neutral ASL at heights of 0.9 m, 1.71 m, 3.5 m, and 5 m. It is found that Uc values are approximately 16% greater than the local mean streamwise velocity component U, and the value of Lx obtained from spatially-separated measurements is greater than the results estimated from Taylor’s hypothesis using the value of U at the four heights. The mean relative difference between the value of Lx and the results estimated by Taylor’s hypothesis using the value of U is approximately 15%. However, the relative difference between the value of Lx and the results estimated from Taylor’s hypothesis using the convection velocity Uc instead of the mean streamwise velocity component U is reduced to 1 ± 6% (≈ zero). Thus, the convection velocity Uc is more appropriate than the mean streamwise velocity component U in obtaining Lx values in the near-neutral ASL.

An application of quadrant and octant analysis to the atmospheric surface layer

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., DQ1≈0.19, DQ2≈0.296, DQ3≈0.2 and DQ4≈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.

Determining Longitudinal Integral Turbulence Scales in the Near-Neutral Atmospheric Surface Layer

We briefly assess approaches used to date for the estimation of the longitudinal integral turbulence scale $$L_u^x$$ in the near-neutral atmospheric surface layer, and propose an approach based on recent theory and measurements. A closed-form expression is derived according to which $$L_u^x$$ is proportional to the height z above the surface. The factor of proportionality depends upon two non-dimensional parameters: the measured lowest Monin frequency $$f_s$$ for which the non-dimensional spectrum conforms to Kolmogorovs two-thirds law, and the ratio $$\beta = \overline{u^2}/{u_{*}^2}$$ , where $$\overline{u^2}$$ and $$u_{*}$$ denote the mean square value of the longitudinal velocity fluctuations and the friction velocity, respectively.

An evaluation of the dissimilarity in heat and momentum transport through quadrant analysis for an unstable atmospheric surface layer flow

To elucidate the role of each fluid motion in the transport of momentum and heat fluxes in an unstable atmospheric surface layer (ASL) flow from single point measurements on a micrometeorological tower, we develop a novel method based on quadrant analysis where the contour maps of the turbulent statistics (fluxes, temperature variances and triple order moments between vertical velocity and temperature) are plotted on the quadrant planes between streamwise (u)–vertical (w) velocities, and vertical velocity (w)–temperature (T). We find that the dissimilarities in the heat and momentum transport with atmospheric stability are closely linked to the non-Gaussian nature of the joint probability density function (JPDF) between w and T. To highlight the changes in the fluid motions which cause this dissimilarity, we plot the contour maps of the third order moments between w and T on the u − w quadrant plane, and also of the streamwise momentum flux conditioned on every quadrant of u − w plane onto the T − w plane, referred to as octant analysis. The results indicate that in a highly-convective ASL, the cold downdrafts interspersed with strong ejections of hot fluid, carry a significant amount of both down-gradient and counter-gradient momentum flux, thus making the momentum transport inefficient. However, in a near-neutral ASL, the heat and momentum both are carried by the ejection and sweep quadrants of u − w quadrant plane, which indicates the temperature fluctuations are highly correlated with the high-speed and low-speed streaks commonly found in pure shear flows in the laboratory experiments.

Scaling and Similarity of the Anisotropic Coherent Eddies in Near-Surface Atmospheric Turbulence

Abstract The low-wavenumber regime of the spectrum of turbulence commensurate with Townsend’s “attached” eddies is investigated here for the near-neutral atmospheric surface layer (ASL) and the roughness sublayer (RSL) above vegetation canopies. The central thesis corroborates the significance of the imbalance between local production and dissipation of turbulence kinetic energy (TKE) and canopy shear in challenging the classical distance-from-the-wall scaling of canonical turbulent boundary layers. Using five experimental datasets (two vegetation canopy RSL flows, two ASL flows, and one open-channel experiment), this paper explores (i) the existence of a low-wavenumber k−1 scaling law in the (wind) velocity spectra or, equivalently, a logarithmic scaling ln(r) in the velocity structure functions; (ii) phenomenological aspects of these anisotropic scales as a departure from homogeneous and isotropic scales; and (iii) the collapse of experimental data when plotted with different similarity coordinates. The results show that the extent of the k−1 and/or ln(r) scaling for the longitudinal velocity is shorter in the RSL above canopies than in the ASL because of smaller scale separation in the former. Conversely, these scaling laws are absent in the vertical velocity spectra except at large distances from the wall. The analysis reveals that the statistics of the velocity differences Δu and Δw approach a Gaussian-like behavior at large scales and that these eddies are responsible for momentum/energy production corroborated by large positive (negative) excursions in Δu accompanied by negative (positive) ones in Δw. A length scale based on TKE dissipation collapses the velocity structure functions at different heights better than the inertial length scale.

Transition region where the large-scale and very large scale motions coexist in atmospheric surface layer: wind tunnel investigation

The turbulent structures in atmospheric surface layer (ASL) are investigated in wind tunnel with hot-wire anemometers in present study. The results show that there exist two typical turbulent structures characterised by their streamwise length scales, i.e. large-scale motions (LSMs) and very large scale motions (VLSMs) as reported recently in pipe flow, channel flow, zero-pressure-gradient turbulent boundary layers and near-neutral ASLs. Moreover, a transition region containing both LSMs and VLSMs is first reported in present investigation, and this region depends on the Reynolds numbers. In the transition region, as the wall-normal distance increases, the turbulent energy ratio of LSMs is gradually weakened but VSLMs strengthened.

Coherent structures and flux contribution over an inhomogeneously irrigated cotton field

The turbulence data measured at two levels (i.e., 8.7 and 2.7 m) in the Energy Balance Experiment (EBEX), which was conducted in San Joaquin Valley in California during the period from July 20 to August 24, 2000, are used to study the characteristics of coherent structures over an irrigated cotton field. Patch-to-patch irrigation in the field generated the dry-to-wet horizontal advection and the oasis effects, leading to the development of a stably internal boundary layer (SIBL) in the late mornings or the early afternoons. The SIBL persisted in the rest of the afternoons. Under this circumstance, a near-neutral atmospheric surface layer (ASL) developed during the period with a stratification transition from the unstable to stable conditions during the daytime. Therefore, EBEX provides us with unique datasets to investigate the features of coherent structures that were generated over the patches upstream and passed by our site in the unstable ASL, the near-neutral ASL, and the SIBL. We use an objective detection technique and the conditional average method that is developed based on the wavelet analysis. Our data reveal some consistencies and inconsistencies in the characteristics of coherent structures as compared with previous studies. Ramp-like structures and sweep–ejection cycles under the daytime SIBL have similar patterns to those under the nocturnal stable ASL. However, some features (i.e., intermittence) are different from those under the nocturnal stable ASL. Under the three stratifications, thermal and mechanical factors in the ASL perform differently in affecting the ramp intensity for different quantities (i.e., velocity components, temperature, and specific humidity), leading to coherent structures that modulate turbulence flow and alter turbulent transfer differently. It is also found that coherent structures contribute about 10–20% to the total fluxes in our case with different flux contributions under three stratifications and with higher transporting efficiency in sensible heat flux than latent heat and momentum fluxes.

Wind-Tunnel Experiment on Logarithmic-Layer Turbulence under the Influence of Overlying Detached Eddies

A wind-tunnel experiment was carried out to test a hypothesis that the turbulence characteristics in the near-neutral surface layer are largely determined by detached eddies from above. The surrogate detached eddies were generated by using an active turbulence grid installed at the front of the test section and the parameters of the grid were chosen such that the fully developed logarithmic layer downstream consists of a turbulent flow that has similar normalized intensity to that typically observed in the near-neutral atmospheric surface layer. The effects of the detached eddies on turbulence characteristics were investigated by comparison with a second experiment without detached eddies. The influence of the detached eddies on the logarithmic layer was mostly on the coherent structures; the logarithmic layer with the detached eddies revealed a multi-layer structure similar to that found in the atmosphere where the lower part of the surface layer is dominated by sweep-like events and the upper part by ejection-like events. Our experiments show that the mean velocity gradient and the Reynolds shear stress were, however, not affected significantly by the detached eddies and hence the eddy viscosity.

Scaling the characteristic time of the bursting process in the turbulent boundary layer

Characteristic time scales associated with bursting events in the turbulent boundary layer were examined over a very large range of Reynolds numbers based on momentum thickness of 1010, 2870, 4850, and 5 × 10 6 . Well-resolved hot-wire measurements were obtained at the lowest three Reynolds numbers in a low-speed wind tunnel with a long development length and compared to hot-wire and sonic anemometer measurements in the near-neutral atmospheric surface layer over the salt flats of Utah’s western desert. Bursting events were detected using the modified U-level threshold-crossing algorithm outlined by Luchik and Tiederman (1987) [1]. The same procedure and codes were used to analyze all time series records from both the wind tunnel and field experiments. The time between events, T e , and the event duration, Δ T , were calculated and normalized using four different types of scalings: inner, outer, mixed, and Taylor microscales. It was found that both Reynolds number and wall-normal trends in the mode of T e were eliminated when scaled by the Taylor microscale. Furthermore, constant (Reynolds number independent) values of the nondimensional mean T e and Δ T were found in a narrow wall-normal region near the top of the buffer layer when the data were normalized by the Taylor microscale.

Reynolds Number Invariance of the Structure Inclination Angle in Wall Turbulence

Cross correlations of the fluctuating wall-shear stress and the streamwise velocity in the logarithmic region of turbulent boundary layers are reported over 3 orders of magnitude change in Reynolds number. These results are obtained using hot-film and hot-wire anemometry in a wind tunnel facility, and sonic anemometers and a purpose-built wall-shear stress sensor in the near-neutral atmospheric surface layer on the salt flats of Utah's western desert. The direct measurement of fluctuating wall-shear stress in the atmospheric surface layer has not been available before. Structure inclination angles are inferred from the cross correlation results and are found to be invariant over the large range of Reynolds number. The findings justify the prior use of low Reynolds number experiments for obtaining structure angles for near-wall models in the large-eddy simulation of atmospheric surface layer flows.

The near-neutral atmospheric surface layer: turbulence and non-stationarity

The neutrally stable atmospheric surface layer is used as a physical model of a very high Reynolds number, canonical turbulent boundary layer. Challenges and limitations with this model are addressed in detail, including the inherent thermal stratification, surface roughness and non-stationarity of the atmosphere. Concurrent hot-wire and sonic anemometry data acquired in Utah's western desert provide insight to Reynolds number trends in the axial velocity statistics and spectra.

The Structure of the Near-Neutral Atmospheric Surface Layer

Abstract Recent observational data (turbulence variables by sonic anemometers and three-dimensional flow pattern by Doppler lidar), obtained during the Cooperative Atmosphere Surface Exchange Study field campaign in October 1999 (CASES-99), show evidence of a layered structure of the near-neutral surface layer: (i) the eddy surface layer (ESL), which is the lower sublayer where blocking of impinging eddies is the dominating mechanism; and (ii) the shear surface layer (SSL), which is an intermediate sublayer, where shear affects the isotropy of turbulence. The origin of the eddies impinging from aloft (probably from the SSL) down to the ESL is preliminarily addressed in this study, since the Doppler lidar data show evidence of linearly organized eddies embedded in the surface layer (i.e., about 100-m vertical extent) and horizontally spaced by about 300 m. This is consistent with theories predicting that the primary mechanism of eddy motion in high Reynolds number wall layers is “top-down.” The layered str...