共查询到20条相似文献，搜索用时 675 毫秒
1.
Turbulence measurements performed at high frequencies yield data revealing intermittent and multi-scale processes. Analysing time series of turbulent variables thus requires extensive numerical treatment capable, for instance, of performing pattern recognition. This is particularly important in the case of the atmospheric surface layer and specifically in the vicinity of plant canopies, where largescale coherent motions play a major role in the dynamics of turbulent transport processes. In this paper, we examine the ability of the recently developedwavelet transform to extract information on turbulence structure from time series of wind velocities and scalars. It is introduced as a local transform performing a time-frequency representation of a given signal by a specific wavelet function; unlike the Fourier transform, it is well adapted to studying non-stationary signals. After the principles and the most relevant mathematical properties of wavelet functions and transform are given, we present various applications of relevance for our purpose: determination of time-scales, data reconstruction and filtering, and jump detection. Several wavelet functions are inter-compared, using simple artificially generated data presenting large-scale features similar to those observed over plant canopies. Their respective behaviour in the time-frequency domain leads us to assign a specific range of applications for each. 相似文献
2.
Latent and sensible heat flux predictions from a uniform pine forest using surface renewal and flux variance methods 总被引：1，自引：0，他引：1
Gabriel Katul Cheng-I Hsieh Ram Oren David Ellsworth Nathan Phillips 《Boundary-Layer Meteorology》1996,80(3):249-282
A surface renewal model that links organized eddy motion to the latent and sensible heat fluxes is tested with eddy correlation measurements carried out in a 13m tall uniform Loblolly pine plantation in Duke Forest, Durham, North Carolina. The surface renewal model is based on the occurance of ramp-like patterns in the scalar concentration measurements. To extract such ramp-like patterns from Eulerian scalar concentration measurements, a newly proposed time-frequency filtering scheme is developed and tested. The time-domain filtering is carried out using compactly-supported orthonormal wavelets in conjunction with the Universal Wavelet Thresholding approach of Donoho and Johnstone, while the frequency filtering is carried out by a band-pass sine filter centered around the ramp-occurrence frequency as proposed by other studies. The method was separately tested for heat and water vapour with good agreement between eddy correlation flux measurements and model predictions. The usefulness of the flux-variance method to predict sensible and latent heat fluxes is also considered. Our measurements suggest that the simple flux-variance method reproduces the measured heat and momentum fluxes despite the fact that the variances were measured within the roughness sublayer and not in the surface layer. Central to the predictions of water vapour fluxes using the flux-variance approach is the similarity between heat and water vapour transport by the turbulent air flow. This assumption is also investigated for this uniform forest terrain. 相似文献
3.
The presence of coherent structures in turbulent shear flows suggests order in apparently random flows. These coherent structures play an important dynamical role in momentum and scalar transport. To develop dynamical models describing the evolution of such motion, it is necessary to detect and isolate the coherent structures from the background fluctuations. In this paper, we decomposed atmospheric turbulence time series into large-scale eddies, which include coherent structures and small eddies, which are stochastic by using Fourier digital filtering. The wavelet energy computed for the three components of the velocity fluctuations in the large-scale eddies appears to have local maximum values at certain time scales, which correspond to the scales or frequencies of coherent structures. We extract coherent signals from large-scale vortices at this scale by inverse wavelet transform formulae. This method provides an objective technique for examining the turbulence signal associated with coherent structures in the atmospheric boundary layer. The average duration of coherent structures in three directions based on Mexican hat wavelets are 33 s, 34 s and 25 s respectively. Symmetric andanti-symmetric wavelet basis functions give almost the same results. The main features of the structures during the day and night have little difference. The dimensionless durations for u, v and w have linear correlations with each other. These relationships are insensitive to the wavelet basis. 相似文献
4.
介绍了小波变换和维纳滤波相结合的图像小波域维纳滤波去噪算法，在分析该算法最小均方误差期望的基础上提出一种改进算法。指出在进行维纳滤波之前，阈值化处理可以有效提高维纳滤波的性能。用标准测试图像的处理结果来比较改进前后两种算法的效果。 相似文献
5.
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
6.
高风速相干结构对通量输送影响的实验研究 总被引：2，自引：0，他引：2
切变湍流的相干结构是湍流研究中的重大发现,它表明湍流运动并非完全随机,其中具有可检测的有序结构.本文通过处理南京浦口地区大气边界层观测数据,来分析不稳定层结中高风速相干结构特征.本次观测项目包括对场地中央的气象铁塔上2 m和40 m高度上超声风速仪的脉动速度、温度测量以及风廓线雷达对边界层风速廓线的测量.对超声水平风速时间序列数据进行小波变换 (时间尺度400 s),通过阈值来识别这种高风速相干结构.与多普勒风廓线雷达测量结果对比后发现,这种方法确定的相干结构符合常规的认识,具有较长的时间尺度和较大的垂直尺度 (接近边界层厚度).分析三天相干结构特性得到无量纲空间间隔约为6,即每隔6个边界层厚度的水平位置出现一个高速相干结构.通过与垂直风速小波系数的比较,发现高风速相干结构与向下垂直风速之间有较好相关,这与湍流中 “阵风” 现象的研究结论相似.使用四象限分析方法分类得到两种动量通量输送为负的运动:较小水平风速的上扬 (ejection) 运动 (简称为上扬运动) 和较大水平风速的下扫 (sweep) 运动 (简称为下扫运动),这两种运动在整个湍流活动中处于主导地位.高风速相干结构通过促进下扫运动和抑制上扬运动来影响动量通量的输送. 相似文献
7.
Wind-Tunnel Experiment on Logarithmic-Layer Turbulence under the Influence of Overlying Detached Eddies 总被引：1，自引：1，他引：0
Yasuo Hattori Chin-Hoh Moeng Hitoshi Suto Nobukazu Tanaka Hiromaru Hirakuchi 《Boundary-Layer Meteorology》2010,134(2):269-283
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. 相似文献
8.
Impact of Terrain Heterogeneity on Coherent Structure Properties: Numerical Approach 总被引：2，自引：2，他引：0
Clement Fesquet Sylvain Dupont Philippe Drobinski Thomas Dubos Christian Barthlott 《Boundary-Layer Meteorology》2009,133(1):71-92
A three-dimensional large-eddy simulation (LES) model, which includes the effects of plant–atmosphere interactions, is used
to study the effects of surface inhomogeneities on near-surface coherent structures over an open field and behind a forest
canopy. These simulated conditions are representative of two wind sectors of the Site Instrumental de Recherche par Télédétection
Atmosphérique (SIRTA) experimental site at the Institut Pierre Simon Laplace, Palaiseau, France. Coherent structure properties
deduced from wavelet transforms of the simulated near-surface vertical velocity time series are not modified by upstream terrain
heterogeneities, in agreement with site measurements. This feature is related to the nature of structures detected from the
vertical velocity time series. The turbulence close to the surface seems composed of both local coherent structures and large
coherent structures reflecting outer-layer properties, which depend on the overall surface heterogeneity or upstream heterogeneity.
It is argued that the streamwise velocity is representative of these large outer-layer structures that impinge onto the ground
through a top-down mechanism as identified through the space–time correlation of the wind velocity components. In contrast,
the vertical velocity is more representative of small structures resulting from the impingement of the large outer-layer structures.
These small structures represent locally-generated, active turbulence, which adjusts rapidly to local surface conditions,
and consequently they are only weakly dependent on upstream heterogeneities. 相似文献
9.
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
10.
Ship-board measurements and estimations of air-sea fluxes in the western tropical pacific during TOGA COARE 总被引：1，自引：0，他引：1
Direct air-sea flux measurements were made on R/V Kexue #1 at 4 ° S, 156 ° E during the Tropical Ocean Global Atmosphere (TOGA)
Coupled Ocean-Atmospheric Response Experiment (COARE) Intensive Observation Period (IOP). An array of six accelerometers was
used to measure the motion of the anchored ship, and a sonic anemometer and Lyman-α hygrometer were used to measure the turbulent
wind vector and specific humidity. The contamination of the turbulent wind components by ship motion was largely removed by
an improvement of a procedure due to Shao based on the acceleration signals. The scheme of the wind correction for ship motion
is briefly outlined. Results are presented from data for the best wind direction relative to the ship to minimize flow distortion
effects. Both the time series and the power spectra of the sonic-measured wind components show swell-induced ship motion contamination,
which is largely removed by the accelerometer correction scheme. There was less contamination in the longitudinal wind component
than in the vertical and transverse components. The spectral characteristics of the surface-layer turbulence properties are
compared with those from previous land and ocean results. Momentum and latent heat fluxes were calculated by eddy correlation
and compared to those estimated by the inertial dissipation method and the TOGA COARE bulk formula. The estimations of wind
stress determined by eddy correlation are smaller than those from the TOGA COARE bulk formula, especially for higher wind
speeds, while those from the bulk formula and inertial dissipation technique are generally in agreement. The estimations of
latent heat flux from the three different methods are in reasonable agreement. The effect of the correction for ship motion
on latent heat fluxes is not as large as on momentum fluxes. 相似文献
11.
郑州近54年降水变化的多时间尺度分析 总被引：5，自引：0，他引：5
利用Morlet小波分析方法,对1951~2004年郑州市年降水距平时间序列的小波变化特征进行分析,揭示降水变化的多时间尺度结构,分析其中存在的主要周期振荡和突变点,并与功率谱分析结果进行比较。结果表明:郑州市年降水存在多时间尺度的周期变化特征,郑州市年平均降水存在3 a、准5 a、准9 a和准21 a的主周期。郑州市降水变化存在着明显的多时间尺度,即年代际尺度和年际尺度的周期性变化。郑州市降水变化还显示出自己的突变点分布及其位相结构。在降水量总趋势下降的前提下,2004年后的一段时间还将处在一个偏多期。 相似文献
12.
多尺度大气湍流的扩散及扩散率 总被引：5，自引：3，他引：5
本文首先将大气湍流多尺度化,而将不同尺度的脉动量的Monte Carlo关系转换为Lange-vin方程,在平稳及均匀条件下可得一高阶常系数Lagrange自相关函数的常微分方程。其通解恰为一组衰减实指数函数的线性组合,系数为各尺度湍能对总湍能的比。以此可通过Taylor积分对野外实测扩散参数作出尺度分解。并可方便地将扩散参数表达为湍流参数的函数使其物理意义明确。 相似文献
13.
14.
该文分析了Torrence等人研究的墨西哥帽 (Mexican Hat) 小波变换的影响域和高频失真问题并探讨了解决方案。平移点b处、小波尺度为a的墨西哥帽小波函数的有效定义域为[ b-2.12a, b+2.12a], 以墨西哥帽为母函数的小波变换的真正影响域是2.12a, 小波尺度a的最大值应为N/4.24(N为时间序列长度)。提出了充分利用小波函数速降性质和立方样条插值小波系数的新计算方案, 消除了该计算方案产生虚假的显著高频周期振荡问题。利用正弦函数型时间序列小波变换的解析式检验了文中提出的新计算方案的合理性。利用新计算方案对冬季Ni?o3.4指数进行分析, 冬季Ni?o3.4指数的平均整体小波功率谱表明, 存在约12年的年代际变化和准4年的年际变化, 不存在显著的准两年周期振荡。 相似文献
15.
Theory And Measurements For Turbulence Spectra And Variances In The Atmospheric Neutral Surface Layer 总被引：20，自引：17，他引：3
Predictions from a new theory for high Reynolds number turbulent boundary layers during near-neutral conditions are shown to agree well with measurements of atmospheric surface-layer variances and spectra. The theory suggests surface-layer turbulence is determined by detached eddies that largely originate in the shearing motion immediately above the surface layer; as they descend into this layer, they are strongly distorted by the local shear and impinge onto the surface. Because the origin of these eddies is non-local, they are similar to those described in previous studies as `inactive' turbulence. However, they are, in fact, dynamically highly active, supplying the major mechanism for the momentum transport, including upward bursting on the time scale of the larger eddies. The vertical velocity results show that the variance and the low frequency parts of spectra increase with height in the surface layer, while in the self similar (k1
-1) range the streamwise low frequency components are approximately constant with height. These large-scale longitudinal eddies extend to a length s, which is equal to the boundary-layer height near the surface andincreases linearly to a maximum of about three times the boundary-layer height at roughly 15 m and decreases in the upper parts of the surface layer. This lower part of the surface layer, the eddy surface layer, is the region in which the eddies impinging from layers above are strongly distorted. This new result for the atmospheric boundary layer has practical application for calculating fluctuating wind loads on structures and lateral dispersion of pollution from local sources. 相似文献
16.
基于大涡模拟评估GRAPES模式对对流边界层的模拟性能 总被引：3，自引：1，他引：2
为检验GRAPES半拉格朗日动力框架在大涡尺度上的模拟性能,为未来发展千米及其以下高分辨尺度的数值模式奠定基础,并构造GRAPES大涡模式以检验和发展边界层湍流参数化提供科学工具。通过在GRAPES模式中加入Smagorinsky-Lilly小尺度湍涡参数化,并将模式分辨率提高至50 m,构建GRAPES大涡模式(GRAPES_LES),以便分析GRAPES模式在大涡尺度上的适用性。同时利用广泛应用的已有大涡模式UCLA_LES作为参考,通过对干对流边界层湍流的模拟分析及与UCLA_LES模拟结果的对比,得出如下主要结论:GRAPES半拉格朗日动力框架能够模拟出与已有的大涡模式相似的边界层湍流特征;同时,通过分析也证明GRAPES存在由于采用半拉格朗日平流计算而带来过度耗散的问题:当使用相同的滤波尺度(Smagorinsky 常数)时,GRAPES_LES模拟出的速度场更为平滑,小尺度湍流结构过于光滑,通过对湍流能量的能谱分析更清楚地表明了这一点。进一步,对不同的Smagorinsky常数(对应不同的滤波尺度)进行了敏感性试验,表明可以通过改变滤波尺度,有效地缓解半拉格朗日框架隐含的耗散问题,得到更接近UCLA_LES所模拟的湍流特征。 相似文献
17.
Mitsuaki Horiguchi Taiichi Hayashi Ahoro Adachi Shigeru Onogi 《Boundary-Layer Meteorology》2012,144(2):179-198
Large-scale turbulence structures in the near-neutral atmospheric boundary layer (ABL) are investigated on the basis of observations made from the 213-m tall meteorological tower at Tsukuba, Japan. Vertical profiles of wind speed and turbulent fluxes in the ABL were obtained with sonic anemometer-thermometers at six levels of the tower. From the archived data, 31 near-neutral cases are selected for the analysis of turbulence structures. For the typical case, event detection by the integral wavelet transform with a large time scale (180 s) from the streamwise velocity component (u) at the highest level (200 m) reveals a descending high-speed structure with a time scale of approximately 100 s (a spatial scale of 1 km at the 200-m height). By applying the wavelet transform to the u velocity component at each level, the intermittent appearance of large-scale high-speed structures extending also in the vertical is detected. These structures usually make a large contribution to the downward momentum transfer and induce the enhancement of turbulent kinetic energy. This behaviour is like that of “active” turbulent motions. From the analysis of the two-point space–time correlation of wavelet coefficients for the u velocity component, the vertical extent and the downward influence of large-scale structures are examined. Large fluctuations in the large-scale range (wavelet variance at the selected time scale) at the 200-m level tend to induce the large correlation between the higher and lower levels. 相似文献
18.
Characteristics of intermittent turbulence events in the stably stratified nocturnal boundary layer are investigated with data collected in the CASES-99 tower array of 300-m radius. The array consists of a central 60-m tower with eddy covariance measurements at eight levels and six satellite towers with eddy covariance measurements at 5 m. A significant increase in the magnitude of vertical wind velocity () and spectral information are used to define the onset of an intermittent turbulence event. Normally, only a subset of 5 m-levels in the tower network experience an intermittent turbulence event concurrent with one at the 5 m-level on the main tower. This behaviour reveals the small horizontal extent of most events. Intermittent turbulence events at the main tower 5-m level are normally confined to a layer much thinner than the 60-m tower height. The turbulent kinetic energy budget is evaluated for intermittent turbulence events observed at the 5-m level on the main tower. Generally, the onset of an intermittent turbulence event is not closely related to the reduction of the gradient Richardson number below 0.25, the critical Richardson number of turbulence generation for linear instability. Possible explanations including the influence of advected turbulence patches are discussed. 相似文献
19.
Orthonormal wavelet expansions are applied to surface-layer measurements of vertical wind speed under various atmospheric, stability conditions. The orthonormal wavelet transform allows for the unfolding of these measurements into space and scale simultaneously to reveal the large intermittent behavior in space for the turbulent production wavenumbers. Both Fourier and wavelet power spectra indicated the existence of a –1 power law for the vertical velocity measurements at the production wavenumbers. The –1 power law in the turbulent production range was derived from surface-layer similarity theory. A dimensionless skewness, structure function is applied to the wavelet decomposed vertical velocity field to trace the destruction of the shear-or buoyancy-induced anisotropy under various stability conditions. The structure skewness function revealed shear- or buoyancy-induced eddy asymmetry dependence on stability at each scale within the –1 power-law wavenumber range with more isotropy during propagation from smaller to larger wavenumbers. The asymmetry of these events at the turbulent production wavenumbers appeared very localized in space, as well as in scale, and could be described with a simple eddy-overturning model. It is demonstrated that the wavelet transform is suitable for such analysis. 相似文献
20.
Organised Motion and Radiative Perturbations in the Nocturnal Canopy Sublayer above an Even-Aged Pine Forest 总被引：2，自引：3，他引：2
Using time series measurements of velocity, carbon dioxideand water vapour concentration, and temperature collected justabove a 15 m tall even-aged pine forest, we quantify the roleof organized motion on scalar and momentum transport withinthe nocturnal canopy sublayer (CSL). We propose a frameworkin which the nocturnal CSL has two end-members, bothdominated by organised motion. These end-members representfully developed turbulent flows at near-neutral or slightly stablestratification and no turbulence for very stable stratification.Our analysis suggests that ramps dominate scalar transport fornear-neutral and slightly stable conditions, while linear canopywaves dominate the flow dynamics for very stable conditions.For intermediate stability, the turbulence is highly damped andoften dominated by fine scale motions. Co-spectral analysissuggests that ramps are the most efficient net scalar mass-transportingagent while linear canopy waves contribute little to net scalartransport between the canopy and atmosphere for averagingintervals that include complete wave cycles. However, canopywaves significantly contribute to the spectral properties of thescalar time series. Ramps are the most frequently occurringorganised motion in the nocturnal CSL for this site.Numerous night-time runs, however, resided between thesetwo end-members. Our analysis suggests that whenradiative perturbations are sufficient large (>20 W m-2 innet radiation), the flow can switch from being highly dampedfine-scale turbulence to being organized with ramp-like properties. We also found that when ramps are already the dominant eddymotion in the nocturnal CSL, radiative perturbations have aminor impact on scalar transport. Finally, in agreement withprevious studies, we found that ramps and canopy waves havecomparable length scales of about 30–60 metres. Consequencesto night-time flux averaging are also discussed. 相似文献