Stability Dependence and Diurnal Change of Large-Scale Turbulence Structures in the Near-Neutral Atmospheric Boundary Layer Observed from a Meteorological Tower

Based on the analysis of observations from a 213-m tall meteorological tower at Tsukuba, Japan, we have investigated the favourable conditions for the predominant existence of large-scale turbulence structures in the near-neutral atmospheric boundary layer (ABL). From the wavelet variance spectrum for the streamwise velocity component ( $u$ ) measured by a sonic anemometer-thermometer at the highest level (200 m), large-scale structures (time-scale range of 100–300 s) predominantly exist under slightly unstable and close to neutral conditions. The emergence of large-scale structures also can be related to the diurnal evolution of the ABL. The large-scale structures play an important role in the overall flow structure of the lower boundary layer. For example, $u$ velocity components at the 200-m and 50-m levels show relatively high correlation with the existence of large-scale structures. Under slightly unstable (near-neutral) conditions, a low-speed region in advance of the high-speed structure shows a positive deviation of temperature and appears as the plume structure that is forced by buoyancy in the heated lower layer. In spite of the difference in buoyancy effects between the near-neutral and unstable cases, large-scale structures are frequently observed in both cases and the same vertical correlation of $u$ components is indicated. However, the vertical wind shear is smaller in the unstable cases. On the other hand, in near-neutral cases, the transport efficiency of momentum at the higher level and the flux contribution of sweep motions are larger than those in the unstable cases.     

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.     

北京夏季O3日变化及通量研究

2002年夏季，以北京325m气象塔为观测平台，进行了大气污染物臭氧(O3)及其前体物氮氧化物(NOx)和气象要素加强期的同步观测，并对观测资料做了详尽分析。结果表明：边界层内存在明显的臭氧浓度垂直差异；低层(120m)O3浓度呈明显的日变化，且昼夜振幅较大；夜间高层(280m)O3的化学消耗较弱，可维持较高的浓度；稳定度(Ri)在低层以中性态居多，振幅较小，而在高层以不稳定态居多，振幅较大。两层O3湍流输送通量都呈单峰变化。白天，在O3前体物和局地光化学反应共同作用下，120m左右处的O3污染最大。     

北京夏季O3日变化及通量研究

2002年夏季,以北京325m气象塔为观测平台,进行了大气污染物臭氧(O₃)及其前体物氮氧化物(NO_x)和气象要素加强期的同步观测,并对观测资料做了详尽分析。结果表明:边界层内存在明显的臭氧浓度垂直差异;低层(120m)O₃浓度呈明显的日变化,且昼夜振幅较大;夜间高层(280m)O₃的化学消耗较弱,可维持较高的浓度;稳定度(Ri)在低层以中性态居多,振幅较小,而在高层以不稳定态居多,振幅较大。两层O₃湍流输送通量都呈单峰变化。白天,在O₃前体物和局地光化学反应共同作用下,120m左右处的O₃污染最大。     

Near-Surface Coherent Structures and The Vertical Momentum Flux in a Large-Eddy Simulation of the Neutrally-Stratified Boundary Layer

The near-surface flow of a well-resolved large-eddy simulation of the neutrally-stratified planetary boundary layer is used to explore the relationships between coherent structures and the vertical momentum flux. The near-surface flow is characterized by transient streaks, which are alternating bands of relatively higher and lower speed flow that form parallel to the mean shear direction in the lower part of the boundary layer. Although individual streaks are transient, the overall flow is in a quasi-equilibrium state in which the streaks form, grow, decay and regenerate over lifetimes on the order of tens of minutes. Coupled with the streaky flow is an overturning circulation with alternating bands of updrafts and downdrafts approximately centered on the streaks. The surface stress is dominated by upward ejections of slower moving near-surface air and downward sweeps of higher speed air from higher in the boundary layer. Conditional sampling of the ejection and sweep events shows that they are compact, coherent structures and are intimately related to the streaks: ejections (sweeps) preferentially form in the updrafts (downdrafts) of the three-dimensional streak flow. Hence, consistent with other recent studies, we propose that the streak motion plays an important role in the maintenance of the surface stress by establishing the preferential conditions for the ejections and sweeps that dominate the surface stress. The velocity fluctuation spectra in the model near the surface have a k ⁻¹ spectral slope over an intermediate range of wavenumbers. This behaviour is consistent with recent theoretical predictions that attempt to evaluate the effects of organized flow, such as near-surface streaks, on the variance spectra.     

Analysis of Boundary Layer Structure,Turbulence, and Flux Variations before and after the Passage of a Sea Breeze Front Using Meteorological Tower Data

A detailed analysis of a sea breeze front(SBF) that penetrated inland in the Beijing–Tianjin–Hebei urban agglomeration of China was conducted. We focused on the boundary layer structure, turbulence intensity, and fluxes before and after the SBF passed through two meteorological towers in the urban areas of Tianjin and Beijing, respectively.Significant changes in temperature, humidity, winds, CO₂, and aerosol concentrations were observed as the SBF passed. Differences in these changes ...     

城市边界层动量和保守物通量的特征

利用2005年1-5月北京325 m气象塔47 m高度的湍流脉动资料(风速、温度、水汽和CO2),对城市边界层冠层内的湍流运动统计特征(相似关系、高阶矩、通量和谱等)进行了分析。其中,谱分析的结果表明,城市冠层内稳定度对湍流谱的影响比较小,而水平风速的影响比较大。因此,速度和温度的相似关系在夜间稳定条件下也成立。但是,由于水汽和CO2还受其他因素的影响,相似关系并不适用。更高阶矩的研究表明它们的陡峭度与偏斜度之间存在平方关系。而水汽和CO2之间也存在差异,它们的通量日变化特征明显不同,CO2通量的日变化更能体现人类活动的影响。同时,感热通量、潜热通量和CO2通量存在季节变化,尤其是潜热通量季节差异很大。     

The Effects of Inner- and Outer-Layer Turbulence in a Convective Boundary Layer on the Near-Neutral Inertial Sublayer Over an Urban-Like Surface

A large-eddy simulation of the atmospheric boundary layer, large enough to contain both an urban surface layer and a convective mixed layer, was performed to investigate inner-layer and outer-layer scale motions. The objective was to determine the applicability of Monin–Obukhov similarity theory to inner-layer motions, to investigate the influence of outer-layer motions on surface-layer structure, as well as to assess the interaction of the two scales of motion. The urban surface roughness consisted of square-patterned cubic buildings of dimension H (40 m). A spatial filter was used to decompose the two scales in the inertial sublayer. The horizontal square filter of size 10H was effective in separating the inner-layer (surface-layer height ≈ 2 H) and outer-layer scales (boundary-layer height δ ≈ 30H), where the Reynolds stress contribution of the inner layer dominates in the logarithmic layer (depth 2H). Similarity coefficients for velocity fluctuations were successfully determined for inner-layer motions in the surface layer, proving the robustness of Monin–Obukhov similarity for surface-layer turbulence. The inner-layer structures exhibit streaky structures that have similar streamwise length but narrower spanwise width relative to the streamwise velocity fluctuation field, consistent with observations from an outdoor scale model. The outer-layer motions to some extent influence the location of ejections and sweeps through updraft and downdraft motions, respectively, thus, disturbing the homogeneity and similarity of inner-layer motions. Although the horizontal averages of the variances and covariance of motions reveal that the Reynolds stresses are dominated by inner-layer structures, the localized influence of the interaction of outer-layer horizontal and inner-layer vertical motions on the Reynolds stress is not insignificant.     

Role of Convective Structures and Background Turbulence in the Dry Convective Boundary Layer

We quantify the role of the convective buoyant structures and the remainder turbulence, here called background turbulence, in the convective atmospheric boundary layer in horizontally homogeneous, dry and barotropic conditions. Three filtering methods to separate the structures and the background turbulence are first evaluated. These are: short-time averaging, Fourier filtering and proper orthogonal decomposition. The Fourier method turns out to be the most appropriate for the present purpose. The decomposition is applied to two cases: one with no mean flow and another with moderate mean wind speed. It is shown that roughly 85 % of the vertical flux of the potential temperature and about 72 % of the kinetic energy is carried by the structures in the mixed layer in both cases. The corresponding percentage for the potential temperature variance is 81 % in the zero mean-wind case and 76 % in the moderate mean-wind case. The structures are responsible for as much as 94 % of the momentum flux in the mixed layer of the moderate mean-wind case. In the surface layer the background turbulence is generally more important than the structure contribution in both cases. The budget of the potential temperature flux is analyzed in detail and it is shown that its turbulent transport term is mostly built up by the structures but also the interaction between the structures and the background turbulence plays a significant role. The other important budget terms are shown to be dominated by the structures except for the pressure–temperature gradient covariance.     

Large-Scale Structures over a Single Street Canyon Immersed in an Urban-Type Boundary Layer

An analysis of the dynamics of the flow over a street canyon immersed in an atmospheric boundary layer is presented, using particle image velocimetry measurements in a wind tunnel. Care was taken to generate a 1:200 model scale urban type boundary layer that is correctly scaled to the size of the canyon buildings. Using proper orthogonal decomposition (POD) of the velocity field and conditional averaging techniques, it is first shown that the flow above the opening of the canyon consists of a shear layer separating from the upstream obstacle, animated by a coherent flapping motion and generating large-scale vortical structures. These structures are alternately injected into the canyon or shed off the obstacle into the outer flow. It is shown that unsteady fluid exchanges between the canyon and the outer flow are mainly driven by the shear layer. Finally, using POD, the non-linear interaction between the large-scale structures of the oncoming atmospheric boundary layer and the flow over the canyon is demonstrated.     

Intermittency of Turbulence in the Atmospheric Boundary Layer: Scaling Exponents and Stratification Influence

We show the relationship between the intermittency of turbulence and the type of stratification for different atmospheric situations during the SABLES98 field campaign. With this objective, we first demonstrate the scaling behaviour of the velocity structure functions corresponding to these situations; next, we analyze the curvature of the scaling exponents of the velocity structure functions versus the order of these functions (ζ _p vs. p), where ζ _p are the exponents of the power relation for the velocity structure function with respect to the scale. It can be proved that this curve must be concave, under the assumption that the incompressible approximation does not break down at high Reynolds numbers. The physical significance of this kind of curvature is that the energy dissipation rate increases as the scale of the turbulent eddies diminishes (intermittency in the usual sense). However, the constraints imposed by stability, preventing full development of the turbulence, allow the function ζ _p versus p to show any type of curvature. In this case, waves of high frequency trapped by the stability, or bursts of turbulence caused by the breaking up of internal waves, may produce a redistribution of energy throughout the scaling range. Due to this redistribution, the variation with the scale of the energy dissipation rate may be smaller (decreasing the intermittency) and, even in more stable situations, this rate may diminish (instead of increasing) as the scale diminishes (convex form of the curve ζ _p vs. p).     

Turbulence Structure in the Wake Region of a Meteorological Tower

A meteorological tower significantly modifies the air flow, the mean windspeed and wind direction as well as the turbulencestructure of the air. Suchchanges can be noticed in particular in the wake region of the tower.Measurementson the 200 m tower ofForschungszentrum Karlsruhewere carried outusing Solent sonic anemometers in the lee of the towerand cup anemometers on both sides.In the wake region, spectral energydensity is increased in the high-frequency range. Superposition of this disturbance spectrum on the undisturbedspectrum yields a `knee' in the resulting spectrum. In the case of low turbulence intensity with stable stratification,a plateau with a constant energy content is observed in front of the knee.This effect is caused by the new production of turbulence energy from the mean flow as well as by an energy transfer fromlarger to smaller vortices. Power spectra in strongly stable conditionsshow a more rapid decrease of intensity in the region where the inertialsubrange is expected.The relevant scales of wake turbulence are derived from the maximum of the disturbance spectrum.Locations of the high-frequency peak do not depend on atmospheric stability,but are controlled mainly by mean wind speed.Apart from the reduction of the mean wind speed, the spectra and cospectra exhibit a strong anisotropy for such cases.The results demonstrate the significant influence of a tower on turbulence spectra in the wake region.     

Scaling of Wall-Normal Turbulence Intensity and Vertical Eddy Structures in the Atmospheric Surface Layer

Adequate high-quality data on three-dimensional velocities in the atmospheric surface layer (height \(\delta \)) were acquired in the field at the Qingtu Lake Observation Array. The measurement range occupies nearly the entire logarithmic layer from approximately \(0.006\delta \)–\(0.2\delta \). The turbulence intensity and eddy structures of the velocity fluctuations in the logarithmic region were primarily analyzed, and their variations in the z (wall-normal) direction were revealed. The primary finding was that the turbulent intensity of wall-normal velocity fluctuations exhibits a sharp upswing in the logarithmic region, which differs from classic scaling law and laboratory results. The upswing of the wall-normal turbulence intensity in the logarithmic region is deemed to be linear based on an ensemble of 20 sets of data. In addition, the wall-normal extent of the correlated structures and wall-normal spectra were compared to low Reynolds number results in the laboratory.     

在北京气象塔上测量城市边界层辐射特征

利用2004年10～12月在北京325m气象铁塔上所测量的长波辐射和短波辐射垂直分层资料,分析了晴空、霾(烟、尘、轻雾)和浓雾天气条件下塔上两层的辐射特征。结果表明,城市边界层大气对短波总辐射有明显的衰减作用,2m处的总辐射280m处的总辐射。在晴好天气下,衰减比例为11.2%;在霾和雾天气下,衰减比例分别达到22.3%和36.6%。在塔上280m处测量的晴天城市地表反照率为0.12,而在霾和雾天气条件下分别为0.17和0.29。大气逆辐射(向下长波辐射)的日变化没有短波辐射的明显且有规律,晴好天气的辐射通量密度最小,霾天较大,雾天最大,2m处的大气逆辐射一般高于280m处的。向上长波辐射呈明显的日变化,且2m处的向上长波辐射日变化幅度280m处的。净辐射具有典型的日变化,晴天日变化幅度最大,霾和雾天的日变化幅度减小。晴天辐射加热最显著,278m厚气层的辐射加热达1.35℃.h-1,霾天为1.13℃.h-1,雾天为0.40℃.h-1。个例分析表明,在近地面气层内,霾中的气溶胶粒子和雾中的水滴对辐射产生程度不同的影响,并影响到辐射平衡,进而影响不同高度气层的加热和冷却,导致边界层结构的改变。     

非均匀网格的过渡湍流理论及其在大气边界层数值模拟中的应用

本文将Stull提出的均匀网格下的过渡湍流理论推广到非均匀网格情形，推广的非均匀网格的过渡湍流理论满足Stull提出的对过渡矩阵系数的要求并具有清晰的物理意义。然后，将非均匀网格的过渡湍流理论应用于一维大气边界层数值模式中，对Wangara资料进行了模拟，并与均匀网格情形进行了对比。计算表明，非均匀网格的过渡湍流模式能很好地模拟Wangara大气边界层平均量与湍流量的变化；本文非均匀网格的过渡湍流理论的推广是可行的，它可能会在大气边界层数值模拟及其他方面（如：中尺度模式）得到应用。     

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.     

Some Basic Properties of the Surrogate Subgrid-Scale Heat Flux in the Atmospheric Boundary Layer

The development of improved subgrid-scale (SGS) models for large-eddy simulation of scalar transport in the atmospheric boundary layer requires an improved understanding of basic properties of the SGS fluxes. High frequency atmospheric wind speed and temperature data sampled at a height of 1.7 m are used to measure SGS heat fluxes and dissipation of temperature variance, by means of one-dimensional filtering and invoking Taylor's hypothesis. Conditional averaging is used to isolate interesting features of the SGS signals, and to relate them to the large-scale characteristics of the flow, such as the presence of coherent structures. Both mean and conditionally averaged SGS quantities are compared with those obtained using a standard eddy-diffusivity model. Within the limitations imposed by the one-dimensional data analysis, we observe that the model appears unable to reproduce important features of the real signals, such as the negative dissipation of temperature variance associated with strong negative resolved temperature gradients due to the ejection of warm air under unstable atmospheric stability conditions.     

在北京气象铁塔上测量城市边界层辐射的研究

测量边界层辐射对研究地气系统相互作用有重要意义,城市边界层辐射的变化影响着城市局地气候和环境.目前辐射测量仪器的精度不能满足城市大气边界层辐射分层测量的特殊要求.2004年10～12月,在北京325 m气象铁塔上的280 m和2 m高度上分别安装了长波辐射表和短波总辐射表,进行辐射测量.为提高辐射测量精度,采取多种方法,进行了较细致的误差分析和资料订正,使两层不同高度上仪器的一致性得到明显提高,计算订正了铁塔塔体发射的长波辐射对辐射测量的影响,取得了比较合理的结果.辐射观测资料分析表明,边界层内辐射通量密度随高度的变化是明显的,在不同环境条件下对边界层结构有着重要作用.