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1.
A Lagrangian stochastic model for the motion of heavy particles has been developed by coupling a stochastic model for the motion of fluid elements to the Stokes equations of motion of a particle in a turbulent flow. The effects of crossing trajectories and continuity are incorporated by generalising Csanady's (1963) ideas developed for stationary homogeneous turbulence; effects of turbulence inhomogeneity and nonstationarity are embodied in the stochastic model for the fluid motion.The model has been used particularly to examine the effects of turbulence nonstationarity through simulations of the dispersion of heavy particles in the decaying homogeneous turbulence which is obtained by Taylor-transforming grid turbulence. Significant differences from the stationary case occur, mainly as a result of the growth of the turbulent time scale with time.The importance of the source location in influencing both passive scalar and particle dispersion in grid turbulence is highlighted by the model and can be simply accounted for by nondimensionalisation using the r.m.s. turbulence velocity at the source and the mean travel time from the grid to the source as velocity and time scales, respectively. Reconciliation of the three different experiments of Snyder and Lumley (1971), Wells and Stock (1983) and Ferguson (1986) reporting heavy particle flow and dispersion statistics in wind tunnel grid turbulence has been attempted using this nondimensionalisation. A good correspondence between the various data sets was not obtained because the source in the Wells and Stock, and Ferguson experiments was located at the grid where the self-similar development of the turbulence which underlies the scaling is not appropriate.The model matches the data for the heaviest particles used by Snyder and Lumley reasonably well. For very light particles, it correctly reverts to the passive scalar limit, while the experimental data in general do not properly approach this limit. 相似文献
2.
A Lagrangian Stochastic Model for Heavy Particle Dispersion in the Atmospheric Marine Boundary Layer
The dispersion of heavy particles and pollutants is often simulated with Lagrangian stochastic (LS) models. Although these
models have been employed successfully over land, the free surface at the air-sea interface complicates the implementation
of traditional LS models. We present an adaptation of traditional LS models to the atmospheric marine boundary layer (MBL),
where the bottom boundary is represented by a realistic wavy surface that moves and deforms. In addition, the correlation
function for the turbulent flow following a particle is extended to the anisotropic, unsteady case. Our new model reproduces
behaviour for Lagrangian turbulence in a stratified air flow that departs only slightly from the expected behaviour in isotropic
turbulence. When solving for the trajectory of a heavy particle in the air flow, the modelled turbulent forcing on the particle
also behaves remarkably well. For example, the spectrum of the turbulence at the particle location follows that of a massless
particle for time scales approximately larger than the Stokes’ particle response time. We anticipate that this model will
prove especially useful in the context of sea-spray dispersion and its associated momentum, sensible and latent heat, and
gas fluxes between spray droplets and the atmosphere. 相似文献
3.
Hao Liu Renmin Yuan Jie Mei Jianning Sun Qi Liu Yu Wang 《Boundary-Layer Meteorology》2017,165(2):277-294
The scale properties of anisotropic and isotropic turbulence in the urban surface layer are investigated. A dimensionless anisotropic tensor is introduced and the turbulent tensor anisotropic coefficient, defined as C, where \(C = 3d_{3}\,+\,1 (d_{3}\) is the minimum eigenvalue of the tensor) is used to characterize the turbulence anisotropy or isotropy. Turbulence is isotropic when \(C \approx 1\), and anisotropic when \(C \ll 1\). Three-dimensional velocity data collected using a sonic anemometer are analyzed to obtain the anisotropic characteristics of atmospheric turbulence in the urban surface layer, and the tensor anisotropic coefficient of turbulent eddies at different spatial scales calculated. The analysis shows that C is strongly dependent on atmospheric stability \(\xi = (z-z_{\mathrm{d}})/L_{{\textit{MO}}}\), where z is the measurement height, \(z_{\mathrm{d}}\) is the displacement height, and \(L_{{\textit{MO}}}\) is the Obukhov length. The turbulence at a specific scale in unstable conditions (i.e., \(\xi < 0\)) is closer to isotropic than that at the same scale under stable conditions. The maximum isotropic scale of turbulence is determined based on the characteristics of the power spectrum in three directions. Turbulence does not behave isotropically when the eddy scale is greater than the maximum isotropic scale, whereas it is horizontally isotropic at relatively large scales. The maximum isotropic scale of turbulence is compared to the outer scale of temperature, which is obtained by fitting the temperature fluctuation spectrum using the von Karman turbulent model. The results show that the outer scale of temperature is greater than the maximum isotropic scale of turbulence. 相似文献
4.
《Dynamics of Atmospheres and Oceans》2006,41(1):1-27
An analytical model is developed for the initial stage of surface wave generation at an air–water interface by a turbulent shear flow in either the air or in the water. The model treats the problem of wave growth departing from a flat interface and is relevant for small waves whose forcing is dominated by turbulent pressure fluctuations. The wave growth is predicted using the linearised and inviscid equations of motion, essentially following Phillips [Phillips, O.M., 1957. On the generation of waves by turbulent wind. J. Fluid Mech. 2, 417–445], but the pressure fluctuations that generate the waves are treated as unsteady and related to the turbulent velocity field using the rapid-distortion treatment of Durbin [Durbin, P.A., 1978. Rapid distortion theory of turbulent flows. PhD thesis, University of Cambridge]. This model, which assumes a constant mean shear rate Γ, can be viewed as the simplest representation of an oceanic or atmospheric boundary layer.For turbulent flows in the air and in the water producing pressure fluctuations of similar magnitude, the waves generated by turbulence in the water are found to be considerably steeper than those generated by turbulence in the air. For resonant waves, this is shown to be due to the shorter decorrelation time of turbulent pressure in the air (estimated as ∝ 1/Γ), because of the higher shear rate existing in the air flow, and due to the smaller length scale of the turbulence in the water. Non-resonant waves generated by turbulence in the water, although being somewhat gentler, are still steeper than resonant waves generated by turbulence in the air. Hence, it is suggested that turbulence in the water may have a more important role than previously thought in the initiation of the surface waves that are subsequently amplified by feedback instability mechanisms. 相似文献
5.
切变湍流的相干结构是湍流研究中的重大发现,它表明湍流在表面上看来不规则运动中具有可检测的有序运动,这种相干结构在切变湍流的脉动生成和发展中起着主宰作用.因此识别和提取相干结构对于认识和研究湍流是非常重要的.用数字滤波法将包含相干结构的大尺度信号提取出来以后,再用子波分析,根据子波能量极大值的判别方法,分别确定出大气湍流三个方向上的速度脉动信号相干结构的频率或时间尺度,然后由确定尺度上的连续子波反演公式,提取出大气湍流三个方向上的速度脉动信号相干结构所对应的波形. 相似文献
6.
Measurements of concentration fluctuation intensity, intermittency factor, and integral time scale were made in a water channel
for a plume dispersing in a well-developed, rough surface, neutrally stable, boundary layer, and in grid-generated turbulence
with no mean velocity shear. The water-channel simulations apply to full-scale atmospheric plumes with very short averaging
times, on the order of 1–4 min, because plume meandering was suppressed by the water-channel side walls. High spatial and
temporal resolution vertical and crosswind profiles of fluctuations in the plume were obtained using a linescan camera laser-induced
dye tracer fluorescence technique. A semi-empirical algebraic mean velocity shear history model was developed to predict these
concentration statistics. This shear history concentration fluctuation model requires only a minimal set of parameters to
be known: atmospheric stability, surface roughness, vertical velocity profile, and vertical and crosswind plume spreads. The
universal shear history parameter used was the mean velocity shear normalized by surface friction velocity, plume travel time,
and local mean wind speed. The reference height at which this non-dimensional shear history was calculated was important,
because both the source and the receptor positions influence the history of particles passing through the receptor position. 相似文献
7.
A Wind-Tunnel Investigation of Wind-Turbine Wakes: Boundary-Layer Turbulence Effects 总被引：1，自引：1，他引：0
Wind-tunnel experiments were performed to study turbulence in the wake of a model wind turbine placed in a boundary layer
developed over rough and smooth surfaces. Hot-wire anemometry was used to characterize the cross-sectional distribution of
mean velocity, turbulence intensity and kinematic shear stress at different locations downwind of the turbine for both surface
roughness cases. Special emphasis was placed on the spatial distribution of the velocity deficit and the turbulence intensity,
which are important factors affecting turbine power generation and fatigue loads in wind energy parks. Non-axisymmetric behaviour
of the wake is observed over both roughness types in response to the non-uniform incoming boundary-layer flow and the effect
of the surface. Nonetheless, the velocity deficit with respect to the incoming velocity profile is nearly axisymmetric, except
near the ground in the far wake where the wake interacts with the surface. It is found that the wind turbine induces a large
enhancement of turbulence levels (positive added turbulence intensity) in the upper part of the wake. This is due to the effect
of relatively large velocity fluctuations associated with helicoidal tip vortices near the wake edge, where the mean shear
is strong. In the lower part of the wake, the mean shear and turbulence intensity are reduced with respect to the incoming
flow. The non-axisymmetry of the turbulence intensity distribution of the wake is found to be stronger over the rough surface,
where the incoming flow is less uniform at the turbine level. In the far wake the added turbulent intensity, its positive
and negative contributions and its local maximum decay as a power law of downwind distance (with an exponent ranging from
−0.3 to −0.5 for the rough surface, and with a wider variation for the smooth surface). Nevertheless, the effect of the turbine
on the velocity defect and added turbulence intensity is not negligible even in the very far wake, at a distance of fifteen
times the rotor diameter. 相似文献
8.
Experiments on integral length scale control in atmospheric boundary layer wind tunnel 总被引：2，自引：1，他引：1
Accurate predictions of turbulent characteristics in the atmospheric boundary layer (ABL) depends on understanding the effects of surface roughness on the spatial distribution of velocity, turbulence intensity, and turbulence length scales. Simulation of the ABL characteristics have been performed in a short test section length wind tunnel to determine the appropriate length scale factor for modeling, which ensures correct aeroelastic behavior of structural models for non-aerodynamic applications. The ABL characteristics have been simulated by using various configurations of passive devices such as vortex generators, air barriers, and slot in the test section floor which was extended into the contraction cone. Mean velocity and velocity fluctuations have been measured using a hot-wire anemometry system. Mean velocity, turbulence intensity, turbulence scale, and power spectral density of velocity fluctuations have been obtained from the experiments for various configuration of the passive devices. It is shown that the integral length scale factor can be controlled using various combinations of the passive devices. 相似文献
9.
大气边界层强风的阵性和相干结构 总被引：14，自引：5，他引：9
我国北方春季冷锋过境后,常骤发强风,甚至起沙扬尘,持续数小时甚至一二天,通过对边界层超声风温仪的资料分析,可知大风常叠加有周期为3～6 min的阵风,较有规律,且有明显的相干结构:阵风风速峰期有下沉运动,谷期有上升运动;阵风扰动以沿平均流的顺风方向分量为主,横向和垂直方向的分量都较小,其本质是低频次声波和重力波的混合;阵风沿顺风向且向下传播.周期小于1 min的脉动在水平面上基本是各向同性的不规则的湍涡.大风期间,无论是平均流、阵风和湍流脉动,至少在120 m高度以下,主要都有西风和北风动量下传,感热上传.平均流的动量下传强于由脉动下传的量,与一般天气情况不同,而且阵风与湍流的动量下传的量值差不多.平均流和阵风在动量传送上起相当大的作用. 相似文献
10.
The dispersion of heavy particles subjected to a turbulent forcing is often simulated with Lagrangian stochastic models. Although
these models have been employed successfully over land, the implementation of traditional LS models in the marine boundary
layer is significantly more challenging. We present an adaptation of traditional Lagrangian stochastic models to the atmospheric
marine boundary layer with a particular focus on the representation of the scalar turbulence for temperature and humidity.
In this new model, the atmosphere can be stratified and the bottom boundary is represented by a realistic wavy surface that
moves and deforms. Hence, the correlation function for the turbulent flow following a particle is extended to the inhomogenous,
anisotropic case. The results reproduce behaviour for scalar Lagrangian turbulence in a stratified airflow that departs only
slightly from the expected behaviour in isotropic turbulence. When solving for the surface temperature and the radius of evaporating
heavy water droplets in the airflow, the modelled turbulent forcing on the particle also behaves remarkably well. We anticipate
that this model will prove especially useful in the context of sea-spray dispersion and its associated sensible heat, latent
heat, and gas fluxes between spray droplets and the atmosphere. 相似文献
11.
J. R. Chasnov 《Dynamics of Atmospheres and Oceans》1996,23(1-4)
The decay of statistically homogeneous velocity and density fluctuations in a stably stratified fluid is considered. Over decay times long compared with the turbulence time scale but short compared with the period of internal gravity waves, three distinct high Reynolds number similarity states may develop. These. similarity states are a consequence of the invariance of the low wavenumber coefficients of the three-dimensional kinetic or potential energy spectrum, and their preferential development depends on the relative magnitudes of the initial kinetic and potential energy per unit mass of the fluid. When the turbulence has decayed over a time comparable with the period of the gravity waves, the three similarity states mentioned above are disrupted. Evidence will be presented of a new similarity state which then develops asymptotically. In this similarity state, the time decay exponent of the total energy per unit mass of the turbulence is reduced by a factor of two from its value for decaying isotropic turbulence, and the associated vertical integral scale approaches a constant independent of time. 相似文献
12.
We utilized a Doppler lidar to measure integral scale and coherence of vertical velocity w in the daytime convective boundary layer (CBL). The high resolution 2 μm wavelength Doppler lidar developed by the NOAA Environmental Technology Laboratory was used to detect the mean radial velocity of aerosol particles. It operated continuously in the zenith-pointing mode for several days in the summer 1996 during the “Lidars in Flat Terrain” experiment over level farmland in central Illinois. We calculated profiles of w integral scales in both the alongwind and vertical directions from about 390 m height to the CBL top. In the middle of the mixed layer we found, from the ratio of the w integral scale in the vertical to that in the horizontal direction, that the w eddies are squashed by a factor of about 0.65 as compared to what would be the case for isotropic turbulence. Furthermore, there is a significant decrease of the vertical integral scale with height. The integral scale profiles and vertical coherence show that vertical velocity fluctuations in the CBL have a predictable anisotropic structure. We found no significant tilt of the thermal structures with height in the middle part of the CBL.The National Center for Atmospheric Research is sponsored by the National Science Foundation. 相似文献
13.
David Pino Harm J. J. Jonker Jordi Vilà-Guerau de Arellano Alessandro Dosio 《Boundary-Layer Meteorology》2006,121(3):537-556
The role of shear and inversion strength on the decay of convective turbulence during sunset over land is systematically studied by means of large-eddy simulations. Different decay rates have been found for the vertical and horizontal velocity fluctuations, resulting in an increase of the anisotropy for all the studied cases. Entrainment, which persists during the decay process, favours the appearance of vertical upward movements associated with a conversion from kinetic to potential energy. Particular attention is paid to the evolution of the characteristic length scale of the various turbulent variables during this process. The length scale evolution is found to depend on the wind shear characteristics, but not on the strength of the inversion. In general the length scales of the variables grow during decay because small-scale fluctuations dissipate faster than large-scale fluctuations. Only the length scale of the vertical velocity component remains nearly constant during decay. Spectral analysis of the variance budgets shows that pressure correlations are responsible for fixing this length scale, effectively compensating the strong but oscillating influence of buoyancy. In the shear cases, after an initial period of growth, the length scales start to decrease once the buoyancy-generated variance has sufficiently subsided. Also here the effect of pressure redistribution is crucial, as it transfers the spectral influence of shear to the other velocity components. 相似文献
14.
Yih-Ho Pao 《Boundary-Layer Meteorology》1973,5(1-2):177-193
A turbulent stratified shear flow is generated in a towing tank by towing a grid or a circular cylinder through a tank of stratified salt water. The internal waves and turbulence generated in these flows are visualized with shadowgraphs and measured with quartz-coated hot-film probes (up to four probes for velocity fluctuations) and single-electrode conductivity probes (up to four probes for salinity fluctuations) which are towed at the same speed as the obstacle. The velocity and salinity signals are recorded on magnetic tapes. A portion of these signals is processed directly-on-line with a digital computer. From these shadowgraphs and probe measurements, we observe that
- Far downstream of the obstacle where the turbulence has already subsided, the stratified fluid always has a layered structure. This layered structure persists for a long time, and is a result of the convection of turbulently mixed layers by the mean flow. These results indicate that in the regions of a stably stratified atmosphere and ocean where the turbulence has subsided, one could often find layered structure.
- There are spectral peaks and valleys in the measured velocity and salinity autospectra when the stratifications are sufficiently strong. Under certain conditions, these spectral peaks tend to lift up the spectral curves to show substantialf ?5/3 subranges, although the turbulence Reynolds numbers are too low for the flows to have recognizable inertial subranges. This anomalousf ?5/3 subrange demonstrates the pitfalls of using spectral measurements in thef ?5/3 subrange to predict the turbulent energy dissipation rate through the Kolmogorov hypothesis.
- A diagnostic method is developed for distinguishing internal waves from turbulence, utilizing their phase characteristics. The phase characteristics can be conveniently examined from the cospectra and quadrature spectra measurements of: (a), two vertically separated velocity probes; (b), two vertically separated density probes; and (c), a velocity probe and a density probe. This method is demonstrated to be useful in the laboratory and can be applied directly to atmospheric and oceanic measurements to distinguish internal waves from turbulence.
- From the coherency measurements, it is found that the entire turbulent stratified wake is actually whipping up and down at a frequency corresponding to the Brunt-Väisälä frequency. This indicates that similar stratified shear flows in the atmosphere and in the ocean, such as the jet streams in the atmosphere and the Cromwell current in the ocean, may oscillate vertically, which in turn can induce horizontal oscillation and meandering.
15.
R. H. Shaw K. T. U Paw X. J. Zhang W. Gao G. Den Hartog H. H. Neumann 《Boundary-Layer Meteorology》1990,50(1-4):319-338
The relationship between surface pressure fluctuations and the velocity field associated with turbulent coherent structures is examined for flow within and above a deciduous forest. Measurements were taken with tower-mounted sonic anemometer/thermometers at six heights, Lyman-alpha humidiometers at three heights, and a pressure sensor at the forest floor. We find a strong, near-linear relationship between the mean square turbulent velocity and the standard deviation of the high-pass-filtered pressure fluctuations. Lagged cross-correlations between vertical velocity fluctuations and those of pressure show maximum correlations of ± 0.5 but with a phase offset. Examination of surface pressure during the passage of coherent structures, which are characterized by a transition from ejection to sweep, reveals a period of overpressure about 20 s in duration roughly centered on the time of passage of the scalar microfront at the top of the canopy. Pressure patterns associated with coherent structures appear to be largely responsible for the form of the correlations stated above.Pressure patterns calculated from an integrated Poisson equation, using observed velocity and temperature signals during coherent structures, match the main features of the observed pressure. Retrieval of the pressure fluctuations in this manner reveals that the mean wind shear/turbulence interaction term is dominant, but that important contributions arise from two other terms in the equation. Buoyancy effects are negligible. We show that the surface pressure signal is mainly created by the velocity field near the top of the forest, and present evidence to suggest that features of the sub-crown air movement result directly from this pressure field. 相似文献
16.
E. Van Gorsel A. Christen C. Feigenwinter E. Parlow R. Vogt 《Boundary-Layer Meteorology》2003,109(3):311-329
Six levels of simultaneously sampled ultrasonic data are used to analyse the turbulence structure within a mixed forest of 13 m height on a steep slope (35°) in an alpine valley. The data set is compared to other studies carried out over forests in more ideal, flat terrain. The analysis is carried out for 30-min mean data, joint probability distributions, length scales and spectral characteristics.Thermally induced upslope winds and cold air drainage lead to a wind speed maximum within the trunk space. Slope winds are superimposed on valley winds and the valley-wind component becomes stronger with increasing height. Slope and valley winds are thus interacting on different spatial and time scales leading to a quite complex pattern in momentum transport that differs significantly from surface-layer characteristics. Directional shear causes lateral momentum transports that are in the same order or even larger than the longitudinal ones. In the canopy, however, a sharp attenuation of turbulence is observed. Skewed distributions of velocity components indicate that intermittent turbulent transport plays an important role in the energy distribution.Even though large-scale pressure fields lead to characteristic features in the turbulent structure that are superimposed on the canopy flow, it is found that many statistical properties typical of both mixing layers and canopy flow are observed in the data set. 相似文献
17.
Large-eddy Simulation of the Dispersion of Solid Particles in a Turbulent Boundary Layer 总被引：2，自引：2，他引：2
Ivana Vinkovic Cesar Aguirre Michel Ayrault Serge Simoëns 《Boundary-Layer Meteorology》2006,121(2):283-311
A large-eddy simulation (LES) with the dynamic Smagorinsky-Germano subgrid-scale (SGS) model is used to study the dispersion of solid particles in a turbulent boundary layer. Solid particles are tracked in a Lagrangian way. The instantaneous velocity of the surrounding fluid is considered to have a large-scale part (directly computed by the LES) and a small-scale part. The SGS velocity of the surrounding fluid is given by a three-dimensional Langevin model written in terms of SGS statistics at a mesh level. An appropriate Lagrangian correlation time scale is considered in order to include the influences of gravity and inertia of the solid particle. Inter-particle collisions and the influence of particles on the mean flow are also taken into account. The results of the LES are compared with the wind-tunnel experiments of Nalpanis et al. (1993 J Fluid Mech 251: 661–685) and of Tanière et al. (1997 Exp in Fluids 23:463–471) on sand particles in saltation and in modified saltation, respectively. 相似文献
18.
Hiroshi Takimoto Ayumu Sato Janet F. Barlow Ryo Moriwaki Atsushi Inagaki Shiho Onomura Manabu Kanda 《Boundary-Layer Meteorology》2011,140(2):295-314
We investigate the spatial characteristics of urban-like canopy flow by applying particle image velocimetry (PIV) to atmospheric
turbulence. The study site was a Comprehensive Outdoor Scale MOdel (COSMO) experiment for urban climate in Japan. The PIV
system captured the two-dimensional flow field within the canopy layer continuously for an hour with a sampling frequency
of 30 Hz, thereby providing reliable outdoor turbulence statistics. PIV measurements in a wind-tunnel facility using similar
roughness geometry, but with a lower sampling frequency of 4 Hz, were also done for comparison. The turbulent momentum flux
from COSMO, and the wind tunnel showed similar values and distributions when scaled using friction velocity. Some different
characteristics between outdoor and indoor flow fields were mainly caused by the larger fluctuations in wind direction for
the atmospheric turbulence. The focus of the analysis is on a variety of instantaneous turbulent flow structures. One remarkable
flow structure is termed ‘flushing’, that is, a large-scale upward motion prevailing across the whole vertical cross-section
of a building gap. This is observed intermittently, whereby tracer particles are flushed vertically out from the canopy layer.
Flushing phenomena are also observed in the wind tunnel where there is neither thermal stratification nor outer-layer turbulence.
It is suggested that flushing phenomena are correlated with the passing of large-scale low-momentum regions above the canopy. 相似文献
19.
We analyse single-point velocity statistics obtained in a wind tunnel within and above a model of a waving wheat crop, consisting of nylon stalks 47 mm high and 0.25 mm wide in a square array with frontal area index 0.47. The variability of turbulence measurements in the wind tunnel is illustrated by using a set of 71 vertical traverses made in different locations, all in the horizontally-homogeneous (above-canopy) part of the boundary layer. Ensemble-averaged profiles of the statistical moments up to the fourth order and profiles of Eulerian length scales are presented and discussed. They are consistent with other similar experiments and reveal the existence of large-scale turbulent coherent structures in the flow. The drag coefficient in this canopy as well as in other reported experiments is shown to exhibit a characteristic height-dependency, for which we propose an interpretation. The velocity spectra are analysed in detail; within and just above the canopy, a scaling based on fixed length and velocity scales (canopy height and mean horizontal wind speed at canopy top) is proposed. Examination of the turbulent kinetic energy and shear stress budgets confirms the role of turbulent transport in the region around the canopy top, and indicates that pressure transport may be significant in both cases. The results obtained here show that near the top of the canopy, the turbulence properties are more reminiscent of a plane mixing layer than a wall boundary layer. 相似文献
20.
An atmospheric surface-layer (ASL) experiment conducted at a meteorological site in the Oostelijk-Flevoland polder of the Netherlands is described. Turbulent fluctuations of wind velocity, air temperature and static pressure were measured, using three 10 m towers.Simultaneous turbulent signals at several heights on the towers were used to investigate the properties of the turbulent structures which contribute most significantly to the turbulent vertical transports in the unstable ASL. These turbulent structures produce between 30 and 50% of the mean turbulent vertical transport of horizontal alongwind momentum and they contribute to between 40 and 50% of the mean turbulent vertical heat transport; in both cases this occurs during 15 to 20% of the total observation time.The translation speed of the turbulent structures equals the wind speed averaged over the depth of the ASL, which scales on the surface friction velocity. The inclination angle of the temperature interface at the upstream edge of the turbulent structures to the surface is significantly smaller than that of the internal shear layer, which is associated with the temperature interface. The turbulent structures in the unstable ASL are determined by a large-scale temperature field: Convective motions, which encompass the whole depth of the planetary boundary layer (PBL), penetrate into the ASL. The curvature of the vertical profile of mean horizontal alongwind velocity forces the alignment of the convective cells in the flow direction (Kuettner, 1971), which have an average length of several hundreds of metres and an average width of a few tens of metres. This mechanism leads to the formation of turbulent structures, which extend throughout the depth of the ASL. 相似文献