大气边界层湍流涡旋结构的小波分解

利用离散正交小波在若干物理判别准则下对边界层湍流脉动信号进行去噪和多尺度分解,从而有效地区分出均匀各向同性小涡成份和大尺度含能涡旋成份。能谱分析发现,小涡的谱动力学行为具有非常好的标度不变性,标度关系满足Kolmogorov的“-5／3”律。       

用连续子波变换提取城市冠层大气湍流的相干结构

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.     

大气边界层湍流多尺度分形特征的研究

运用离散正交小波变换将湍流信号分解为不同尺度，计算其分数维。考察其分数维的变化得出：随着分解层次得增加，提取湍流信号得低频部分趋于简单光滑，分数维不断减小，高频部分呈现复杂，分数维趋于定值，平均为1．70左右。说明大气边界层湍流信号在某些尺度上，存在明显的自相似性特征。     

高风速相干结构对通量输送影响的实验研究

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.     

0814号强台风“黑格比”风场相干结构统计和演化特征研究

为了探究台风风场相干结构统计特性以及在台风不同空间结构部位的演化规律,采用小波系数谱分析(Spavelet Analysis)方法对0814号强台风“黑格比”风速场的湍流相干结构统计特征及演化特性进行了系统分析。研究结果表明：对于0814号强台风“黑格比”过程,顺风向脉动风速与横风向脉动风速的相干结构呈现出多尺度分布特征;顺风向、横风向和竖向风速分量的相干结构主周期均值分别为70.2 s、50.3 s、22.8 s,主尺度均值分别为54.4 s、38.9 s、18.2 s,三个方向相干结构的周期尺度比均值分别为1.28、1.27、1.29;台风“黑格比”风场水平向空间部位中,前外环流区(FOV)的顺风向、横风向和竖向的相干结构主尺度均值分别为68.8 s、41.7 s、14.4 s;前眼壁区(FEW)的三个方向脉动风速的相干结构主尺度均值分别为69.9 s、32.5 s、19.5 s;风眼区三个方向脉动风速的相干结构主尺度分别为83.5 s、50.3 s、15.3 s;后眼壁区(BEW)其值为55.1 s、42.3 s、11.2 s;后外环流区(BOV)三个方向脉动风速相干结构的主尺度分别为53.6 s、39.1 s、10.9 s;整体上表现为前外环流区(FOV)和前眼壁区(FEW)相干结构主尺度大于后眼壁区(BEW)和后外环流区(BOV),风眼区相干结构主尺度则为最大。     

大气边界层阵风相干结构的产生条件

壁湍流相干结构的发现是近代湍流研究的重大进展之一,从20世纪50年代开始,在大气边界层湍流中也发现了相干结构——对流云街,并进行了系统的研究。近些年来,人们发现在近地层湍流中也存在相干结构。利用北京325 m气象塔对城市下垫面中大风和小风天气的风速分析,发现较有规律的周期3~6 min的阵风,且有明显的相干结构,而对不同下垫面的阵风研究,均发现存在这种相干结构,这种阵风相干结构对通量输送有不可忽视的作用。本文利用2012年4月甘肃省民勤县巴丹吉林沙漠观测塔的超声风速和平均场风速、温度观测资料,对阵风相干结构的产生条件进行了分析。采用傅立叶变换,将三维超声风速按频率分成基流(周期10分钟以上)、阵风扰动(周期1到10分钟)、湍流脉动(周期小于1分钟)三部分,结合平均场的资料分析发现:阵风相干结构出现在静力中性、不稳定甚至略微稳定的条件下,或者说机械作用主导的大气边界层,阵风区就会出现相干结构,热力作用对其有抑制和干扰的作用。从而,阵风的相干结构和壁面相干结构都出现在中性条件下,是机械湍流的现象,都主导着动量能量的输运。阵风区的相干结构并不等同于对流云街,他们出现在不同的大气稳定度条件下且尺度不同。     

大气边界层剪切湍流统计特性的风洞实验及其层次相似律

对风洞模拟的大气边界层的剪切湍流场的速度脉动时间序列信号以及某一高度位置上的小波变换分解重构的信号进行了系统的统计分析.结果表明, 在非均匀下垫面大气边界层的不同垂直高度上的信号, She-Leveque湍流层次相似律都成立, 层次相似指数β和最高激发态的奇异标度指数γ随垂直高度的变化而变化, 并给出不同尺度的分解重构信号的一些层次结构特点.     

正交小波变换研究复杂下垫面边界层的湍流特征

小波变换方法具有较好的时频局部特性,非常适合于分析非平稳的湍流信号。本文对35 m铁塔的超声风速测量数据进行了离散正交小波变换,计算了各向同性系数(ISO isotropy coefficient)及小波功率谱,以此对实验场所代表的水陆交际复杂下垫面的近地面层湍流特征进行了研究。ISO系数可以很好地描述实际大气在不同尺度的各向同性特征。根据ISO系数的分布,我们可以通过设定一定的阈值(ISO=0.7)来得到各向同性小涡的分离尺度(记为ISO0.7),即湍流的各向同性尺度范围。研究表明,下垫面对于边界层湍流各向同性特性具有相当大的影响,当风从不同下垫面吹过时ISO0.7尺度的均值具有较明显的差异。同时湍流度和稳定度对湍流各向同性也有一定的影响。对湍流小波功率谱研究表明,功率谱谱幂率在分离出的小涡所处的频段接近于-5/3。而当风从陆面吹过且湍流度较弱时,湍流小波功率谱小涡以分离尺度所对应的频率(fms)为界具有明显的两段趋势:高于fms的频段,谱幂率一般接近-5/3;而低于fms的频段的谱幂率接近-3/3。这反映了下垫面特征对湍流功率谱的影响。同时在不同的风向转变过程中下垫面对功率谱的影响具有差异。     

Hilbert方法在大气边界层湍流特征分析中的有效性

介绍一种新的建立在经验模态分解(EMD)方法基础上的非线性、非平稳数据分析技术一Hilbert分析技术,并首次将其应用于大气边界层(PBL)湍流数据的分析,初步探讨了其在PBL湍流研究中的有效性.通过对城市与森林冠层上湍流资料的能量分布特征和统计平稳度进行分析、比较,结果表明:Hilbert谱分析能有效地对PBL湍流信号进行分析.它的边缘谱分析能够有效地探测PBL湍流信号的能量分布特征,统计平稳度分析也能有效地给出PBL湍流信号平稳性的定量化测量,这些将有助于建立合适的数据质量控制方法,以及对现有空气质量与扩散模式中扩散参数的计算加以改进.文中个例分析中,城市和森林冠层上空的湍流有一定相似性,湍流混合都比较充分,但森林冠层上湍流信号的能量更多地集中在大尺度湍涡,且扰动风速的高频部分具有更强的间歇性.对于相近高度的湍流信号来说,多数情况下,森林冠层上相同尺度的湍涡表现得比城市冠层上更不稳定,但湍涡的含能量要更低.     

用小波系数谱方法分析湍流湿度脉动的相干结构

小波系数谱分析方法是结合小波分析和高分辨率谱分析的一种统计方法,可以用来同时识别时间序列中相干结构的生命尺度和出现周期,可以很好地描述相干结构的演变过程。基于此方法,作者分析了2004年11月在河北省白洋淀地区的陆地和岛上两个观测点（分别代表陆地和水面两种不同下垫面）湍流湿度脉动的相干结构特征,结果表明陆地和水上湿度序列的相干结构尺度分布相似,并且尺度与周期之间的关系一致:小于5 s的相干结构不连续出现,而且通常伴有更大尺度的相干结构,而5~30s的相干结构有与其尺度差不多的周期。在寻找更大尺度的相干结构时发现存在一个尺度,当大于某个周期时,在各个周期上这个尺度的相干结构都显著;与正交小波变换识别相干结构主尺度的方法识别的相干结构主尺度一致。另外,小尺度结构不连续出现也可以解释小尺度湍流能量变化比较大。     

The Effects of Vegetation Density on Coherent Turbulent Structures within the Canopy Sublayer: A Large-Eddy Simulation Study

Large-eddy simulation has become an important tool for the study of the atmospheric boundary layer. However, since large-eddy simulation does not simulate small scales, which do interact to some degree with large scales, and does not explicitly resolve the viscous sublayer, it is reasonable to ask if these limitations affect significantly the ability of large-eddy simulation to simulate large-scale coherent structures. This issue is investigated here through the analysis of simulated coherent structures with the proper orthogonal decomposition technique. We compare large-eddy simulation of the atmospheric boundary layer with direct numerical simulation of channel flow. Despite the differences of the two flow types it is expected that the atmospheric boundary layer should exhibit similar structures as those in the channel flow, since these large-scale coherent structures arise from the same primary instability generated by the interaction of the mean flow with the wall surface in both flows. It is shown here that several important similarities are present in the two simulations: (i) coherent structures in the spanwise-vertical plane consist of a strong ejection between a pair of counter-rotating vortices; (ii) each vortex in the pair is inclined from the wall in the spanwise direction with a tilt angle of approximately 45°; (iii) the vortex pair curves up in the streamwise direction. Overall, this comparison adds further confidence in the ability of large-eddy simulation to produce large-scale structures even when wall models are used. Truncated reconstruction of instantaneous turbulent fields is carried out, testing the ability of the proper orthogonal decomposition technique to approximate the original turbulent field with only a few of the most important eigenmodes. It is observed that the proper orthogonal decomposition reconstructs the turbulent kinetic energy more efficiently than the vorticity.     

Analysis of Coherent Structures Within the Atmospheric Boundary Layer

Large-eddy simulation has become an important tool for the study of the atmospheric boundary layer. However, since large-eddy simulation does not simulate small scales, which do interact to some degree with large scales, and does not explicitly resolve the viscous sublayer, it is reasonable to ask if these limitations affect significantly the ability of large-eddy simulation to simulate large-scale coherent structures. This issue is investigated here through the analysis of simulated coherent structures with the proper orthogonal decomposition technique. We compare large-eddy simulation of the atmospheric boundary layer with direct numerical simulation of channel flow. Despite the differences of the two flow types it is expected that the atmospheric boundary layer should exhibit similar structures as those in the channel flow, since these large-scale coherent structures arise from the same primary instability generated by the interaction of the mean flow with the wall surface in both flows. It is shown here that several important similarities are present in the two simulations: (i) coherent structures in the spanwise-vertical plane consist of a strong ejection between a pair of counter-rotating vortices; (ii) each vortex in the pair is inclined from the wall in the spanwise direction with a tilt angle of approximately 45°; (iii) the vortex pair curves up in the streamwise direction. Overall, this comparison adds further confidence in the ability of large-eddy simulation to produce large-scale structures even when wall models are used. Truncated reconstruction of instantaneous turbulent fields is carried out, testing the ability of the proper orthogonal decomposition technique to approximate the original turbulent field with only a few of the most important eigenmodes. It is observed that the proper orthogonal decomposition reconstructs the turbulent kinetic energy more efficiently than the vorticity.     

Coherent Structures Detected in Atmospheric Boundary-layer Turbulence using Wavelet transforms at Huaihe River Basin, China

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.     

Detecting Near-Surface Coherent Structure Characteristics Using Wavelet Transform with Different Meteorological Elements

In the present study, three wavelet basis functions (Mexican-hat, Morlet, and Wave) were used to analyze atmospheric turbulence data obtained from an eddy covariance system in order to determine effect of six meteorological elements (three-dimensional wind speed, temperature, and CO2 and H2O concentrations) on the time scale of coherent structures. First, we used the degree of correlation between original and reconstructed waveforms to test the three wavelets’ performance when determining the time scale of coherent structures. The Wave wavelet’s reconstructed coherent structure signal best matched the original signal; thus, it was used for further analysis of the time scale, number, and time cover of the meteorological elements. We found similar results for all elements, though there was some internal variation, suggesting that coherent structures are not inherently dependent on these elements. Our results provide a basis for proper coherent structure detection in atmospheric turbulence and improve the understanding of similarities and differences between coherent structure characteristics of different meteorological elements, which is helpful for further research into atmospheric turbulence and boundary layers.     

A Physically-Based Turbulent Velocity Time Series Decomposition

Single-point, three-component turbulent velocity time series data obtained in the atmospheric boundary layer over the ocean reveal coherent structures that are consistent with a model of a steady linearly varying spatial velocity field that translates past the measurement point at constant velocity. The kinematic model includes both strain and rotation rates and has implications regarding vortex generation, vortex pairing, vortex break-up, and stability. While the complete specification of the dimensions, spatial velocity gradients, and translational velocity of the linear coherent structure (LCS) cannot be made from the single-point, three-component measurements, the model LCS velocity time series can be determined from least- squares fits to the data. The total turbulent kinetic energy is used to find in the record the initial and final times of a model LCS in the data, i.e., the time interval over which a model LCS is passing over the anemometer. Maxima in the kinetic energy removed from the data (by subtraction of the model LCS velocity functions from the data) are used to identify the most-energetic model LCSs. These model LCS velocity functions replicate the essential large-scale features of the time series of the three-component velocity fluctuations, most noticeably in the streamwise component. The model LCS decomposition was used to perform a scale analysis of the data, which was compared to the usual Fourier method. Time intervals of model LCSs were found successively in the data, after subtracting the previous fits. This process resulted in a series of 'levels with a number of LCSs found at each level. About six levels account for most of the kinetic energy. The model also allows the computation of the Reynolds stress components, for which six levels also are sufficient. The recomposition of the time series on a LCS-by-LCS basis compares well with the mode-by-mode Fourier recomposition for the average momentum fluxes and kinetic energy.     

Structure Function Analysis of Two-Scale Scalar Ramps. Part II: Ramp Characteristics and Surface Renewal Flux Estimation

Ramp features in the turbulent scalar field are associated with turbulent coherent structures, which dominate energy and mass fluxes in the atmospheric surface layer. Although finer scale ramp-like shapes embedded within larger scale ramp-like shapes can readily be perceived in turbulent scalar traces, their presence has largely been overlooked in the literature. We demonstrate the signature of more than one ramp scale in structure functions of the turbulent scalar field measured from above bare ground and two types of short plant canopies, using structure-function time lags ranging in scale from isotropic to larger than the characteristic coherent structures. Spectral analysis of structure functions was used to characterize different scales of turbulent structures. By expanding structure function analysis to include two ramp scales, we characterized the intermittency, duration, and surface renewal flux contribution of the smallest (i.e., Scale One) and the dominant (i.e., Scale Two) coherent structure scales. The frequencies of the coherent structure scales increase with mean wind shear, implying that both Scale One and Scale Two are shear-driven. The embedded Scale One turbulent structure scale is ineffectual in the surface-layer energy and mass transport process. The new method reported here for obtaining surface renewal-based scalar exchange works well over bare ground and short canopies under unstable conditions, effectively eliminating the α calibration for these conditions and forming the foundation for analysis over taller and more complex surfaces.     

Detection of long-term coherent exchange over spruce forest using wavelet analysis

Summary This study presents a discussion of a method for automated and quasi-online analysis of coherent structures using wavelet transform. The method is optimised for rapid processing of vector and scalar variables obtained over tall vegetation. It has been designed to assess long-term statistics of coherent structures, as it is applicable over a wide range of atmospheric conditions. Data of artificial and real turbulent signals are used to perform the analysis and to evaluate the presented method.Different wavelet functions are used for filtering the original signals, determining characteristic time scales, and detecting individual coherent structures. On this basis, statistics of temporal separation of coherent structures and phase shift between different variables can be calculated.Background turbulence and spikes are found to be efficiently removed without changing the shape, particularly the sharp localised gradients, of coherent structures. The determined peak in the calculated wavelet variance spectrum is observed to correspond very well to characteristic event durations and to satisfy the definition of coherent structures present in vector and scalar variables. The detection algorithm was successful in analysing data covering a wide range of atmospheric conditions. Detected individual coherent structures provide a parallel temporal pattern for scalar variables, but a phase shift between scalar and vector components.