Near-neutral Surface Layer Research Articles

Gusty wind disturbances and large-scale turbulent structures in the neutral atmospheric surface layer

This study analyzes the contribution of large-scale turbulent structures, including very large-scale and large-scale motions, to the streamwise turbulent kinetic energy and momentum flux in comparison with the contribution of the gusty wind disturbances based on the high-quality data obtained from the field measurements conducted in the near-neutral surface layer. The results of this study denote that the gusty wind disturbances contain only a portion of the energy contained in very large-scale motions and do not contain any of the information contained in large-scale motions. The amount of lost contributions to the streamwise turbulent kinetic energy and momentum flux increases linearly with the friction velocity, eventually becoming 53% and 67%, respectively. This indicates that large-scale turbulent structures (very large-scale motions and large-scale motions) better describe the coherent structures in the atmospheric surface layer when compared with the gusty wind disturbances.

The Share of the Mean Turbulent Kinetic Energy in the Near-Neutral Surface Layer for High and Low Wind Speeds

We examine the dependence on wind speed of the share of the mean turbulent kinetic energy among the three velocity components in the near-neutral surface layer. To contrast the general behaviour and the local effects, four datasets are considered, corresponding to different surfaces and environmental conditions. For high wind speeds (i.e., wind speed $$\approx {10}\,{\hbox {ms}^{-1}}$$ ), the shares are well-defined and about the same for all sites. As wind speed decreases (becoming $$\approx {1}\,{\hbox { ms}^{-1}}$$ ), large record-to-record variability occurs giving, on average, an almost isotropic state for the horizontal velocity components. Through spectral analysis, we relate this behaviour to the low-frequency, submeso motions and to the lack of conditions required by Reynolds averaging. The implications for modelling are also discussed, showing that the wind speed, or a related quantity, must be accounted for, besides stability, in second-order closures.

Amplitude modulation between multi-scale turbulent motions in high-Reynolds-number atmospheric surface layers

Long-term measurements were performed at the Qingtu Lake Observation Array site to obtain high-Reynolds-number atmospheric surface layer flow data ($Re_{\unicode[STIX]{x1D70F}}\sim O(10^{6})$). Based on the selected high-quality data in the near-neutral surface layer, the amplitude modulation between multi-scale turbulent motions is investigated under various Reynolds number conditions. The results show that the amplitude modulation effect may exist in specific motions rather than at all length scales of motion. The most energetic motions with scales larger than the wavelength of the lower wavenumber peak in the energy spectra play a vital role in the amplitude modulation effect; the small scales shorter than the wavelength of the higher wavenumber peak are strongly modulated, whereas the motions with scales ranging between these two peaks neither contribute significantly to the amplitude modulation effect nor are strongly modulated. Based on these results, a method of decomposing the fluctuating velocity is proposed to accurately estimate the degree of amplitude modulation. The corresponding amplitude modulation coefficient is much larger than that estimated by establishing a nominal cutoff wavelength; moreover, it increases log-linearly with the Reynolds number. An empirical model is proposed to parametrize the variation of the amplitude modulation coefficient with the Reynolds number and the wall-normal distance. This study contributes to a better understanding of the interaction between multi-scale turbulent motions and the results may be used to validate and improve existing numerical models of high-Reynolds-number wall turbulence.

Scaling of turbulence spectra measured in strong shear flow near the Earth’s surface

Within the lowest kilometer of the Earth’s atmosphere, in the so-called atmospheric boundary layer, winds are often gusty and turbulent. Nearest to the ground, the turbulence is predominately generated by mechanical wall-bounded wind shear, whereas at higher altitudes turbulent mixing of heat and moisture also play a role. The variance (square of the standard deviation) of the fluctuation around the mean wind speed is a measure of the kinetic energy content of the turbulence. This kinetic energy can be resolved into the spectral distributions, or spectra, as functions of eddy size, wavenumber, or frequency. Spectra are derived from Fourier transforms of wind records as functions of space or time corresponding to wavenumber and frequency spectra, respectively. Atmospheric spectra often exhibit different subranges that can be distinguished and scaled by the physical parameters responsible for: (1) their generation; (2) the cascade of energy across the spectrum from large- to small-scale; and (3) the eventual decay of turbulence into heat owing to viscosity effects on the Kolmogorov microscale, in which the eddy size is only a fraction of a millimeter. This paper addresses atmospheric turbulence spectra in the lowest part of the atmospheric boundary layer—the so-called surface layer—where the wind shear is strong owing to the nonslip condition at the ground. Theoretical results dating back to Tchen’s early work in 1953 ‘on the spectrum of energy in turbulent shear flow’ led Tchen to predict a shear production subrange with a distinct inverse-linear power law for turbulence in a strongly sheared high-Reynolds number wall-bounded flow, as is encountered in the lowest sheared part of the atmospheric boundary layer, also known as the eddy surface layer. This paper presents observations of spectra measured in a meteorological mast at Høvsøre, Denmark, that support Tchen’s prediction of a shear production subrange following a distinct power law of degree −1 in the lowest part of the atmospheric surface layer with the form where is the surface friction velocity and is the wavenumber. Tchen’s turbulence theory is shown to be able to predict the measured spectra of the wind velocity component parallel to the mean wind direction for eddy sizes larger than the measurement height above the ground. An amended analytical model for the near-neutral surface layer spectrum is then proposed. This model, which is applicable to the scaling of the spectrum at all heights in the surface layer, is obtained by a combination of Kaimal’s classical spectral model for scaling the inertial subrange with Tchen’s 1953 and 1954 proposed shear production subrange theory. The shear production-amended spectral model is compared with observations of ensemble-averaged near-neutral spectra selected during a nine-month measurement period from recordings from six sonic anemometers at heights of 10, 20, 40, 60, 80, and 100 m in the meteorological tower at the test site for large wind turbines in Høvsøre, Denmark. Finally, potential applications of the new spectral model are discussed, in particular for use within the lowest one-third of the surface layer in which the production subrange component of the spectrum is most prominent. The new spectral model can supply wavenumber-resolved turbulent kinetic energies for the prediction of wind loads on buildings, bridges, and wind turbines, and its spectral parameterization can also be used for scale-dependent parameterization of, e.g., surface-released atmospheric dispersion calculations for regions close to the ground.

Spatial length scales of large-scale structures in atmospheric surface layers

Synchronous multipoint measurements were performed in the atmospheric surface layer at theQingtu LakeObservationArray site to obtain high-Reynolds-number [Re-tau similar to O(10(6))] data. Based on the selected high-quality data in the near-neutral surface layer, the spatial length scales of the large-scale dominant structures in the outer region of the turbulent boundary layer are investigated. The characteristic length scales are extracted from the two-point streamwise velocity correlations. Results show that the spatial length scales are invariant over a three order of magnitude change in Reynolds number [Re-tau similar to O(103) - O(10(6))]. However, they increase significantly with the wall-normal distance, showing reasonable collapses on outer-scaled axes. The variation of the spanwise width scale in the logarithmic region follows a linear increase, with the rate of increase much larger than that in the wake region. Moreover, the variation of the wall-normal length scale is also revealed, which displays a qualitative behavior similar to that of the spanwise width scale. The universal laws revealed in the present work contribute to a better understanding of the dominant structures in the outer region of the turbulent boundary layer at high Reynolds numbers.

Improving the Representation of the Nocturnal Near-Neutral Surface Layer in the Urban Environment with a Mesoscale Atmospheric Model

A new approach to improve the representation of surface processes in the Global Environmental Multiscale (GEM) atmospheric model associated with the exchanges between the urban canopy and the atmosphere is presented. Effects of the urban canopy on the evolution of surface-layer wind, temperature, moisture, and turbulence are directly parametrized in order to allow realistic interactions between the canopy elements (i.e., roofs, roads, and walls) and the atmosphere at GEM’s multiple vertical levels that are positioned inside the canopy. Surface energy budgets as implemented in the Town Energy Balance (TEB) scheme have been used to determine temperatures of the urban canopy elements for the proposed multilayer scheme. Performance of the multilayer scheme is compared against standard implementations of the TEB scheme for one nighttime intensive observation period of the Joint Urban 2003 experiment held in Oklahoma City, USA. Although the new approach is found to have a negligible impact on urban surface-layer wind profiles, it improves the prediction of near-neutral nocturnal profiles of potential temperature close to the surface. The urban heat island effect is simulated with a better accuracy by the multilayer approach. The horizontal temperature gradient across the central business district of the city along the direction of flow is also reasonably well captured by the proposed scheme.

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.

Land-Atmosphere Energy Transfer and Surface Boundary Layer Characteristics in the Rongbu Valley on the Northern Slope of Mt. Everest

The land-atmosphere interaction processes are unique at Mt. Everest as a result of high elevation. Based on turbulent data collected from April 2005 to March 2006 with the eddy covariance method at Quzong in the Rongbu Valley on the northern slope of Mt. Everest, land-atmosphere energy budget before and after the southwest monsoon onset and surface layer turbulent characteristics are studied for the first time. It is found that energy budget components (net radiation flux, sensible heat flux, latent heat flux, and soil heat flux) and surface heating field have strong diurnal and seasonal variations. In particular, under the influence of the southwest monsoon, the characteristics of surface parameters can be clearly identified. From pre-monsoon to monsoon season, the sensible heat flux decreases whereas the latent heat flux increases. The latent heat flux and Evaporative Fraction at Quzong are relatively high most of the year. Furthermore, the intensity of heating source in the wet season (from June to August) is much greater than that in the dry season (from October to December). The relationship between normalized standard deviation of wind speed and atmospheric stability, variations of normalized standard deviation of temperature, and humidity with atmospheric stability are analyzed using the Monin-Obukhov similarity theory. The result reveals that the normalized standard deviation of velocity components of the three-dimensional wind speed follows similarity relationships in the convective and near-neutral surface layer, but does not seem to be valid in stable surface layer. However, the normalized standard deviation of temperature and humidity does not obey Monin-Obukhov similarity theory in the entire interval of atmospheric stratifications.

Turbulence spectra in the near-neutral surface layer over the Loess Plateau in China

We present the power spectra of wind velocity and the cospectra of momentum and heat fluxes observed for different wind directions over flat terrain and a large valley on the Loess Plateau. The power spectra of longitudinal (u) and lateral (v) wind speeds satisfy the −5/3 power law in the inertial subrange, but do not vary as observed in previous studies within the low frequency range. The u spectrum measured at 32 m height for flow from the valley shows a power deficit at intermediate frequencies, while the v spectrum at 32 m downwind of the valley reaches another peak in the low frequency range at the same frequency as the u spectrum. The corresponding peak wavelength is consistent with the observed length scale of the convective outer layer at the site. The v spectrum for flat terrain shows a spectral gap at mid frequencies while obeying inner layer scaling in its inertial subrange, suggesting two sources of turbulence in the surface layer. All the spectra and cospectra from the valley direction show a height dependency over the three levels.

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...

Near-Neutral Surface Layer Turbulence at the Boulder Atmospheric Observatory Tower: Evidence of Increasing Vertical Turbulence with Height

Abstract Wind and turbulence profiles were analyzed during breezy, near-neutral conditions at the Boulder Atmospheric Observatory tower to quantify the effects of an abrupt 20- to 30-m increase in terrain located 3–5 km west of the tower. Results indicate that regional roughness length z0 is greatest and local z0 is smallest in sectors downwind of a long, shallow bluff, suggesting that distant, more complex, upwind terrain enhances horizontal turbulence but has little effect on local wind speed profiles. Turbulence parameter profiles are nearly constant up to 200 m above ground level (AGL), and friction velocity u∗ is nearly constant up to 150–200 m for simple fetch with z0 equal to 4 cm and near-surface wind speed approximately equal to 7 m s−1 at 10 m AGL. Horizontal turbulence parameters, however, increase by 50% to nearly 100% at all tower levels downwind of the distant terrain bluff when compared with those with simple fetch. The effect of the bluff on vertical turbulence increases with height. Although the effect on the 10-m standard deviation of vertical wind speed σw is negligible, the 200-m σw increases by about 0.5 m s−1, or about twice the 10-m value. The u∗ also increases by 16% to nearly 100% between the 10- and 200-m heights downwind of the bluff. Primarily because of the enhanced σw downwind of the bluff, atmospheric dispersion could be underestimated by factors of about 2.5, 2.0, and 1.3 at the 200-, 100-, and 50-m levels, respectively, if the vertical and transverse standard deviations of the wind angle fluctuations are estimated from 10-m values.

Stability and wind shear effects on meso-scale cellular convection

A three-dimensional numerical model has been used to assess the effects of vertical stability and wind shear on the nature and form of meso-scale cellular convection (MCC). The model was shown to be capable of simulating a real occasion of MCC before it was used in idealised cases. These cases revealed different regimes in MCC: open cells, longitudinal bands and closed cells/transverse bands. Open cells were favoured by the existence of instability in the surface layer and a lack of wind shear in the Ekman layer. Longitudinal bands were favoured by similar conditions in the surface layer plus wind shear in the Ekman layer. A near-neutral surface layer favoured the occurrence of closed cells/transverse bands. The depth of convection in the longitudinal bands was a function of the stability in both the surface and Ekman layers and of the wind shear in the Ekman layer. The regimes are related to the instability and shear through bulk Richardson numbers in the surface and Ekman layers.

The Temperature-Humidity Covariance Budget in the Convective Boundary Layer

Abstract The behavior of the temperature-humidity covariance (θq) budget in the convectively driven boundary layer is determined through analysis of data from AMTEX and (to a lesser extent) Kansas and Minnesota. In the near-neutral surface layer a balance is found between production and molecular destruction; in the mixed layer, transport is also important. We extend the Corrsin theory for inertial subrange scalar spectral behavior to the temperature-humidity cospectrum, and thus relate the molecular destruction rate of θq to its inertial range level. Destruction rates inferred from AMTEX cospectra agree with those found from the imbalance of production and transport terms. The budgets within the surface layer and the mixed layer are parameterized separately with appropriate scales. Both temperature and humidity fluctuations contribute to the small-scale refractive index variations which affect acoustic and electromagnetic wave propagation in the atmosphere. Our results indicate that their joint contribut...