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1.
The spatial variability of turbulence in a fully-leafed almond orchard was studied. Two three-dimensional sonic anemometers were used to measure turbulence spectra and coherence at different vertical and lateral separations inside the canopy. Peak frequencies of the horizontal velocity components, normalized by local horizontal wind speed, are greater in the canopy crown than in the trunkspace. Peak-normalized frequencies for the vertical velocity power spectra are similar in the canopy crown and in the subcanopy trunkspace. Spectral slopes in the inertial subrange are more negative than those predicted with Kolmogorov's -2/3 theory. It is thought that the foliage elements act to short-circuit the eddy cascade. Lateral separation of the instruments in the subcanopy trunkspace has little effect on the shape of the velocity spectra. On the other hand, lateral and vertical velocity coherences between spatially separated sensors are low inside the canopy. These low coherences are due to the Eulerian length scales being of the same order of magnitude as the separation distances of the anemometers. Phase angles between velocity components are about zero for small separation distances. When the two instruments are separated by 9 m and one instrument is positioned in a row while the other is between two rows, vertical velocities are about 180 deg out of phase and the streamwise velocities are about 40 to 60 deg out of phase. These data support the contention that preferred differences occur between within- and between-row wind flow regimes. 相似文献
2.
Numerical Simulation of the Airflow Across Trees in a Windbreak 总被引：2，自引：0，他引：2
The flow across a three-dimensional (3-D) windbreak comprising individual cypress trees is studied to establish the significance and extent of the 3-D flow patterns. The cypress tree is modelled as a solid cylindrical stem and a conic porous canopy. Cases with a single row of trees or two rows of trees with different distances between the rows are considered; in the case of a single row, several densities of the canopy are used. The steady Reynolds-averaged Navier–Stokes (RANS) approximation is solved using a commercial computational fluid dynamics (CFD) package and a high-resolution mesh. Three-dimensional flow is found in the vicinity of the windbreak up to a leeward distance of 1–2 tree-heights, depending on the density of the canopy, and is manifest as significant lateral variations and reduced vertical flow. At larger leeward distances, a two-dimensional (2-D) flow is established with characteristics similar to existing 2-D studies; the flow leeward of the last row is insensitive to the distance between the rows. Homogeneous 2-D windbreak models are found to be inaccurate in the vicinity of the windbreak. This is exactly the region that needs to be sheltered in many cases, since the inner vegetation is anyway protected by the outer vegetation. 相似文献
3.
Antonio Segalini Jens H. M. Fransson P. Henrik Alfredsson 《Boundary-Layer Meteorology》2013,148(2):269-283
An analysis of velocity statistics and spectra measured above a wind-tunnel forest model is reported. Several measurement stations downstream of the forest edge have been investigated and it is observed that, while the mean velocity profile adjusts quickly to the new canopy boundary condition, the turbulence lags behind and shows a continuous penetration towards the free stream along the canopy model. The statistical profiles illustrate this growth and do not collapse when plotted as a function of the vertical coordinate. However, when the statistics are plotted as function of the local mean velocity (normalized with a characteristic velocity scale), they do collapse, independently of the streamwise position and freestream velocity. A new scaling for the spectra of all three velocity components is proposed based on the velocity variance and integral time scale. This normalization improves the collapse of the spectra compared to existing scalings adopted in atmospheric measurements, and allows the determination of a universal function that provides the velocity spectrum. Furthermore, a comparison of the proposed scaling laws for two different canopy densities is shown, demonstrating that the vertical velocity variance is the most sensible statistical quantity to the characteristics of the canopy roughness. 相似文献
4.
Three-dimensional wind velocity components were measured above and within a uniform almond orchard. Turbulent statistics associated with the turbulent flow inside the canopy are examined in detail. Turbulence in an almond orchard is characterized by relatively high turbulent intensities and large skewness and kurtosis values. These results indicate that the frequency distribution of wind velocity components is non-Gaussian. Conditional sampling of the turbulent measurements show that large, infrequent sweeps provide the predominant mechanism for tangential momentum stress in the canopy crown. Deep inside the canopy, a secondary wind maximum and small, but positive, tangential momentum stresses are observed. 相似文献
5.
A neighbourhood-scale multi-layer urban canopy model of shortwave and longwave radiation exchange that explicitly includes the radiative effects of tall vegetation (trees) is presented. Tree foliage is permitted both between and above buildings, and mutual shading, emission and reflection between buildings and trees are included. The basic geometry is a two-dimensional canyon with leaf area density profiles and probabilistic variation of building height. Furthermore, the model accounts for three-dimensional path lengths through the foliage. Ray tracing determines the receipt of direct shortwave irradiance by building and foliage elements. View factors for longwave and shortwave diffuse radiation exchange are computed once at the start of the simulation using a Monte Carlo ray tracing approach; for subsequent model timesteps, matrix inversion rapidly solves infinite reflections and interception of emitted longwave between all elements. The model is designed to simulate any combination of shortwave and longwave radiation frequency bands, and to be portable to any neighbourhood-scale urban canopy geometry based on the urban canyon. Additionally, the model is sufficiently flexible to represent forest and forest-clearing scenarios. Model sensitivity tests demonstrate the model is robust and computationally feasible, and highlight the importance of vertical resolution to the performance of urban canopy radiation models. Full model evaluation is limited by the paucity of within-canyon radiation measurements in urban neighbourhoods with trees. Where appropriate model components are tested against analytic relations and results from an independent urban radiation transfer model. Furthermore, system response tests demonstrate the ability of the model to realistically distribute shortwave radiation among urban elements as a function of built form, solar angle and tree foliage height, density and clumping. Separate modelling of photosynthetically-active and near-infrared shortwave bands is shown to be important in some cases. Increased canyon height-to-width ratio and/or tree cover diminishes the net longwave radiation loss of individual canyon elements (e.g., floor, walls), but, notably, has little effect on the net longwave loss of the whole urban canopy. When combined with parametrizations for the impacts of trees on airflow and hydrological processes in the urban surface layer, the new radiation model extends the applicability of urban canopy models and permits more robust assessment of trees as tools to manage urban climate, air quality, human comfort and building energy loads. 相似文献
6.
森林下垫面陆面物理过程及局地气候效应的数值模拟试验 总被引：5，自引：0，他引：5
文中基于大气边界层和植被冠层微气象学基本原理 ,建立了一个森林植被效应的陆面物理过程和二维大气边界层数值模式。并应用该模式进行了植被和土壤含水量等生物和生理过程在陆面过程和局地气候效应方面的数值模拟试验。所得数值模拟试验结果与实际情况相吻合。结果表明 ,应用该模式可获得植被温度、植被冠层内空气温度、地表温度日变化特征 ;森林下垫面大气边界层风速、位温、比湿、湍流交换系数的时空分布和日变化特征。该模式还可应用于不同下垫面 ,模拟陆面物理过程与大气边界层相互作用机制及其局地气候效应的研究 ,这将为气候模式与生物圈的耦合研究奠定一个良好的基础。 相似文献
7.
Numerical Simulation Experiment of Land Surface Physical Processes and Local Climate Effect in Forest Underlying Surface 总被引：1，自引：0，他引：1 
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下载免费PDF全文 LIU Shuhu PAN Ying DENG Yi MA Mingmin JIANG Haimei LIN Hongtao JIANG Haoyu LIANG Fuming LIU Heping WANG Jianhua 《Acta Meteorologica Sinica》2006,20(1):72-85
Based on the basic principles of atmospheric boundary layer and plant canopy micrometeorology, a forest underlying surface land surface physical process model and a two-dimensional atmospheric boundary layer numerical model are developed and numerical simulation experiments of biosphere and physiological processes of vegetation and soil volumetric water content have been done on land surface processes with local climate effect. The numerical simulation results are in good agreement with realistic observations, which can be used to obtain reasonable simulations for diurnal variations of canopy temperature, air temperature in canopy, ground surface temperature, and temporal and spatial distributions of potential temperature and vertical wind velocity as well as relative humidity and turbulence exchange coefficient over non-homogeneous underlying surfaces. It indicates that the model developed can be used to study the interaction between land surface process and atmospheric boundary layer over various underlying surfaces and can be extended to local climate studies. This work will settle a solid foundation for coupling climate models with the biosphere. 相似文献
8.
Dennis D. Baldocchi Shashi B. Verma Norman J. Rosenberg 《Boundary-Layer Meteorology》1983,25(1):43-54
Air flow was observed above and within canopies of a number of kinds of soybeans. The Clark cultivar and two isolines of the Harosoy cultivar were studied in 1979 and 1980, respectively. Wind speed above the canopy was measured with cup anemometers. Heated thermistor anemometers were used to measure air flow within the canopy. Above-canopy air flow was characterized in terms of the zero-plane displacement (d), roughness parameter (z o) and drag coefficient (C d). d and z o were dependent on canopy height but were independent of friction velocity in the range 0.55 to 0.75 m s?1 · C d for the various canopies ranged from 0.027 to 0.035. Greater C d values were measured over an erectophile canopy than over a planophile canopy. C d was not measurably affected by differences in leaf pubescence. Within-canopy wind profiles were measured at two locations: within and between rows. The wind profile was characterized by a region of great wind shear in the upper canopy and by a region of relatively weak wind shear in the middle canopy. Considerable spatial variability in wind speed was evident, however. This result has significant implications for canopy flow modeling efforts aimed at evaluating transport in the canopy. In the lower canopy, wind speed within a row increased with depth whereas wind speed between two rows decreased with depth. The wind speeds at the two locations tended to converge to a common value at a height near 0.10 m. The attenuation of within-canopy air flow was stronger in canopies with greater foliage density. Canopy flow attenuation seemed to decrease with increasing wind speed, suggesting that high winds distorted the shape of the canopy in such a manner that the penetration of wind into the canopy increased. 相似文献
9.
A large-eddy simulation study was performed to characterize turbulence in sparse, row-oriented canopies. This was accomplished by simulating a set of heterogeneous row-oriented canopies with varying row vegetation density and spacing. To determine the effects of heterogeneity, results were compared to horizontally homogeneous canopies with an equivalent ‘effective’ leaf area index. By using a proper effective leaf area index, plane-averaged mean velocities and bulk scaling parameters contained only small errors when heterogeneity was ignored. However, many cases had significantly larger second- and third-order velocity moments in the presence of heterogeneity. Some heterogeneous canopies also contained dispersive fluxes in the lower canopy that were over 20 % as large as the turbulent flux. Impacts of heterogeneity were most pronounced in the cases of large row leaf area density and widely spaced rows. Despite the substantial amount of open space in the sparse canopies, vertical velocity skewness and quadrant-hole analysis indicated that the flow behaved predominantly as a canopy layer even though integral length scales at the canopy top no longer followed mixing-layer scaling. This was supported by the fact that similar composite-averaged coherent structures could be readily identified in both the heterogeneous and homogeneous canopies. Heterogeneity had an effect on coherent structures, in that structure detection events were most likely to occur just upwind of the vegetation rows. In simulations with large row spacing, these structures also penetrated deeper into the canopy when compared to the equivalent homogeneous canopy. 相似文献
10.
利用石家庄SA多普勒天气雷达和饶阳X波段双偏振雷达探测资料，结合常规综合观测资料，对2018年6月13日下午发生于太行山东麓的雹云的天气背景、降雹特征、雷达回波演变特征及回波三维立体结构进行了综合分析，重点利用双多普勒雷达径向速度资料反演出格点的三维风速（流场），并结合回波特征分析了雹云云体结构。结果显示，高空强劲的偏北风急流促使涡后横槽转竖，槽后冷空气沿偏北气流南下，形成上干冷、下暖湿的不稳定层结，在低空低涡附近及地面辐合线上发展造成这次风雹天气；双多普勒雷达反演风场表明，雹云的中层强回波中心呈明显的“S”形水平流场和悬挂回波配置特征，构成了具有成雹的“0线”结构；不仅佐证了雹云降雹“0线”结构的存在，而且证明其呈现形式具有多样性。 相似文献
11.
Large-Eddy Simulation of Inhomogeneous Canopy Flows Using High Resolution Terrestrial Laser Scanning Data 总被引：1，自引：1，他引：0
Fabian Schlegel Jörg Stiller Anne Bienert Hans-Gerd Maas Ronald Queck Christian Bernhofer 《Boundary-Layer Meteorology》2012,142(2):223-243
The effect of sub-tree forest heterogeneity in the flow past a clearing is investigated by means of large-eddy simulation
(LES). For this purpose, a detailed representation of the canopy has been acquired by terrestrial laser scanning for a patch
of approximately 190 m length in the field site “Tharandter Wald”, near the city of Dresden, Germany. The scanning data are
used to produce a high resolution plant area distribution (PAD) that is averaged over approximately one tree height (30 m)
along the transverse direction, in order to simplify the LES study. Despite the smoothing involved with this procedure, the
resulting two-dimensional PAD maintains a rich vertical and horizontal structure. For the LES study, the PAD is embedded in
a larger domain covered with an idealized, horizontally homogeneous canopy. Simulations are performed for neutral conditions
and compared to a LES with homogeneous PAD and recent field measurements. The results reveal a considerable influence of small-scale
plant distribution on the mean velocity field as well as on turbulence data. Particularly near the edges of the clearing,
where canopy structure is highly variable, usage of a realistic PAD appears to be crucial for capturing the local flow structure.
Inside the forest, local variations in plant density induce a complex pattern of upward and downward motions, which remain
visible in the mean flow and make it difficult to identify the “adjustment zone” behind the windward edge of the clearing. 相似文献
12.
Horizontal and Vertical Co2 Advection In A Sloping Forest 总被引：7，自引：1，他引：7
A system measuring the horizontal and vertical advection was devised and installed in a sloping forest at the Vielsalm site, Belgium. The measurements showed that under stable conditions a flow regime established below the canopy: air flowed horizontally along the slope and entrained the air above the canopy vertically. This movement occurs during stable nights characterised by strongly negative net radiation. It creates negative air concentration gradients in both the vertical and horizontal directions. The advection fluxes associated with these movements are opposite and of a similar order of magnitude. This implies that the horizontal advection cannot be ignored in the carbon budget equation at night. Unfortunately, the large variability of, and considerable uncertainty about, advection fluxes does not enable one to produce estimates of the source term from these equations. Advection measurement systems should be improved in order to enable such estimates to be made. Particular attention should be paid to the estimation of the vertical velocity above the canopy and to the vertical profiles of the horizontal velocity and horizontal CO2 gradient below the canopy. 相似文献
13.
Takenobu Michioka Hiroshi Takimoto Hiroki Ono Ayumu Sato 《Boundary-Layer Meteorology》2018,168(2):247-267
The effects of fetch on turbulent flow and pollutant dispersion within a canopy formed by regularly-spaced cubical objects is investigated using large-eddy simulation. Six tracer gases are simultaneously released from a ground-level continuous pollutant line source placed parallel to the spanwise axis at the first, second, third, fifth, seventh and tenth rows. Beyond the seventh row, the standard deviations of the fluctuations in the velocity components and the Reynolds shear stresses reach nearly equivalent states. Low-frequency turbulent flow is generated near the bottom surface around the first row and develops as the fetch increases. The turbulent flow eventually passes through the canopy at a near-constant interval. The mean concentration within the canopy reaches a near-constant value beyond the seventh row. In the first and second rows, narrow coherent structures frequently affect the pollutant escape from the top of the canopy. These structures increase in width as the fetch increases, and they mainly affect the removal of pollutants from the canopy. 相似文献
14.
Ramp patterns in scalar traces such as temperature are the signature of coherent structures. A pseudo-wavelet analysis technique was developed in which ideal saw-tooth patterns of varying size were used as basis functions and fitted to temperature and velocity data. Data recorded from three very different vegetation stands were examined in this study. It was found that the most probable structure duration for the forest canopy was in the range 35–40 s, for the orchard canopy it was 20–25 s and for the maize it was 15–20 s. When expressed in non-dimensional form, the structure duration probability distribution for the maize canopy was about a decade larger than for the forest canopy, with the orchard canopy intermediate. The mean eddy duration versus wind shear relation falls on a narrow band for all three canopies, indicating that wind shear at the canopy top is the determining factor for the scale of the coherent eddies. The inverse of duration and intermittency of coherent structures exhibits a tendency of independence from wind shear at higher wind shear values. Coherent structures transport heat in a more efficient way than do smaller scale, less coherent motions. In all the canopies, the heat flux fractions associated with coherent structures are at least 10% higher than the corresponding time fraction. 相似文献
15.
In order to study the turbulence structure behind a multiscale tree-like element in a boundary layer, detailed particle image
velocimetry measurements are carried out in the near-wake of a fractal-like tree. The tree is a pre-fractal with five generations,
each consisting of three branches and a scale-reduction factor of 1/2 between consecutive generations. Detailed mean velocity
and turbulence stress profiles are documented, as well as their downstream development. Scatter plots of mean velocity gradient
(transverse shear in the wake) and Reynolds shear stress exhibit a good linear relation at all locations in the flow. Therefore,
in the transverse direction of the wake evolution, the data support the Boussinesq eddy-viscosity concept. The measured mixing
length increases with streamwise distance, in agreement with classic wake expansion rates. Conversely, the measured eddy viscosity
and mixing length in the transverse direction decrease with increasing elevation, which differs from the behaviours measured
in the vertical direction in traditional boundary layers or in canopy flows studied before. In order to find an appropriate
single length scale to describe the wake evolution behind a multiscale object, two models are proposed, based on the notion
of superposition of scales. One approach is based on the radial spectrum of the object while the second is based on its length-scale
distribution evaluated using fractal geometry tools. Both proposed models agree well with the measured mixing length. The
results suggest that information about multiscale clustering of branches must be incorporated into models of the mixing length
for flows through single or sparse canopies of multiscale trees. 相似文献
16.
L. H. Allen Jr. 《Boundary-Layer Meteorology》1975,8(1):39-79
Model predictions of CO2 concentrations downwind from a line source were calibrated using experimental data. Agreement between the model and experimental data was improved by adjusting for wind direction meander and cup anemometer overshoot. The model predictions showed that by using a negative exponential wind speed profile within the crop canopy, predictions were closer to observed CO2 concentration profiles than when experimentally-observed wind speed profiles, which were constant with height in the lower canopy, were used. This finding suggests that much of the lower canopy airflow was not direct mass flow in the downwind direction. Eddy diffusivity profiles which showed a within-canopy local minimum resulted in arestriction in the predicted loss of CO2 out of the canopy system. Two-dimensional plots of predicted null vertical flux and CO2 concentration portrayed vividly the turbulent diffusion and mass flow transport of CO2 from the line source. 相似文献
17.
18.
P. J. Walklate 《Boundary-Layer Meteorology》1993,63(1-2):1-22
The integral length-scalesL for the three orthogonal components of diffusivityK=L are derived from spectral analysis of velocity time series measurements. A 3-D sonic anemometer was used to make these velocity measurements at heights in the range 0.7–7.0 m in and above a 2 m orchard canopy with near-neutral atmospheric boundary-layer stability conditions. The integral length-scale is compared with another length-scale of diffusionL
obtained by fitting an exponential model to the auto-correlation spectrumR
E
(t) in the region 0.95<R
E
(t)<0.5 for smallt. This length-scale is appropriate to a high frequency region of the energy spectrum where turbulent momentum transport becomes diffusion-like and the turbulent energy varies with the inverse square of frequence. This region has been shown by others to determine the magnitude of the dissipation rate of turbulent energy by the action of viscosity even though the dominant dynamics are inviscid. Within the crop, the ratio of the length-scalesL/L
were found to be smaller than the values measured above the crop for vertical turbulence. This was attributed to the enhanced decay rate of turbulent energy due to the effect of the airflow interaction with the crop. It is unclear whether similar effects are present in the horizontal plane because of greater scatter in the data, resulting from the more variable nature of the wind direction in the horizontal plane. 相似文献
19.
An Investigation of Higher-Order Closure Models for a Forested Canopy 总被引：11，自引：10，他引：1
Gabriel Katul 《Boundary-Layer Meteorology》1998,89(1):47-74
Simultaneous triaxial sonic anemometer velocity measurements vertically arrayed at six levels within and above a uniform pine forest were used to examine two parameterization schemes for the triple-velocity correlation tensor employed in higher-order closure models. These parameterizations are the gradient-diffusion approximation typically used in second-order closure models, and the full budget for the triple-velocity correlation tensor typically employed in third-order closure models. Both second- and third-order closure models failed to reproduce the measured profiles of the triple-velocity correlation within and above the canopy. However, the Reynolds stress tensor profiles (including velocity variances) deviated greatly from the measurements only within the lower levels of the canopy. It is shown that the Reynolds stresses are most sensitive to the parameterization of the triple-velocity correlation in these lower canopy regions where local turbulent production is negligible and turbulence is mainly sustained by the flux transport term. The failure of the third-order closure model to reproduce the measured third moments in the upper layers of the canopy-top contradicts conclusions from a previous study over shorter vegetation but agrees with another study for a deciduous forest. Whether the third-order closure model failure is due to the zero-fourth-cumulant closure approximation is therefore considered. Comparisons between measured and predicted quadruple velocity correlations suggest that the zero-fourth-cumulant approximation is valid close to the canopy-atmosphere in agreement with recent experiments. 相似文献
20.
A new method to estimate tree biomass heat storage from thermal infrared (TIR) imaging of biomass surface temperature is presented.
TIR images of the canopy are classified into trunk, branches, and leaves. The one-dimensional heat equation in cylindrical
coordinates is forced with trunk and branch surface temperatures to simulate the temperature distribution and heat storage
in tree trunks and branches. Assuming uniform leaf temperatures, heat storage in leaves is computed from the surface temperature
of the leaves separately for the sunlit upper and shaded lower canopy. The sum of trunk, branches, leaf, and air heat storage
gives the canopy heat storage. Measurements in a walnut orchard near Davis, California, in early June 2007 showed that biomass
heat storage was of the same order as air heat storage and about 1% of daytime and 9% of nighttime net radiation. 相似文献