Wind-Tunnel Experiment on Logarithmic-Layer Turbulence under the Influence of Overlying Detached Eddies

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.     

Spectra and Cospectra of Turbulence in an Internal Boundary Layer over a Heterogeneously Irrigated Cotton Field

During the Energy Balance Experiment, patch-to-patch irrigation generated gradients in soil moisture in a north-south oriented cotton field. An internal boundary layer (IBL) developed as a result of strong horizontal advection from relatively dry upstream patches to relatively wet downstream patches associated with the prevailing northerly winds. This generated large eddies of multiple sizes, which had significant influences on the structure of turbulence in the IBL. The power spectra and cospectra of wind speed, temperature, humidity, and energy fluxes measured at two heights within the IBL are presented and used to investigate the influence of the IBL on surface layer turbulence. The spectra and cospectra were greatly enhanced by external disturbances at low frequencies. The peak frequencies of these disturbances did not change with height. The spectra and cospectra typically converged and were parallel to the Kansas spectrum at high frequencies (in the inertial subrange). A clear gap in the spectra of horizontal wind velocity existed at intermediate frequencies when the surface layer was stable. The results indicate that large eddies that originated in the upstream convective boundary layer had considerable impacts on the spectra and cospectra of surface layer turbulence. The influence of these large eddies was greater (1) when the IBL was well-developed in the near surface layer than when the IBL did not exist, (2) at higher levels than at lower levels, and (3) when the atmospheric surface layer (ASL) was unstable than when the ASL was stable. The length scales of these large eddies were consistent with the dominant scales of surface heterogeneity at the experiment site.     

Stability Dependence of Canopy Flows over a Flat Larch Forest

The dependence on atmospheric stability of flow characteristics adjacent to a very rough surface was investigated in a larch forest in Japan. Micrometeorological measurements of three-dimensional wind velocity and air temperature were taken at two heights above the forest, namely 1.7 and 1.2 times the mean canopy height h. Under near-neutral and stable conditions, the observed turbulence statistics suggest that the flow was likely to be that of the atmospheric surface layer (ASL) at 1.7h, and of the roughness sublayer (RSL) at 1.2h. However, in turbulence spectra, canopy-induced large coherent motions appeared clearly at both heights. Even under strongly stable conditions, the large-scale motions were retained at 1.2h, whereas they were overwhelmed by small-scale motions at 1.7h. This phenomenon was probably due to the enhanced contribution of the ASL turbulence associated with nocturnal decay of the RSL depth, because the small-scale motions appeared at frequencies close to the peak frequencies of well-known ASL spectra. This result supports the relatively recent concept that canopy flow is a superimposition of coherent motions and the ASL turbulence. The large-scale motions were retained in temperature spectra over a wider region of stability compared to streamwise wind spectra, suggesting that a canopy effect extended higher up for temperature than wind. The streamwise spacing of dominant eddies according to the plane mixing-layer analogy was only valid in a narrow range at near neutral, and it was stabilised at nearly half its value under stable conditions.     

Theory And Measurements For Turbulence Spectra And Variances In The Atmospheric Neutral Surface Layer

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.     

Turbulence Spectra and Cospectra Under the Influence of Large Eddies in the Energy Balance EXperiment (EBEX)

During the Energy Balance EXperiment, the patch-by-patch, flood irrigation in a flat cotton field created an underlying surface with heterogeneous soil moisture, leading to a dry (warm)-to-wet (cool) transition within the cotton field under northerly winds. Moreover, the existence of an extremely dry, large bare soil area upstream beyond the cotton field created an even larger step transition from the bare soil region to the cotton field. We investigated the turbulence spectra and cospectra in the atmospheric surface layer (ASL) that was disturbed by large eddies generated over regions upstream and also influenced by horizontal advection. In the morning, the ASL was unstable while in the afternoon a stable internal boundary layer was observed at the site. Therefore, the turbulence data at 2.7 and 8.7 m are interpreted and compared in terms of interactions between large eddies and locally generated turbulence under two atmospheric conditions: the unstable ASL beneath the convective boundary layer (CBL) (hereafter the unstable condition) and the stable ASL beneath the CBL (hereafter the stable condition). We identified the influences of multiple sizes of large eddies on ASL turbulence under both stratifications; these large eddies with multiple sizes were produced over the dry patches and dry, large bare soil areas upstream. As a consequence of the disturbance of large eddies, the broadening, erratic variability, and deviation of spectra and cospectra, relative to those described by Monin–Obukhov similarity theory, are evident in the low- to mid-frequencies. Transfer of momentum, heat, and water vapour by large eddies is distinctly observed from the turbulence cospectra and leads to significant run-to-run variations of residuals of the surface energy balance closure. Our results indicate that these large eddies have greater influences on turbulence at higher levels compared to lower levels, and in the unstable ASL compared to the stable ASL.     

Two-Point Properties Of Atmospheric Turbulence Very Close To The Ground: Comparison Of A High Resolution Les With Theoretical Models

Using the tools of large-eddy simulation (LES), rapid distortion theory andEulerian kinematic simulation, two-point properties of turbulenceare studied. It is shown how the boundary conditions in thepresent LES model are fully consistent with the new top-down understanding ofatmospheric turbulence close to the ground, wherethe turbulence is analysed in terms of turbulent eddies impinging ontothe ground, being distorted by the shear and the blocking by the wall,and experiencing the creation of an internal boundary layer within aneddy surface layer (ESL) with a height of a dozen metres.The question of very large streamwise structures is addressed, and itis shown that the k^-1 range measured in spectra for the streamwisevelocity in the ESL in several recent field experiments can be found in LES, and corresponds to streaky structures. However, thesestreaky structures are different from the low Reynolds number streaks,which induce a k^-2 range in the spectrum. Finally, two-point verticalcorrelations are studied. In particular, effects of shear and of the slope of thespectrum are analysed.     

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.     

Turbulence Structure of the Unstable Atmospheric Surface Layer and Transition to the Outer Layer

We present a new model of the structure of turbulence in the unstable atmospheric surface layer, and of the structural transition between this and the outer layer. The archetypal element of wall-bounded shear turbulence is the Theodorsen ejection amplifier (TEA) structure, in which an initial ejection of air from near the ground into an ideal laminar and logarithmic flow induces vortical motion about a hairpin-shaped core, which then creates a second ejection that is similar to, but larger than, the first. A series of TEA structures form a TEA cascade. In real turbulent flows TEA structures occur in distorted forms as TEA-like (TEAL) structures. Distortion terminates many TEAL cascades and only the best-formed TEAL structures initiate new cycles. In an extended log layer the resulting shear turbulence is a complex, self-organizing, dissipative system exhibiting self-similar behaviour under inner scaling. Spectral results show that this structure is insensitive to instability. This is contrary to the fundamental hypothesis of Monin--Obukhov similarity theory. All TEAL cascades terminate at the top of the surface layer where they encounter, and are severely distorted by, powerful eddies of similar size from the outer layer. These eddies are products of the breakdown of the large eddies produced by buoyancy in the outer layer. When the outer layer is much deeper than the surface layer the interacting eddies are from the inertial subrange of the outer Richardson cascade. The scale height of the surface layer, z _s, is then found by matching the powers delivered to the creation of emerging TEAL structures to the power passing down the Richardson cascade in the outer layer. It is z _s = u _* ³ /k_s, where u _* is friction velocity, k is the von Kármán constant and _s is the rate of dissipation of turbulence kinetic energy in the outer layer immediately above the surface layer. This height is comparable to the Obukhov length in the fully convective boundary layer. Aircraft and tower observations confirm a strong qualitative change in the structure of the turbulence at about that height. The tallest eddies within the surface layer have height z _s, so z _s is a new basis parameter for similarity models of the surface layer.     

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.     

Organized Structure of Active Turbulence Over an Array of Cubes within the Logarithmic Layer of Atmospheric Flow

We investigate the coherent structure of atmosphere turbulence over very large roughness within a fully rough, high Reynolds number turbulent flow. The horizontal distributions of coherent turbulence were determined by multipoint measurements of velocity fluctuations using sonic anemometers in a comprehensive outdoor scale model experiment for urban climate (COSMO). COSMO is made up of 512 cubical obstacles, each 1.5 m on a side, arranged in a rectangular pattern on a flat 50 m × 100 m concrete plate. A total of 15 sets of sonic anemometers were aligned horizontally within the logarithmic layer above this site. The velocity fluctuations observed in COSMO were decomposed into active and inactive contributions by applying a spatial-filtering method, and which used a simple moving average along the spanwise direction of the predominant flow as a filter function. The size of the filter should be between the sizes of the active and inactive fluctuations. This method potentially eliminates the considerable portion of low frequency modes included in the horizontal velocity fluctuation, while preserving well the Reynolds stress. The structural characteristics of the active turbulence were qualitatively similar to those measured over various surface configurations. Overall, the observed structures of the active turbulence are composed of very large streaks of low momentum fluid elongated in the streamwise direction with some sub-structures included in the streaks. The sub-structures were the main cause of the ejections, which accompany horizontal vortices. The active motion, including the streaky structures, did not reproduce the lower frequency peak of the bi-modal distribution of the horizontal velocity spectra, but reproduced the higher frequency mode that robustly follows inner-layer similarity (i.e. Monin–Obukhov similarity).     

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。     

Characteristics of canopy turbulence over a deciduous forest on complex terrain

Canopy turbulence plays an important role in mass and energy exchanges at the canopy-atmosphere interface. Despite extensive studies on canopy turbulence over a flat terrain, less attention has been given to canopy turbulence in a complex terrain. The purpose of this study is to scrutinize characteristics of canopy turbulence in roughness sublayer over a hilly forest terrain. We investigated basic turbulence statistics, conditionally sampled statistics, and turbulence spectrum in terms of different atmospheric stabilities, wind direction and vertical structures of momentum fluxes. Similarly to canopy turbulence over a homogeneous terrain, turbulence statistics showed coherent structure. Both quadrant and spectrum analysis corroborated the role of intermittent and energetic eddies with length scale of the order of canopy height, regardless of wind direction except for shift of peak in vertical wind spectrum to relatively high frequency in the down-valley wind. However, the magnitude of the momentum correlation coefficient in a neutral condition was smaller than typical value over a flat terrain. Further scrutiny manifested that, in the up-valley flow, temperature skewness was larger and the contribution of ejection to both momentum and heat fluxes was larger compared to the downvalley flow, indicating that thermal instability and weaker wind shear in up-valley flow asymmetrically affect turbulent transport within the canopy.     

Turbulence Spectra And Dissipation Rates Above And Within A Forest Canopy

Three velocity componentsand temperature were measured usingthree-dimensional sonic anemometers/thermometers attwo levels, above and within a forest canopy, in theChangbai Mountains of northeast China. Turbulencespectral structure, local isotropy anddissipation rates above and within the forest canopywere calculated using the eddy correlation method.Results show that the normalized turbulent spectralcurves have -2/3 slopes in the inertial subrange.While the shapes of the spectra are in good agreementwith the Kansas flat terrain results, the atmosphericturbulence is anisotropic above the forest canopy. Dueto breaking down of large eddies by the foliage,branches and trunks, the spectral peak frequencies forvelocity and temperature are higher withinthan above the forest canopy. Compared withmeasurements from previous studies over flat terrain,the velocity and temperature spectra above andinside the forest canopy appear to shift toward higherfrequencies. The turbulence is approximately isotropicin the inertial subrange within the forest canopy, and isanisotropic above the forest canopy. The turbulentkinetic energy and heat energy dissipation rates aboveand inside the forest canopy are much larger thanthose obtained by Kaimal and Hogstrom over grasslandand grazing land. The distinct features in the resultsof the present experiment may be attributed to thedynamic forcing caused by the rough surface of the forestcanopy.     

强风天气下边界层结构特征

近地层观测的强风运动表明,叠加在平均流动之上的脉动通常有两种,一种是随机的湍流脉动,还有一种具有相干结构的阵风扰动。分析表明,上层强风的剪切运动产生阵风,并向下传递能量,对近地层的通量传输起到重要作用。本文利用北京325 m气象塔、位于海拔1257 m的妙峰山测风塔和位于海拔1688 m的灵山测风塔的资料,分析了强风天气下,边界层上层出现阵风并向下传递的过程,进一步证实无论在近地层还是边界层上层,强风期间,叠加在平均流动上除了高频湍流脉动之外,还有周期为1~10分钟的阵风,即相干结构。阵风峰期有下沉运动,阵风谷期有上升运动。这些相干结构在边界层上层产生,向下运动和传播过程中受到平均气流梯度的切变作用和地面摩擦,破碎为湍流结构。边界层上层的阵风和湍流产生的动量通量向下传递,使得强风期间,边界层中阵风和湍流对通量具有同样的输送能力,对边界层中沙尘、污染物等气溶胶的传输具有重要作用。本研究为模式中进行通量输送参数化方案的修正提供了观测和理论依据。     

HUBEX试验区近地面层的湍流输送

1998年淮河流域能量和水循环试验(HUBEX)期间曾进行了1个月的近地面层湍流观测.分析不稳定条件下湍流的统计特性和谱特征,并与Monin-Obukhov相似理论进行了比较.结果表明,不稳定的时候各湍流量的统计特征与相似理论的预期相符.虽然不稳定条件下温度和湿度涨落的相关系数很高,谱的式样也相近,但湿度谱的峰值频率高于温度谱.协谱曲线的形状显示感热通量的谱峰较宽,表现出w和T在较宽范围的强相关性,而水汽通量谱在高频段下降很快,说明水汽的输送更多地出现在低频部分.从谱相关系数可见,在近中性的时候,各尺度湍流涡的热量输送效率普遍较低,随着不稳定性增强而显著提高.分析还发现,不论不稳定性的程度如何,小尺度湍流的水汽输送效率都较低.水汽通量谱的相关系数随稳定度的变化不如热通量的谱相关系数大,表明近中性时除小尺度湍流外其他湍流涡的水汽输送效率高于热量输送.     

Influence from Surrounding Land on the Turbulence Measurements Above a Lake

Turbulence measurements taken at a Swedish lake are analyzed. Although the measurements took place over a relatively large lake with several km of undisturbed fetch, the turbulence structure was found to be highly influenced by the surrounding land during daytime. Variance spectra of both horizontal velocity and scalars during both unstable and stable stratification displayed a low frequency peak. The energy at lower frequencies showed a daily variation, increasing in the morning and decreasing in the afternoon. This behaviour is explained by spectral lag, where the low frequency energy due to large eddies that originate from the convective boundary layer above the surrounding land. When the air is advected over the lake the small eddies rapidly equilibrate with the new surface forcing. However, the large eddies remain for an appreciable distance and influence the turbulence in the developing lake boundary layer. The variances of the horizontal velocity and scalars are increased by these large eddies, while the turbulent fluxes are mainly unaffected. The drag coefficient, Stanton number and Dalton number used to parametrize the momentum flux, heat flux and latent heat flux respectively all compare well with current parametrizations developed for open sea conditions. The diurnal cycle of the partial pressure of methane, $p\mathrm{CH}_{4}$ , observed at this site is closely related to the diurnal cycle of the lake-air methane flux. An idealized two-dimensional model simulation of the boundary layer at a lake site indicates that the strong response of $p\mathrm{CH}_{4}$ to the surface methane flux is due to the shallow internal boundary layer that develops above the lake, allowing methane to accumulate in a relatively small volume.     

日间对流边界层中的非局地动量混合

日间对流边界层最显著的结构特征是在热力作用下所形成的组织化对流。与小尺度湍涡不同的是,组织化对流具有边界层尺度的垂直相干性,可实现垂直贯穿边界层的非局地物质和能量传输。本文针对对流边界层中的动量混合,探究组织化对流对动量输送的贡献。以高精度大涡模拟数据为研究资料,通过傅里叶变换、本征正交分解和经验模态分解3种滤波方法,分离组织化对流和背景湍涡,计算与两者相关的非局地和局地动量通量,发现与组织化对流相关的非局地动量通量是总通量的重要组成部分,并主导混合层中的垂直动量输送。而后,基于协谱和相位谱分析,探究组织化对流的空间结构对动量传输的影响,发现在热力主导的不稳定环境中,单体型环流结构对动量的传输效率较低。而在风切较强的近中性环境中,滚涡型组织化结构可使垂直和水平流向扰动速度的相位差减小,从而提升动量传输效率。研究结果表明,边界层方案需要包含非局地动量通量项,其参数化应考虑整体稳定度对传输效率的影响。     

Townsend's Hypothesis, Coherent Structures And Monin–Obukhov Similarity

Townsend's hypothesis states that turbulence near a wall can be divided into an activepart that transports momentum, and an inactive part that does not, and that these twokinds of turbulence do not interact. Active turbulence is generated by wind shear and has properties that scale on local parameters of the flow, while inactive turbulence isthe product of energetic processes remote from the surface and scales on outer-layerparameters. Both kinds of motion can be observed in the atmospheric surface layer, soMonin–Obukhov similarity theory, which is framed in terms of local parameters only,can apply only to active motions. If Townsend's hypothesis were wrong, so that activeand inactive motions do interact in some significant way, then transport processes nearthe ground would be sensitive to outer-layer parameters such as boundary-layer depth,and Monin–Obukhov theory would fail.Experimental results have shown that heat transport near the ground does depend onprocesses in the outer layer. We propose a mechanism for this whereby inactive motionsinitiate active, coherent ejection/sweep structures that carry much of the momentum andheat. We give evidence that the inactive motions take the form of streak patterns of fasterand slower air, and argue that these are induced by the pressure effects of large eddiespassing overhead. The streak pattern includes regions where faster streams of air overtakeand engulf slower-moving streaks. Transverse vortices form across the spines of the streaksat these places and some of them develop into horseshoe vortices. These horseshoe vorticesgrow rapidly and are rotated forward in the sheared flow so they soon contact the ground,squirting the air confined between the legs of the horseshoe vortex outwards as a forcefulejection. This model is consistent with a wide range of results from the field and laboratoryexperiments. Heat transport is significantly affected, so undermining the dimensionalassumptions of Monin–Obukhov similarity theory.