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.     

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.     

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。     

近壁湍流边界层相干结构形成的动力学分析

以壁面脉冲扰动来构建湍流边界层近壁区对称与非对称相干结构理论模式,采用直接数值模拟的方法,研究了湍流边界层近壁区对称与非对称相干结构形成的理论机制和演化规律.计算结果显示非对称相干结构比对称相干结构更容易增长,这是由于非对称型结构能在湍流边界层近壁区域诱导产生较大的展向扰动速度,且不同初始结构类型分布对湍流边界层近壁区相干结构形成的动力学机制不同所致,从而加深了对近壁湍流边界层单个相干结构形成的某种动力学机制的认识理解.     

Observations of Coherent Turbulence Structures in the Near-Neutral Atmospheric Boundary Layer

Turbulence structures of high Reynolds number flow in the near-neutral atmospheric boundary layer (ABL) are investigated based on observations at Shionomisaki and Shigaraki, Japan. A Doppler sodar measured the vertical profiles of winds in the ABL. Using the integral wavelet transform for the time series of surface wind data, the pattern of a descending high-speed structure with large vertical extent (from the surface to more than 200-m level) is depicted from the Doppler sodar data. Essentially this structure is a specific type of coherent structure that has been previously shown in experiments on turbulent boundary-layer flows. Large-scale high-speed structures in the ABL are extracted using a long time scale (240 s) for the wavelet transform. The non-dimensional interval of time between structures is evaluated as 3.0–6.2 in most cases. These structures make a large contribution to downward momentum transfer in the surface layer. Quadrant analyses of the turbulent motion measured by the sonic anemometer (20-m height) suggest that the sweep motion (high-speed downward motion) plays a substantial role in the downward momentum transfer. In general, the contribution of sweep motions to the momentum flux is nearly equal to that of ejection motions (low-speed upward motions). This contribution of sweep motions is related to the large-scale high-speed structures.     

Techniques for the Eduction of Coherent Structures from Flow Measurements in the Atmospheric Boundary Layer

Two empirical methods to detect coherent motions embedded in the flow field have been compared, namely the variable interval time average (VITA) method and a wavelet-based technique, both with artificial signals as well as velocity measurements from the atmospheric boundary layer over a forest canopy. It has been found that the wavelet method is slightly better than the VITA approach in coherent structure eduction, even if the results of both techniques are comparable. Also the application of the present approach to simultaneous conditionally sampled wind data has highlighted some important features of coherent structures and gust generation in canopy flows.     

Impact of terrain heterogeneity on near-surface turbulence structure

This study investigates the impact of terrain heterogeneity on local turbulence measurements using 18 months of turbulence data taken on a 30 m tower at the SIRTA mixed land-use observatory under varying stability conditions and fetch configurations. These measurements show that turbulence variables such as the turbulent kinetic energy or momentum fluxes are strongly dependent on the upstream complexity of the terrain (presence of trees or buildings, open field). However, using a detection technique based on wavelet transforms which permits the isolation of the large-scale coherent structures from small-scale background fluctuations, the study shows that, for all stability conditions, whatever the upstream complexity of the terrain, the coherent structures display universal properties which are independent of the terrain nature: the frequency of occurrence, time duration of the coherent structures, the time separation between coherent structures and the relative contribution of the coherent structures to the total fluxes (momentum and heat) appear to be independent of the upstream roughness. This is an important result since coherent structures are known to transport a large portion of the total energy. This study extends to all stability conditions a numerical study by Fesquet et al. [Fesquet, C., Dupont, S., Drobinski, P., Barthlott, C., Dubos, T., 2008. Impact of terrain heterogeneities on coherent structures properties: experimental and numerical approaches. In: 18th Symposium on Boundary Layers and Turbulence. No. 11B.1. Stockholm, Sweden., Fesquet, C., Dupont, S., Drobinski, P., Dubos, T., Barthlott, C., in press. Impact of terrain heterogeneity on coherent structure properties: numerical approach. Bound.-Layer Meteorol.] conducted in neutral conditions which shows that a reason for such behavior is that the production of local active turbulence in an internal boundary layer associated with coherent structure originating from the outer layer and impinging onto the ground is not sensitive to the nature of the terrain.     

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。     

Coherent structures and flux contribution over an inhomogeneously irrigated cotton field

The turbulence data measured at two levels (i.e., 8.7 and 2.7?m) in the Energy Balance Experiment (EBEX), which was conducted in San Joaquin Valley in California during the period from July 20 to August 24, 2000, are used to study the characteristics of coherent structures over an irrigated cotton field. Patch-to-patch irrigation in the field generated the dry-to-wet horizontal advection and the oasis effects, leading to the development of a stably internal boundary layer (SIBL) in the late mornings or the early afternoons. The SIBL persisted in the rest of the afternoons. Under this circumstance, a near-neutral atmospheric surface layer (ASL) developed during the period with a stratification transition from the unstable to stable conditions during the daytime. Therefore, EBEX provides us with unique datasets to investigate the features of coherent structures that were generated over the patches upstream and passed by our site in the unstable ASL, the near-neutral ASL, and the SIBL. We use an objective detection technique and the conditional average method that is developed based on the wavelet analysis. Our data reveal some consistencies and inconsistencies in the characteristics of coherent structures as compared with previous studies. Ramp-like structures and sweep?Cejection cycles under the daytime SIBL have similar patterns to those under the nocturnal stable ASL. However, some features (i.e., intermittence) are different from those under the nocturnal stable ASL. Under the three stratifications, thermal and mechanical factors in the ASL perform differently in affecting the ramp intensity for different quantities (i.e., velocity components, temperature, and specific humidity), leading to coherent structures that modulate turbulence flow and alter turbulent transfer differently. It is also found that coherent structures contribute about 10?C20% to the total fluxes in our case with different flux contributions under three stratifications and with higher transporting efficiency in sensible heat flux than latent heat and momentum fluxes.     

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.     

Impact of Terrain Heterogeneity on Coherent Structure Properties: Numerical Approach

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.     

Observations of planview flux patterns within convective structures of the marine atmospheric surface layer

Air/sea flux variability on horizontal scales from 50 m to several km results, in part, from the presence of coherent convective structures within the atmospheric boundary layer. The horizontal distribution of fluxes within these convective updrafts and downdrafts is, therefore, central to studies of air/sea interaction and remote sensing of sea surface wind and wave fields. This study derives these flux patterns from observations of the Marine Atmospheric Surface Layer (MASL).Research aircraft flights through the MASL provide an optimal means for sampling large numbers of the above-mentioned coherent structures. The NCAR Electra flew numerous legs through the MASL at a height of 50 m during the 1987 stratocumulus phase of Project FIRE (First ISSCP (International Satellite Cloud Climatology Program) Regional Experiment).In situ measurements from these legs serve as the dataset for this paper. The data are processed in such a way as to retain only the turbulence fluctuations. Conditional sampling, based on the vertical velocity field, results in the isolation of convective updrafts and downdrafts. Compositing of the data for these two classes of convective drafts results in horizontal planviews of the vertical fluxes of buoyancy, absolute humidity, along-meanwind component of momentum, and vertical velocity. To ensure dynamical similarity, these horizontal planviews are oriented in a coordinate system aligned with the mean wind.     

Coherent Structures at a Forest Edge: Properties, Coupling and Impact of Secondary Circulations

Little is known about the influence of coherent structures on the exchange process, mainly in the case of forest edges. Thus, in the framework of the ExchanGE processes in mountainous Regions (EGER) project, measurements of atmospheric turbulence were taken at different heights between a forest and an adjacent clear cutting using sonic anemometers and high-frequency optical gas analyzers. From these turbulence data, dominant coherent structures were extracted using an already existing wavelet methodology, which was developed for homogeneous forest canopies. The aim of this study is to highlight differences in properties of coherent structures between a forest and a clear cutting. Distinct features of coherent exchange at the forest edge are presented and a careful investigation of vertical and horizontal coupling by coherent structures around the surface heterogeneity is made. Within the forest, coherent structures are less frequent but possess larger time scales, indicating that only the largest coherent motions can penetrate through the forest canopy. At the forest edge, there is no crown layer that can hinder the vertical exchange of coherent structures, because these exhibit similar time scales at all heights. In contradiction to that, no improved vertical coupling was detected at the forest edge. This is mainly because the structures captured by the applied routine contribute less to total turbulent fluxes at the edge than within the forest. Thus, coherent structures with time scales between 10 and 40 s are not the dominant exchange mechanism at the forest edge. With respect to the horizontal direction, a consistent picture of coherent transport could be derived: along the forest edge there is mainly good coupling by coherent structures, whereas perpendicular to the forest edge there is mainly decoupling. Finally, it was found that there is a systematic modulation of coherent structures directly at the forest edge: strong ejection motions appear in all time series during the daytime, whereas strong sweeps dominate at night. An effect of wind direction relative to the forest edge is excluded. Consequently, it is hypothesized that this might be an indication of a quasi-stationary secondary circulation above the clear cutting that develops due to differences in surface temperature and roughness. Such circulations might be a relevant turbulent transport mechanism for ecosystem-atmosphere exchange in heterogeneous landscapes.     

Use of obliquely rotated principal component analysis to identify coherent structures

Principal component analysis (PCA) with oblique rotation is applied to Large Eddy Simulation (LES) results to discern and quantify coherent structures within the convective boundary layer (CBL). Sensitivity tests are first conducted on a moderately convective LES run. Once the ability of PCA to generate robust results is verified, the method is applied to LES runs spanning a range of stability regimes. Interregime similarities and differences in the coherent structures are discussed. For the moderately convective LES run, three-dimensional convective cells are arrayed in two-dimensional bands aligned with the geostrophic wind. The resulting gravity waves in the free atmosphere and convective inflow and outflow in the boundary layer are also captured by the PCA. Convective modes are more sensitive to the ratio of w _* to u _* than are the dynamic modes.PCA has demonstrated advantages over previous analysis methods. PCA score maps provide information on the spatial distribution of phenomena that has not been available from traditional conditional sampling studies. Principal components provide information on the vertical structures of phenomena that would be obscured by life-cycle effects or erratic tilts from the vertical in the conventional approaches to either conditional sampling or composite analysis. Future work includes application of this technique to multi-level observational time series from a surface-layer tower for the Risø Air/Sea Experiment (RASEX).     

Coherent Structures in the Atmospheric Surface Layer under Stable and Unstable Conditions

Simultaneous measurements of the instantaneous values of absolute temperatureat seven heights within the lower 36 m of the atmospheric boundary layer underdifferent stability conditions were carried out, accompanied by measurements ofthe wind velocity components at two levels and of solar radiation flux at the surface.The data obtained allow one to investigate individual convective cells known ascoherent structures (CS). Outside the CS, i.e., during quiet periods, an instanttemperature profile is in close agreement with the dry-adiabatic lapse rate, butwithin CS the temperature changes much faster with height, and the shape ofthe profile varies significantly.A method was developed to transform temperature records from sensors atseveral heights into an other form, namely, into temporal variations of theheights of isothermal surfaces. Since coherent structures were found to advectwith the mean wind velocity, these temporal height variations may be transformedinto the spatial ones, i.e., into the xoz-plane section of the temperature field.In such a pictorial presentation coherent structures look like asymmetric columnsof heat, penetrating the whole atmospheric surface layer.Coherent structures also exist in the stable stratified surface layer, but they have aninverse asymmetry and occupy only the lower several metres. Wavelike activitydominates in the upper part of the stable surface layer.The characteristic time of surface-layer adjustment to the rapid changes of solarradiation (due to cloud shadows or cloud gaps) was found to be on the order ofone minute. Such a time interval is required for coherent structure to reach the topof surface layer.     

Large-Eddy Simulation Of The Stably Stratified Planetary Boundary Layer

In this work, we study the characteristics of a stably stratifiedatmospheric boundary layer using large-eddy simulation (LES).In order to simulate the stable planetary boundary layer, wedeveloped a modified version of the two-part subgrid-scalemodel of Sullivan et al. This improved version of themodel is used to simulate a highly cooled yet fairly windy stableboundary layer with a surface heat flux of(W)_o = -0.05 m K s^-1and a geostrophic wind speed of U_g = 15 m s^-1.Flow visualization and evaluation of the turbulencestatistics from this case reveal the development ofa continuously turbulent boundary layer with small-scalestructures. The stability of the boundary layercoupled with the presence of a strong capping inversionresults in the development of a dominant gravity wave atthe top of the stable boundary layer that appears to be relatedto the most unstable wave predicted by the Taylor–Goldsteinequation. As a result of the decay of turbulence aloft,a strong-low level jet forms above the boundary layer.The time dependent behaviour of the jet is compared with Blackadar'sinertial oscillation analysis.     

A Numerical Study Of Nocturnal Wavelike Motion In Forests

In this paper, we use a two-dimensional eddy-resolved model to investigate the instability of a parallel shear flow in a stably stratified boundary layer whose lower domain is occupied by a canopy. The results support our contention that wave motion in the canopy is initiated by shear in an air layer near the treetops. Significant modification by the wave motion of the mean velocity and temperature fields is found even before the wave reaches saturation. The wave fluxes of momentum and heat are not constant with height. Downwind tilting braids are found at the finite amplitude stage of the wave growth and could persist after wave breaking; these downwind tilting structures are believed to be the same as the temperature microfronts reported in the literature. We also present an analysis of the velocity and temperature fields of an observed wave event in the time-height domain and show that the simulation has captured the broad features of the observation.