共查询到20条相似文献，搜索用时 31 毫秒
1.
C. A. Biltoft 《Boundary-Layer Meteorology》2001,100(3):393-404
Applications of Kolmogorov's universal equilibrium hypothesis and the Taylor transform to velocity spectra derived from measurements within the low frequency portion of the atmospheric surface layer (ASL) inertial subrange are examined. The measured ratios of lateral to longitudinal velocity component spectra exhibit considerable scatter, but suggest convergence towards 1.0 rather than towards the 4/3 expected from theory. Shear and buoyancy introduce anisotropy to the inertial subrange, thereby contributing to the observed scatter. The apparent discrepancy between the 4/3 velocity component spectrum ratio expected from theory and the measurements could arise as a consequence of the processing used to produce spectra. These processing effects must be considered from the perspective of the propagating eddy. Spectral averaging used with sonic anemometer data is done over time periods that are large with respect to inertial subrange eddy correlation decay times. This averaging causes energy from larger scale eddies to appear as `local convection' that dominates the Taylor transform. Spectrum ratio convergence and cospectra approaching zero are necessary, but not sufficient, conditions for onset of local isotropy. Measurements of spectrum ratios and cospectra over the entire inertial subrange are needed to determine whether or not local isotropy might occur within the ASL. 相似文献
2.
B. D. Amiro 《Boundary-Layer Meteorology》1990,52(3):227-246
Atmospheric turbulence was measured within a black spruce forest, a jack pine forest, and a trembling aspen forest, located in southeastern Manitoba, Canada. Drag coefficients (C
d
) varied little with height within the pine and aspen canopies, but showed some height dependence within the dense spruce canopy. A constant C
d
of 0.15, with the measured momentum flux and velocity profiles, gave good estimates of leaf-area-index (LAI) profiles for the pine and aspen canopies, but underestimated LAI for the spruce canopy.Velocity spectra were scaled using the Eulerian integral time scales and showed a substantial inertial subrange above the canopies. In the bottom part of the canopies, the streamwise and cross-stream spectra showed rapid energy loss whereas the vertical spectra showed an apparent energy gain, in the region where the inertial subrange is expected. The temperature spectra showed an inertial subrange with the expected -2/3 slope at all heights. Cospectra of momentum and heat flux had slopes of about -1 in much of the inertial subrange. Possible mechanisms to explain some of the spectral features are discussed. 相似文献
3.
Eugene Yee R. Chan P. R. Kosteniuk C. A. Biltoft J. F. Bowers 《Boundary-Layer Meteorology》1996,77(2):173-207
The dynamical characteristics of concentration fluctuations in a dispersing plume over the energetic and inertial-convective range of scales of turbulent motion are studied using a multiscale analysis technique that is based on an orthonormal wavelet representation. It is shown that the Haar wavelet concentration spectrum is similar to the Fourier concentration spectrum in that both spectra exhibit an extensive inertial-convective subrange spanning about two decades in frequency, with a scaling exponent of -5/3. Analysis of the statistical properties (e.g., fluctuation intensity, skewness, and kurtosis) of the concentration wavelet coefficients (i.e., the concentration discrete detailed signal) suggests that the small scales are always more intermittent than the large scales. The degree of intermittency increases monotonically with decreasing scale within the inertial-convective subrange, reaching a plateau at the very small scales associated with the beginning of the near-dissipation subrange. The probability density function (pdf) of the concentration discrete detailed signal displays stretched exponential tails with an intermittency exponent (tail slope) q that increases as
a
, where is the scale or dilation and a is a power-law exponent that is dependent on downwind distance, plume height, and stratification strength with typical values in the range from about 0.25 to 0.35. It is shown that the concentration variance cascade process requires a phase coherency of eddies between different scales at the small-scale end of the inertial-convective subrange.The variation of the concentration wavelet statistics with height above the ground is investigated. The increased mean shear near the ground smooths the fine-scale plume structure for scales within the inertial-convective subrange, producing a weaker spatiotemporal intermittency in the concentration field compared to that measured higher up in the plume. The pdf of the concentration detailed signal at a fixed scale possesses less elongated tails with decreasing height z. The intermittency exponent q is found to decrease roughly linearly with increasing z.Finally, the results of the wavelet decomposition are combined to provide a conceptual model of the turbulent transport, stirring, and mixing regimes in a dispersing plume. The implications of the results for contaminant texture in a plume are discussed. 相似文献
4.
Summary ?A lot of work on near surface turbulence characteristics has been done in the experimental study of atmospheric turbulence,
but little work on fluctuating scalar field has been done, especially on some quantities such as moisture and pollution. In
this paper, with the methods of statistical theory, Fourier transform, fractal geometry and wavelet transform, the turbulent
moisture data obtained from Huaihe River Basin Experiment (HUBEX) are studied to explore the statistical, spectral, fractal
and intermittent characteristics of turbulent moisture. The results show that: (1) In the high humid area such as Huaihe river
basin, the moisture plays an important role in the surface energy balance, energy transportation and cycle. (2) The energy
spectra and cospectra are in agreement with the “−2/3” and “−4/3” scaling power law correspondingly in inertial sub-range.
(3) The fractal dimension of turbulent moisture in Huaihe river basin is clearly higher than that in other areas (such as
Beijing). (4) The fluctuating moisture field has strong intermittency, and is not in accordance with the Kolmogorov theory.
With the orthonormal wavelet transform, the dissipative events of the fluctuating moisture field contributing to inertial
sub-range intermittency buildup have been identified. After the dissipative events have been suppressed by introducing a conditioning
wavelet sampling scheme, the intermittency that affects the statistical structure of inertial sub-range is restrained. However
the repressive effects of intermittency on turbulent moisture field are worse than those on turbulent velocity field. The
reason may be either the intermittency of turbulent moisture is strong or some coherence structures exist in the turbulent
moisture field.
Received April 27, 2001; Revised January 23, 2002 相似文献
5.
6.
The optical refractive index fluctuation has been determined from measurements of turbulent temperature and humidity fluctuations in the atmospheric surface layer over the ocean. Probability density, spectral density and even- and odd-order structure functions of the refractive index fluctuation are presented. The spectral density exhibits a significant -5/3 inertial subrange as a result of the existence of a -5/3 subrange in the spectra of temperature and humidity fluctuations and in the temperature-humidity co-spectrum. The behaviour in the inertial subrange of fourth- and sixth-order structure functions of the refractive index is in reasonable agreement with that predicted by the analysis of Antonia and Van Atta (1975, 1978). The third-order structure functions of the refractive index exhibit an approximately linear dependence on separation in the inertial subrange, in agreement with a ramp model for the large coherent structure of the motion. 相似文献
7.
Characterisation and Simulation of the Multiscaling Properties of the Energy-Containing Scales of Horizontal Surface-Layer Winds 总被引：1，自引：1，他引：0
Michael K. Lauren Merab Menabde Alan W. Seed Geoffrey L. Austin 《Boundary-Layer Meteorology》1999,90(1):21-46
The multiscaling statistics of atmospheric surface-layer winds at low wavenumbers above farmland and in the lee of a mountain range were examined using a hot-wire and lightweight cup anemometer. It was found that the horizontal velocity spectra could be broken into high and low-wavenumber regimes according to the parameters given by this analysis. The low-wavenumber end of the spectrum possessed a spectral slope parameter that varied between values of 0.8 and 1.35 at the farmland site during the period of the experiment, and the high-wavenumber end – corresponding to the inertial range – possessed a spectral slope slightly greater than -5/3. The larger values for this parameter for the low-wavenumber end appeared to coincide with unstable conditions. In the lee of the mountain range, the low-wavenumber spectral slope parameter was larger still, at 1.45. The low-wavenumber signals over farmland were much less intermittent than inertial-range signals, but in the lee of the mountain range the intermittency increased. From this analysis, it was shown that the statistical properties of the recorded wind signal could be reproduced using a bounded random multiplicative cascade. The model was successfully used to simulate the wind velocity field directly, rather than simulating the energy dissipation field. Since the spectral slope parameter for low wavenumbers appeared to be a function of atmospheric stability, the method presented is a simple way of generating wind signals characteristic of a variety of atmospheric conditions. 相似文献
8.
Spectral Maxima In A Perturbed Stable Boundary Layer 总被引：4，自引：1，他引：4
Wind velocity data have been collected on Nansen Ice Sheet, Antarctica, close to the base of a steeply sloping glacier along which frequently flow katabatic winds. The aim of this study is to investigate how turbulent energy and momentum flux are perturbed by the flow interaction with topography and by the strong mechanical mixing produced by downslope flows. Spectral and cospectral analyses, performed on the wind velocity components, provide evidence that such a perturbation, at any stability, is restricted to frequencies lower than the inertial subrange. Longitudinal spectra display an energy increment, due to turbulence generated by topography and by mechanical forcing related to the katabatic wind structure. The energy, supplied by the topographic forcing, displaces the turbulent energy maximum toward lower frequencies. In near-neutral stratification the spectral maximum occurs at a reduced frequency, which seems to be consistent with the height of the steepest part of the slope, and seems to shift toward higher frequencies as a linear ,function of the local stability parameter,Ll. The parameterisation of the orographic perturbation by means of a similarity relationship allows us to scale u spectra in the same way as over uniform terrain. The scaled, perturbed spectra collapse onto a unique curve in the mid-frequency as well in the inertial subrange, while maxima are grouped in a cluster. Lateral and vertical velocity spectra exhibit shapes independent of stability, suggesting a topographic perturbation that is predominantly over stability effects. 相似文献
9.
Effects of Mesoscale Surface Thermal Heterogeneity on Low-Level Horizontal Wind Speeds 总被引：2，自引：2，他引：0
Using large-eddy simulation, we investigate characteristics of horizontal wind speed at 100 m above the ground, with surface
heat-flux variations that are sinusoidal with amplitudes of 0, 50, and 200 W m−2 and wavelengths of 16, 32, and 128 km, and no background flow. When the amplitude is 200 W m−2, wind speeds induced by the surface-flux variations on scales of 16 and/or 32 km have multiple temporal oscillations from
0600 to 1800 local standard time. The positive peaks first appear before noon. In contrast, for wind speeds induced by the
128-km surface heterogeneity, a single oscillation occurs in the late afternoon, which is much larger than those generated
by the 16- and 32-km surface heterogeneity. In addition, at the oscillation onset the kurtosis of the velocity increment over
a distance of 1 km significantly increases, which implies intermittency in the generation of 1-km scale eddies. The spatially
intermittent energy cascade generated by surface heterogeneity scaled down to 1-km eddies is analogous to the well-known intermittent
energy cascade in the inertial subrange. The kurtosis of the 1-km eddies is much larger with the 128-km surface heterogeneity
than with the 16- and 32-km heterogeneities. Thus we conclude that localized rapid changes of low-level horizontal wind speed
may be caused by significant local surface heterogeneity on scales between a few tens and a few hundreds of kilometres. 相似文献
10.
Wavelet and quadrant analyses were applied to turbulent velocity data in order to investigate the transition from the anisotropy of energy-containing eddies to the isotropy of the inertial subrange scales. The quadrant analysis of the wavelet coefficients of longitudinal and vertical velocity components allows the evaluation of the velocity structure functions and the momentum cospectrum as a function of the separation distance and of the quadrants. In an isotropic condition the contribution both of ejections and sweeps (even quadrants), and both of reflections and deflections (odd quadrants), has to be equal. The analysis has shown that in neutrally stratified conditions the transition to isotropy occurs in a frequency range (0.2 < r/z < 3) usually referred to as internal to the inertial subrange (r is separation distance, z is height). In the transition region, as in the isotropic region, the velocity structure functions still agree with the 1941 and 1962 Kolmogorov theories; but on the other hand the structure functions of the even and odd quadrants are fitted by power laws of different slopes in the transition region. The proposed analysis allows the investigation within the transition region of the different dynamical structure in the energy transfer from the energy-containing scales to the isotropic scales. 相似文献
11.
Analyses of concentration fluctuation (C) spectra from boundary-layer smoke plume experiments at six separate locations show that the spectra from these experiments generally exhibit an inertial subrange at high frequencies with a slope of -5/3 and indicate peak energy at a time period of about 50 to 100 s. These periods of peak energy are a factor of two to five less than those for the peak of the wind speed fluctuation (u or v) spectra. A general spectral formula fits normalized spectra from the U.S. and Australia, where the frequency, n, is made dimensionless by multiplying by the plume dispersion parameter,
y
, and dividing by the wind speed, u. Peak energy occurs at a dimensionless frequency of n
y/u equal to about 0.15. The Kolmogorov constant in the inertial subrange is estimated from a set of averaged spectra. Cross-spectra indicate little relation between concentration and wind fluctuations. However, most of the correlation that exists is due to periods larger than about 10 or 20 s. 相似文献
12.
13.
During the passage of a front, data from a light-weight cup anemometer and wind vane, sited in a steep-walled glacial valley of the Mt Cook region of the Southern Alps of New Zealand, were analysed to derive a power spectrum of the wind velocity for periods between 0.5 and 16 min. The energy spectrum roughly followed a -5/3 power law over the range of periods from 0.5–4 min — as might be expected in the case of an inertial subrange of eddies. However, any inertial subrange clearly does not extend to periods longer than this. We suggest that the observed eddies were generated in a turbulent wake associated with flow separation at the ridge crests, and large eddies are shed at periods of 4–8 min or more.A compressible fluid-dynamic model, with a Smagorinsky turbulence closure scheme and a law of the wall at the surface, was used to calculate flow over a cross section through this area in neutrally stratified conditions. A range of parameters was explored to assess some of the requirements for simulating surface wind gusts in mountainous terrain in New Zealand.In order to approximate the observed wind spectrum at Tasman aerodrome, Mount Cook, we found the model must be three-dimensional, with a horizontal resolution better than 250 m and with a Reynolds-stress eddy viscosity of less than 5 m2 s-1. In two-dimensional simulations, the eddies were too big in size and in amplitude and at the surface this was associated with reversed flow extending too far downstream. In contrast the three-dimensional simulations gave a realistic gusting effect associated with large scale cat's paws (a bigger variety of those commonly seen over water downstream of moderate hills), with reversed flow only at the steep part of the lee slope. The simulations were uniformly improved by better resolution, at all tested resolutions down to 250 m mesh size.The spectra of large eddies simulated in steep terrain were not very sensitive to the details of the eddy stress formulation. We suggest that this is because boundary-layer separation is forced in any case by terrain-induced pressure gradients. 相似文献
14.
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. 相似文献
15.
Gabriel G. Katul Marc B. Parlange John D. Albertson Chia R. Chu 《Boundary-Layer Meteorology》1995,72(1-2):123-148
Longitudinal velocity and temperature measurements above a uniform dry lakebed were used to investigate sources of eddy-motion anisotropy within the inertial subrange. Rather than simply test the adequacy of locally isotropic relations, we investigated directly the sources of anisotropy. These sources, in a daytime desert-like climate, include: (1) direct interaction between the large-scale and small-scale eddy motion, and (2) thermal effects on the small-scale eddy motion. In order to explore these two anisotropy sources, we developed statistical measures that are sensitive to such interactions. It was found that the large-scale/small-scale interaction was significant in the inertial subrange up to 3 decades below the production scale, thus reducing the validity of the local isotropy assumption. The anisotropy generated by thermal effects was also significant and comparable in magnitude to the former anisotropy source. However, this thermal anisotropy was opposite in sign and tended to counteract the anisotropy generated by the large-scale/smallscale interaction. The thermal anisotropy was attributed to organized ramp-like patterns in the temperature measurements. The impact of this anisotropy cancellation on the dynamics of inertial subrange eddy motion was also considered. For that purpose, the Kolmogorov-Obukhov structure function equation, as derived from the Navier-Stokes equations for locally isotropic turbulence, was employed. The Kolmogorov-Obukhov structure function equation in conjunction with Obukhov's constant skewness closure hypothesis reproduced the measured second- and third-order structure functions. Obukhov's constant skewness closure scheme, which is also based on the local isotropy assumption, was verified and was found to be in good agreement with the measurements. The accepted 0.4 constant skewness value derived from grid turbulence experiments overestimated our measurements. A suggested 0.26 constant skewness value, which we derived from Kolmogorov's constant, was found to be adequate. 相似文献
16.
Unlike previous studies on wind turbulence spectrum in the planetary boundary layer, this investigation focuses on high-altitude （1-5 km） wind energy spectrum and turbulence spectrum under various weather conditions. A fast Fourier transform （FFT） is used to calculate the wind energy and turbulence spectrum density at high altitudes （1-5 km） based on wind profiling radar （WPR） measurements. The turbulence spectrum under stable weather conditions at high altitudes is expressed in powers within a frequency range of 2 × 10-5-10-3 s-1, and the slope b is between -0.82 and -1.04, indicating that the turbulence is in the transition from the energetic area to the inertial sub-range. The features of strong weather are reflected less obviously in the wind energy spectrum than in the turbulence spectrum, with peaks showing up at different heights in the latter spectrum. Cold windy weather appears over a period of 1.5 days in the turbulence spectrum. Wide-range rainstorms exhibit two or three peaks in the spectrum over a period of 15-20 h, while in severe convective weather conditions, there are two peaks at 13 and 9 h. The results indicate that spectrum analysis of wind profiling radar measurements can be used as a supplemental and helpful method for weather analysis. 相似文献
17.
高分辨率GRAPES-GFS的动能谱及其转折特征分析 总被引：1，自引：1，他引：0
大量观测数据的计算表明,在对流层高层和平流层低层,大气动能谱在大尺度的斜率近似为k-3,到中尺度转换为近似k-5/3,动能谱的斜率及其转折特征可作为大气运动的基本规律检验数值模式。利用这一特征对GRAPES-GFS全球中期数值预报模式产品进行了检验评估。采用0.25°分辨率GRAPES-GFS模式2013年5月的预报场,分析了模式动能谱的垂直分布及动能谱的辐散分量和旋转分量。结果表明:在对流层高层和平流层低层(225—10 hPa),模式的动能谱曲线能较好地再现大气动能谱从大尺度过渡到中尺度的斜率转折特征,特别是在100 hPa附近动能谱的转折特征比较明显,但是动能谱斜率的绝对值较通常的观测结果(k-3和k-5/3)偏大;在大尺度范围,旋转分量在总动能谱中占优,随着波数的增大(尺度的减小)辐散分量逐渐与旋转分量相当甚至超过旋转分量。基于条件极值的思想提出一种计算转折点(拐点)的新方法,该方法能够较准确地计算出动能谱曲线拐点的位置,而且拐点位置随高度的变化与辐散和旋转分量谱交点位置随高度的变化一致。此外,还计算了辐散分量谱的斜率,发现其在中尺度范围内非常接近k-5/3, 验证了其对总动能谱转折过程中的关键作用。综上,GRAPES-GFS模式能够准确地再现大气动能谱由大尺度到中尺度的斜率转折特征;与旋转分量相比,模式对动能谱辐散分量的描述更为准确,而且模式对旋转分量的较大误差导致了总动能谱的斜率绝对值偏大。 相似文献
18.
Ann-Sofi Smedman-Högström 《Boundary-Layer Meteorology》1973,3(3):329-347
Temperature and humidity spectra have been measured at 3 and 12 m above the ground, together with profiles of wind, temperature and humidity, and flux measurements. Both temperature and humidity spectra appear to follow Monin-Obukov similarity as well as Kolmogorov's prediction for the inertial subrange. The standard deviations of temperature and humidity fluctuations support Monin-Obukov similarity and the predictions of local free convection. The spectral constants for the inertial subrange have been estimated as 0.8 for temperature and 0.6 for humidity. 相似文献
19.
ENERGY-INERTIAL SCALE INTERACTIONS FOR VELOCITY AND TEMPERATURE IN THE UNSTABLE ATMOSPHERIC SURFACE LAYER 总被引：1，自引：1，他引：0
Triaxial sonic anemometer velocity and temperature measurements were used to investigate the local structure of the velocity and temperature fluctuations in the unstable atmospheric surface layer above a grass-covered forest clearing. Despite the existence of a 2/3 power law in the longitudinal velocity (2 decades) and temperature (1 decade) structure functions, local isotropy within the inertial subrange was not attained by the temperature field, although a near-isotropic state was attained by the velocity field. It was found that sources of anisotropy were due to interactions between the large-scale and small-scaleeddy motion, and due to localvelocity-thermal interactions. Statistical measures were developed and used to quantify these types of interactions. Other types of interactions were also measured but were less significant. The temperature gradient skewness was measured and found to be non-zero in agreement with other laboratory flow types for inertial subrange scales. Despite these interactions and anisotropy sources in the local temperature field, Obukhovs 1949hypothesis for the mixed velocity-temperature structure functions was found to be valid. Finally, our measurements show that while a 2/3 power-law in the longitudinal velocity structure function developed at scales comparable to five times the height from the ground surface (z), near-isotropic conditions wereachieved at scales smaller than z/2. 相似文献
20.
We estimated the turbulent kinetic energy (TKE) dissipation rate for thirty-two 1-h intervals of unstable stratification covering the stability range 0.12 ≤ −z/L ≤ 43 (z/L is the ratio of instrument height to the Obukhov length), by fitting Kolmogorov’s inertial subrange spectrum to streamwise
spectra observed over a desert flat. Estimated values are compatible with the existence of local equilibrium, in that the
TKE dissipation rate approximately equalled the sum of shear and buoyant production rates. Only in the neutral limit was the
turbulent transport term in the TKE budget measured to be small. 相似文献