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1.
Intermittency of turbulence within open canopies 总被引：1，自引：0，他引：1
Young-Hee Lee 《Asia-Pacific Journal of Atmospheric Sciences》2011,47(2):137-149
Eddy covariance data have been analyzed to examine intermittency and clustering properties of turbulence within open canopies. Intermittency consists of two aspects: one is related to amplitude variation and the other to clustering. Using the telegraph approximation (TA), the clustering properties have been separated from amplitude effects. Intermittency of canopy turbulence has been explored via clustering exponent, probability density distribution of inter-pulse period of TA, intermittency exponent and structure kurtosis. Intermittency and clustering properties of turbulence within open canopies show similar features to those within dense canopy but some differences are also noted. Unlike within a dense canopy, temperature does not show larger clustering than velocity, which seems to be due to a different thermal structure of the sub-canopy and larger vertical scale of canopy eddy within open canopies. Within the crown region, the inter-pulse probability distribution of TA does not show the ‘double regime’ which was observed within the crown of a dense canopy, indicating less influence of near-field source on canopy turbulence within open canopies. For TA series of the flow variables, intermittency exponent is higher for temperature than for two velocity components within open canopies, which are opposite within a dense canopy. When comparing intermittency for flow variables and their TA series, it is shown that amplitude variation mitigates intermittency for both velocity components and temperature although amplitude variations play a much larger role in velocity intermittency than in temperature counterpart. Kurtosis analysis demonstrates that structure kurtosis is higher at large scales in stable conditions than in unstable conditions, indicating the existence of global intermittency due to stable stratification. The intermittency features of canopy turbulence within open canopies have been discussed in comparison with those within a dense canopy. 相似文献
2.
北京城市通量足迹及源区分布特征分析 总被引：2，自引：1，他引：1
城市下垫面具有不同于其他下垫面的特殊复杂性,通量的源汇分布十分不均,导致直接测量以及定量的描述城市中水汽、二氧化碳等通量变得非常困难。涡动观测系统在城市通量观测中得到广泛应用,但由于涡动协方差观测系统传感器都架设在一定的高度上,其测量结果只反映特定点下垫面或某部分下垫面的物理过程,不能说明观测结果是否具有下垫面属性的代表性,无法反映观测通量的空间变异性。足迹函数(Footprint)的产生就是为了解决这一问题,其又称源权重函数,是描述近地面层表面源或汇的空间分布和仪器观测通量值之间关系的函数。本文采用北京325 m气象塔近1年7层涡动协方差观测数据、超声数据,分析了不同风向、不同稳定度、不同高度下足迹函数所表达的通量贡献区域范围的变化规律。结果表明,在大气稳定条件下时,通量贡献区范围的大小与主风向无显著差异,而不稳定条件下计算结果与主风向无关。在不同高度下所有稳定条件下通量贡献区范围要大于不稳定条件,中性条件则介于两者之间。8 m、16 m高度上的观测结果不能完全代表城市下垫面通量贡献区,47 m以上能够代表城市下垫面通量贡献区,280 m则已经包含了郊区和城区的权重平均。同一大气稳定度条件下,高度越高通量贡献区范围越大,90%通量贡献区范围与观测高度成线性关系,这种线性关系可以预测没有观测高度或者更高处的通量贡献区范围。 相似文献
3.
本文采用二阶闭合的湍流边界层模式,进行一系列数值试验以模拟边界层中连续线源的扩散状况。试验表明:无论在稳定的或不稳定的边界层中,高源的扩散能力都低于低源;在稳定层中,粗糙地表上的大气扩散能力高于光滑表面;在相同风速和地表净辐射情况下,粗糙表面上的大气扩散能力反而低于光滑表面;对流边界层中存在反梯度输送,因而K理论的应用受到限制。试验还表明,修正的Kazanski-Monin参数可能比Monin-Obukhov长度更能反映大气的扩散能力。 相似文献
4.
The intermittent structure of turbulence within the canopy sublayer (CSL) is sensitive to the presence of foliage and to the
atmospheric stability regime. How much of this intermittency originates from amplitude variability or clustering properties
remains a vexing research problem for CSL flows. Using a five-level set of measurements collected within a dense hardwood
canopy, the clustering properties of CSL turbulence and their dependence on atmospheric stability are explored using the telegraphic
approximation (TA). The binary structure of the TA removes any amplitude variability from turbulent excursions but retains
their zero-crossing behaviour, and thereby isolating the role of clustering in intermittency. A relationship between the spectral
exponents of the actual and the TA series is derived across a wide range of atmospheric stability regimes and for several
flow variables. This relationship is shown to be consistent with a relationship derived for long-memory and monofractal processes
such as fractional Brownian motion (fBm). Moreover, it is demonstrated that for the longitudinal and vertical velocity components,
the vegetation does not appreciably alter fine-scale clustering but atmospheric stability does. Stable atmospheric stability
conditions is characterized by more fine scale clustering when compared to other atmospheric stability regimes. For scalars,
fine-scale clustering above the canopy is similar to its velocity counterpart but is significantly increased inside the canopy,
especially under stable stratification. Using simplified scaling analysis, it is demonstrated that clustering is much more
connected to space than to time within the CSL. When comparing intermittency for flow variables and their TA series, it is
shown that for velocity, amplitude variations modulate intermittency for all stability regimes. However, amplitude variations
play only a minor role in scalar intermittency. Within the crown region of the canopy, a ‘double regime’ emerges in the inter-pulse
duration probability distributions not observed in classical turbulence studies away from boundaries. The double regime is
characterized by a power-law distribution for shorter inter-pulse periods and a log-normal distribution for large inter-pulse
periods. The co-existence of these two regimes is shown to be consistent with near-field/far-field scaling arguments. In the
near-field, short inter-pulse periods are controlled by the source strength, while in the far-field long inter-pulse periods
are less affected by the precise source strength details and more affected by the transport properties of the background turbulence. 相似文献
5.
大气边界层湍流标量场的概率分布及其特征分析 总被引：2，自引：0，他引：2
利用2004年11月在白洋淀地区和2005年1月在中国科学院大气物理研究所北京325 m气象塔的47 m高度由超声风温仪和水汽二氧化碳分析仪观测的湍流脉动资料,分析了大气边界层不同下垫面湍流标量场(温度、水汽和二氧化碳)的概率分布及其特征。标量场的概率分布通常不同于高斯分布,而且还会产生偏斜,可以用指数分布描述。因此,标量的偏斜度通常不为0,陡峭度也往往比3大。非0偏斜度的出现可能是由湍流时间序列中的相干结构和间歇性部分造成的。其中,相干结构的存在使概率分布偏斜,但是,它们对偏斜度的贡献相对较小,而与概率分布的长尾现象有关的间歇性则会使偏斜度大大增加。温度、水汽和二氧化碳的平均偏斜度和陡峭度反应了标量场与稳定度、下垫面、天气条件、源汇等因素之间的关系。在不同下垫面,温度和感热通量的偏斜度随稳定度变化比较一致;水汽通量的偏斜度在稳定和不稳定条件下都为正,而水汽本身在不稳定条件下可能出现负的偏斜度;二氧化碳和二氧化碳通量的偏斜度受下垫面影响很大,在不同下垫面,偏斜度与稳定度之间的关系并不一致;而3个标量的陡峭度随稳定度的变化不显著,它们与相应的偏斜度之间存在平方关系。 相似文献
6.
Turbulence data collected over a total of 25 days during two summers are used to describe processes responsible for the nonstationarity
of turbulent sensible heat fluxes at Summit, Greenland. A stationarity test shows that about 40% of the data are classified
as nonstationary. Three main factors are explored to account for the large fraction of nonstationary runs: (1) intermittency
of turbulence in stable conditions, (2) changes in net all-wave radiation in response to cloud forcing, and (3) diurnal trends
in stability. A classification procedure that accounts for the intermittent nature of turbulence shows that during stable,
nonstationary conditions 50% of the total sensible heat flux is realized in 22% of the sampling time. Intermittency often
occurs at Summit during periods characterized by weak and irregular horizontal winds in combination with strong stability.
Rapid changes in net all-wave radiation in response to cloud forcing results in nonstationarity during unstable conditions.
Between 0930–1130 and 1900–1930 UTC turbulent heat fluxes are not only small in magnitude but also typically change sign,
with nonstationarity during these periods often as high as 65%. These results should help resolve some of the present uncertainties
in obtaining reliable fluxes at this site, in particular under stable atmospheric conditions. 相似文献
7.
Eugene Yee R. Chan P. R. Kosteniuk G. M. Chandler C. A. Biltoft J. F. Bowers 《Boundary-Layer Meteorology》1995,76(1-2):41-67
This paper describes a study of the vertical structure of concentration fluctuations in a neutrally buoyant plume from an elevated point source in slightly convective to moderately stable meteorological conditions at ranges of between 12.5 and 100 m for a range of source heights between 1 and 5 m. Observations were made of concentration fluctuations in a dispersing plume using a vertical array of sixteen very fast-response photoionization detectors placed at heights between 0.5 and 16 m. Vertical profiles of a number of concentration statistics were extracted, namely, mean concentration, fluctuation intensity, intermittency factor, peak-to-mean concentration ratio, mean dissipation rate of concentration variance, and various concentration time and length scales of dominant motions in the plume (e.g., integral macro-scale, in-plume mid-scale and Taylor micro-scale). The profiles revealed a similarity to corresponding crosswind profiles for a fully elevated plume, but showed greater and greater departure from the latter shapes once the plume had grown in the vertical so that its lower dege began to interact progressively more strongly with the ground. The evolution of the concentration probability density function at a fixed range, but with decreasing height from the ground, is similar to that obtained at a fixed height but with increasing distance from the source. Concentration power spectra obtained at different heights all had an extensive inertial-convective subrange spanning at least two decades in frequency, but spectra measured near the ground had a greater proportion of the total concentration variance in the lower frequencies (energetic subrange), with a correspondingly smaller proportion in the higher frequencies (inertial-convective subrange). It is believed that these effects result from the increased mean shear near the surface, and blocking by the surface. The effect of enhanced shear-induced molecular diffusion on concentration fluctuations is examined. 相似文献
8.
S. Sethuraman 《Boundary-Layer Meteorology》1980,19(1):67-80
Formation and breaking of internal gravity waves contributing to a very significant increase in turbulence in the atmospheric surface layer over the Atlantic Ocean off Long Island, New York are reported. Contrary to the bursts that are characteristically of short duration, this increase in turbulence lasted for more than one hour and was typical of what one would observe during unstable atmospheric conditions. However, mean temperature profiles indicated strong stable conditions.This research was performed under the auspices of the United States Department of Energy Under Contract No. EY-76-C-02-0016. 相似文献
9.
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. 相似文献
10.
A model for sensible heat flux probability density function for near-neutral and slightly-stable atmospheric flows 总被引：2，自引：0，他引：2
Gabriel G. Katul 《Boundary-Layer Meteorology》1994,71(1-2):1-20
The probability density function for sensible heat flux was measured above a uniform dry lakebed (Owens lake) in Owens Valley, California. It was found that for moderately stable to near neutral atmospheric stability conditions, the probability density function exhibits well defined exponential tails. These exponential tails are consistent with many laboratory boundarylayer measurements and numerical simulations. A model for the sensible heat flux probability density function was developed and tested. A key assumption in the model derivation was the near Gaussian statistics of the vertical velocity and temperature fluctuations. This assumption was verified from time series measurements of temperature and vertical velocity. The parameters for the sensible heat flux probability density function model were also derived from mean meteorological and surface conditions using surface-layer similarity theory. It was found that the best agreement between modeled and measured sensible heat flux probability density function was at the tails. Finally, a relation between the intermittency parameter, the probability density function, and the mean meteorological conditions was derived. This relation rigorously links the intermittency parameter to mean meteorological conditions. 相似文献
11.
12.
Over the past two decades, several inverse methods have been proposed to estimatescalar source and sink strengths from measured mean concentration profiles withinthe canopy volume (hereafter termed the `inverse' problem). These inverse methodscommonly assumed neutral atmospheric stability conditions for the entire canopyvolume. For non-neutral conditions, atmospheric stability corrections in inverseschemes were limited to adjusting the integral time scale or other flow statistics tomatch well-established surface-layer similarity relations above the canopy. Suchstability corrections do not explicitly consider the local stability effects within thecanopy volume. Currently, there is no satisfactory inverse scheme that explicitlyaccounts for local atmospheric stability for canopy turbulence. A Eulerian inversemethod that explicitly accounts for local atmospheric stability within the canopy isdeveloped using second-order closure principles. Field testing the method is conductedusing temperature measurements from two field experiments collected in an even-ageduniform loblolly pine forest. It is demonstrated that by accounting for local atmospheric stability in the inversion scheme, the agreement between modelled sensible heat flux calculations and measurements improve by 60% for stable conditions, 10% for near-neutral conditions and 20% for unstable conditions 相似文献
13.
Measurements of flow over an elongated ridge and its thermal stability dependence: The mean field 总被引：1，自引：0，他引：1
Measurements of mean wind flow and turbulence parameters have been made over Cooper's Ridge, a 115 m high elongated ridge with low surface roughness. This paper describes measurements of the streamwise and vertical variations in the mean field for a variety of atmospheric stability conditions. In near-neutral conditions, the normalised speedup over the ridge compares well with measurements from Askervein (Mickleet al., 1988). The near-neutral results are also compared to an analytical flow model based on that of Huntet al. (1988a). Measured streamwise variations show less deceleration at the foot of the hill and slightly more acceleration at the crest of the hill than does the model. In non-neutral conditions, the speedup over the ridge reduces slightly in unstable conditions and increases by up to a factor of two in stable conditions. The model is modified to allow boundary-layer stability to change the upwind wind profile and the depths of the inner and middle layers. Such a modification is shown to describe the observations of speedup well in unstable and weakly stable conditions but to overestimate the speedup in moderate to strongly stable conditions. This disagreement can be traced to the model's overestimation of the upstream scaling velocity at the height of the middle layer through its use of a stable wind profile form which has greater shear than that of the observed profiles, in possible combination with the three-dimensionality of the ridge which would allow enhanced flow around, rather than over, the feature in more stable conditions. 相似文献
14.
The effects of source size on plume behaviour have been examined in a 1.2 m wind tunnel boundary layer for isokinetic sources with diameters from 3 to 35 mm at source heights of 230 mm and at ground level. Experimental measurements of mean concentration and the variance, intermittency and probability density functions of the concentration fluctuations were obtained. In addition, a fluctuating Gaussian plume model is presented which reproduces many of the observed features of the elevated emission. The mean plume width becomes independent of source size much more rapidly than the instantaneous plume width. Since it is the meandering of the instantaneous plume which generates most of the concentration fluctuations near the source, these are also dependent on source size. The flux of variance in the plume reaches a maximum, whose value is greatest for the smallest source size, close to the source and thereafter is monotonically decreasing. The intermittency factor reaches a minimum, whose value is lowest for the smallest source, and increases back towards one. Concentration fluctuations for the ground-level source are much less dependent on source size due to the effects of the surface. 相似文献
15.
In recent years field experiments have been undertaken in the lower atmosphere to perform a priori tests of subgrid-scale
(SGS) models for large-eddy simulations (LES). The experimental arrangements and data collected have facilitated studies of
variables such as the filtered strain rate, SGS stress and dissipation, and the eddy viscosity coefficient. However, the experimental
set-ups did not permit analysis of the divergence of the SGS stress (the SGS force vector), which is the term that enters
directly in the LES momentum balance equations. Data from a field experiment (SGS2002) in the west desert of Utah, allows
the calculation of the SGS force due to the unique 4 × 4 sonic anemometer array. The vector alignment of the SGS force is
investigated under a range of atmospheric stabilities. The eddy viscosity model is likely aligned with the measured SGS force
under near-neutral and unstable conditions, while its performance is unsatisfactory under stable conditions. 相似文献
16.
Experimental evaluation of analytical and Lagrangian surface-layer flux footprint models 总被引：3，自引：1，他引：3
Three surface-layer flux footprint models have been evaluated with the results of an SF6 tracer release experiment specifically designed to test such models. They are a Lagrangian stochastic model, an analytical model, and a simplified derivative of the analytical model. Vertical SF6 fluxes were measured by eddy correlation at four distances downwind of a near-surface crosswind line source in an area of homogeneous sagebrush. The mean fluxes were calculated for 136 half-hour test periods and compared to the fluxes predicted by the footprint models. All three models gave similar predictions and good characterizations of the footprint over the stability range -0.01 < z
0/L < 0.005. The predictions of the three models were within the limits of the uncertainty of the experimental measurements in all but a few cases within this stability range. All three models are unconditionally recommended for determining the area defined by the footprint over short vegetative canopies in this range. They are also generally appropriate for estimating flux magnitudes within the limits of experimental uncertainties. Most of the mean differences observed between the measured and predicted fluxes at each of the four towers reflect a tendency for the measured fluxes to be greater than those predicted by the three models. Rigorous verification of the models in strongly stable conditions was complicated by the need to obtain very accurate measurements of small fluxes in only marginally stationary conditions. Verification in strongly unstable conditions was hampered by the limited number of appropriate data. 相似文献
17.
Andrey A. Grachev Laura S. Leo Harindra J. S. Fernando Christopher W. Fairall Edward Creegan Byron W. Blomquist Adam J. Christman Christopher M. Hocut 《Boundary-Layer Meteorology》2018,167(2):181-210
Atmospheric turbulence measurements made at the U.S. Army Corps of Engineers Field Research Facility (FRF) located on the Atlantic coast near the town of Duck, North Carolina during the CASPER-East Program (October–November 2015) are used to study air–sea/land coupling in the FRF coastal zone. Turbulence and mean meteorological data were collected at multiple levels (up to four) on three towers deployed at different landward distances from the shoreline, with a fourth tower located at the end of a 560-m-long FRF pier. The data enable comparison of turbulent fluxes and other statistics, as well as investigations of surface-layer scaling for different footprints, including relatively smooth sea-surface conditions and aerodynamically rough dry inland areas. Both stable and unstable stratifications were observed. The drag coefficient and diurnal variation of the sensible heat flux are found to be indicators for disparate surface footprints. The drag coefficient over the land footprint is significantly greater, by as much as an order of magnitude, compared with that over the smooth sea-surface footprint. For onshore flow, the internal boundary layer in the coastal zone was either stable or (mostly) unstable, and varied dramatically at the land-surface discontinuity. The offshore flow of generally warm air over the cooler sea surface produced a stable internal boundary layer over the ocean surface downstream from the coast. While the coastal inhomogeneities violate the assumptions underlying Monin–Obukhov similarity theory (MOST), any deviations from MOST are less profound for the scaled standard deviations and the dissipation rate over both water and land, as well as for stable and unstable conditions. Observations, however, show a poor correspondence with MOST for the flux-profile relationships. Suitably-averaged, non-dimensional profiles of wind speed and temperature vary significantly among the different flux towers and observation levels, with high data scatter. Overall, the statistical dependence of the vertical gradients of scaled wind speed and temperature on the Monin–Obukhov stability parameter in the coastal area is weak, if not non-existent. 相似文献
18.
Buoyancy-driven boundary currents were generated in the laboratory by releasing buoyant fluid from a source adjacent to a vertical boundary in a rotating container. The boundary removed the Coriolis force parallel to it, allowing the buoyant fluid to spread in a current along the boundary. Ise of a cylindrical boundary and a line source that released fluid uniformly around the circumference enabled an axisymmetric (zonal) current to be produced. With the continuous release of fluid from the source, the current grew in width and depth until it became unstable to non-axisymmetric disturbances. The wavelength and phase velocities of the disturbances were consistent with a model of baroclinic instability of two-layer flow when frictional dissipation due to Ekman layers is included. However, when the current only occupied a small fraction of the total depth, barotropic processes were also thought to be important, with the growing waves gaining energy from the horizontal shear.In other experiments, gravity currents were produced by a point source adjacent to either a zonal (circular) or a meridional (radial) vertical boundary. The currents were also observed to become unstable to the same upstream breaking waves as those on the continuous zonal current. Finally, some comparisons are made with oceanic coastal currents. 相似文献
19.
20.
Massimiliano Burlando Emilia Georgieva Corrado F. Ratto 《Boundary-Layer Meteorology》2007,125(2):389-397
The planetary boundary-layer (PBL) parameterization is a key issue for the definition of initial wind flow fields in diagnostic
models. However, PBL theories usually treat separately stable, neutral, and convective stability conditions, so that their
implementation in diagnostic wind models is not straightforward. In the present paper, an attempt is made to adopt a comprehensive
PBL parameterisation, covering stable/neutral and unstable atmospheric conditions, which appears suitable to diagnostic models.
This parameterisation is implemented into our diagnostic mass-consistent code. A validation of the consistency between the
implemented PBL parameterisations has been checked through an analysis of the sensitivity of the vertical wind profiles to
atmospheric stability. 相似文献