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1.
We analyse single-point velocity statistics obtained in a wind tunnel within and above a model of a waving wheat crop, consisting of nylon stalks 47 mm high and 0.25 mm wide in a square array with frontal area index 0.47. The variability of turbulence measurements in the wind tunnel is illustrated by using a set of 71 vertical traverses made in different locations, all in the horizontally-homogeneous (above-canopy) part of the boundary layer. Ensemble-averaged profiles of the statistical moments up to the fourth order and profiles of Eulerian length scales are presented and discussed. They are consistent with other similar experiments and reveal the existence of large-scale turbulent coherent structures in the flow. The drag coefficient in this canopy as well as in other reported experiments is shown to exhibit a characteristic height-dependency, for which we propose an interpretation. The velocity spectra are analysed in detail; within and just above the canopy, a scaling based on fixed length and velocity scales (canopy height and mean horizontal wind speed at canopy top) is proposed. Examination of the turbulent kinetic energy and shear stress budgets confirms the role of turbulent transport in the region around the canopy top, and indicates that pressure transport may be significant in both cases. The results obtained here show that near the top of the canopy, the turbulence properties are more reminiscent of a plane mixing layer than a wall boundary layer. 相似文献
2.
强风天气下边界层结构特征 总被引：2，自引：0，他引：2
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。 相似文献
3.
In-Hye Lee Young-Hee Lee Jinkyu Hong 《Asia-Pacific Journal of Atmospheric Sciences》2011,47(3):281-291
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. 相似文献
4.
Thara V. Prabha Monique Y. Leclerc Anandakumar Karipot David Y. Hollinger Erich Mursch-Radlgruber 《Boundary-Layer Meteorology》2008,126(2):219-236
Observations of low-level jets (LLJs) at the Howland AmeriFlux site in the USA and the jet’s impact on nocturnal turbulent
exchange and scalar fluxes over a tall forest canopy are discussed. Low-frequency motions and turbulent bursts characterize
moderately strong LLJs, whereas low-frequency motions are suppressed during periods with strong LLJs and enhanced shear. An
analysis based on the shear-sheltering hypothesis seeks to elucidate the effect of LLJs on flux measurements. In the absence
of shear sheltering, large eddies penetrate the roughness sublayer causing enhanced mixing while during periods with shear
sheltering, mixing is reduced. In the absence of the latter, ‘upside-down’ eddies are primarily responsible for the enhanced
velocity variances, scalar and momentum fluxes. The integral length scales over the canopy are greater than the canopy height.
The variance spectra and cospectra from the wavelet analysis indicate that large eddies (spatial scale greater than the low-level
jet height) interact with active canopy-scale turbulence, contributing to counter-gradient scalar fluxes. 相似文献
5.
Progress on practical problems such as quantifying gene flow and seed dispersal by wind or turbulent fluxes over nonflat terrain
now demands fundamental understanding of how topography modulates the basic properties of turbulence. In particular, the modulation
by hilly terrain of the ejection-sweep cycle, which is the main coherent motion responsible for much of the turbulent transport,
remains a problem that has received surprisingly little theoretical and experimental attention. Here, we investigate how boundary
conditions, including canopy and gentle topography, alter the properties of the ejection-sweep cycle and whether it is possible
to quantify their combined impact using simplified models. Towards this goal, we conducted two new flume experiments that
explore the higher-order turbulence statistics above a train of gentle hills. The first set of experiments was conducted over
a bare surface while the second set of experiments was conducted over a modelled vegetated surface composed of densely arrayed
rods. Using these data, the connections between the ejection-sweep cycle and the higher-order turbulence statistics across
various positions above the hill surface were investigated. We showed that ejections dominate momentum transfer for both surface
covers at the top of the inner layer. However, within the canopy and near the canopy top, sweeps dominate momentum transfer
irrespective of the longitudinal position; ejections remain the dominant momentum transfer mode in the whole inner region
over the bare surface. These findings were well reproduced using an incomplete cumulant expansion and the measured profiles
of the second moments of the flow. This result was possible because the variability in the flux-transport terms, needed in
the incomplete cumulant expansion, was shown to be well modelled using “local” gradient-diffusion principles. This result
suggests that, in the inner layer, the higher-order turbulence statistics appear to be much more impacted by their relaxation
history towards equilibrium rather than the advection-distortion history from the mean flow. Hence, we showed that it is possible
to explore how various boundary conditions, including canopy and topography, alter the properties of the ejection-sweep cycle
by quantifying their impact on the gradients of the second moments only. Implications for modelling turbulence using Reynolds-averaged
Navier Stokes equations and plausible definitions for the canopy sublayer depth are briefly discussed. 相似文献
6.
介绍一种新的建立在经验模态分解(EMD)方法基础上的非线性、非平稳数据分析技术一Hilbert分析技术,并首次将其应用于大气边界层(PBL)湍流数据的分析,初步探讨了其在PBL湍流研究中的有效性.通过对城市与森林冠层上湍流资料的能量分布特征和统计平稳度进行分析、比较,结果表明:Hilbert谱分析能有效地对PBL湍流信号进行分析.它的边缘谱分析能够有效地探测PBL湍流信号的能量分布特征,统计平稳度分析也能有效地给出PBL湍流信号平稳性的定量化测量,这些将有助于建立合适的数据质量控制方法,以及对现有空气质量与扩散模式中扩散参数的计算加以改进.文中个例分析中,城市和森林冠层上空的湍流有一定相似性,湍流混合都比较充分,但森林冠层上湍流信号的能量更多地集中在大尺度湍涡,且扰动风速的高频部分具有更强的间歇性.对于相近高度的湍流信号来说,多数情况下,森林冠层上相同尺度的湍涡表现得比城市冠层上更不稳定,但湍涡的含能量要更低. 相似文献
7.
Impact of Eddy Characteristics on Turbulent Heat and Momentum Fluxes in the Urban Roughness Sublayer 总被引：1，自引：0，他引：1
Eddy-covariance observations above the densely built-up Centre of Nanjing were made from December 2011 to August 2012. Separate eddy-covariance systems installed at two levels on a 36-m tower located on a rooftop were operated simultaneously, and observations grouped into two sectors (A, B) according to the prevalent wind directions. For sector A, where the nearby buildings are all below the lower measurement level, the sensible heat and momentum fluxes are generally greater at the upper level. For sector B, where several high-rise buildings are located upwind, the sensible heat and momentum fluxes at the upper level are close to those at the lower level. The analysis shows that the turbulent eddy characteristics differ between the two wind sectors, leading to a different behaviour of turbulent exchange between the two levels. A hypothesis is proposed that addresses the vertical variation of turbulent fluxes in the urban roughness sublayer (RSL). For sector A, the buildings block the flow, change the trajectory of scalars, and distort the footprint of scalar fluxes; this ‘blocking effect’ is believed to lead to a smaller sensible heat flux above the canopy layer. Such an effect should decrease with height in the RSL, explaining the increase of the observed turbulent heat flux with height. In addition, the presence of non-uniform building heights adversely affects turbulence organization around the canopy top, and likely elevates the inflection point of the mean flow to a higher elevation close to the upper measurement level, where larger shear results in a larger momentum flux. For sector B, wake effects from the nearby high-rise buildings strongly reduce turbulence organization at higher elevations, leading to similar sensible heat and momentum fluxes at both measurement levels. 相似文献
8.
A. M. Reynolds 《Boundary-Layer Meteorology》2012,142(2):193-205
Second- and third-order turbulence closure models have met with mixed success when applied to the prediction of turbulent
flows within and above plant canopies and model predictions are typically no better than those obtained using simpler two-equation
(k-e){(k-\varepsilon)} models. This is because the local gradient diffusion approximation (a crucial model requirement) cannot represent accurately
turbulent transport that is dominated by the presence of ejections and sweeps whose length scales are comparable with the
canopy height. To make progress, turbulent transport must be treated without approximation, as in Lagrangian probability density
function (PDF) models. This study is the first to develop and to validate a PDF model of horizontally-homogeneous canopy flow.
The model relies upon a prescribed length-scale that has been used elsewhere in the modelling of turbulent flows. Model predictions
compare favourably with measurements of neutrally stratified turbulent flows within and above canopies of mature corn and
forested eucalypt trees. 相似文献
9.
10.
This paper argues that the active turbulence and coherent motions near the top of a vegetation canopy are patterned on a plane mixing layer, because of instabilities associated with the characteristic strong inflection in the mean velocity profile. Mixing-layer turbulence, formed around the inflectional mean velocity profile which develops between two coflowing streams of different velocities, differs in several ways from turbulence in a surface layer. Through these differences, the mixing-layer analogy provides an explanation for many of the observed distinctive features of canopy turbulence. These include: (a) ratios between components of the Reynolds stress tensor; (b) the ratio K
H/K
M of the eddy diffusivities for heat and momentum; (c) the relative roles of ejections and sweeps; (d) the behaviour of the turbulent energy balance, particularly the major role of turbulent transport; and (e) the behaviour of the turbulent length scales of the active coherent motions (the dominant eddies responsible for vertical transfer near the top of the canopy). It is predicted that these length scales are controlled by the shear length scale % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamitamaaBa% aaleaacaWGtbaabeaakiabg2da9iaadwfacaGGOaGaamiAaiaacMca% caGGVaGabmyvayaafaGaaiikaiaadIgacaGGPaaaaa!3FD0!\[L_S = U(h)/U'(h)\] (where h is canopy height, U(z) is mean velocity as a function of height z, and % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGabmyvayaafa% Gaeyypa0JaaeizaiaadwfacaGGVaGaaeizaiaadQhaaaa!3C32!\[U' = {\rm{d}}U/{\rm{d}}z\]). In particular, the streamwise spacing of the dominant canopy eddies is x = mL
s, with m = 8.1. These predictions are tested against many sets of field and wind-tunnel data. We propose a picture of canopy turbulence in which eddies associated with inflectional instabilities are modulated by larger-scale, inactive turbulence, which is quasi-horizontal on the scale of the canopy. 相似文献
11.
An analysis of the wind data recorded at the fifteen stations in the Salt Lake Valley indicates that the distributions of the kinetic energy of the mean and turbulent motions in the valley are generally inhomogeneous and nonstationary. The mean motion in the valley, which is strongly affected by the mountain-valley winds, shows a southeasterly flow in the evening and early morning, a northwesterly flow in the afternoon, and a transitional flow in the late morning and after sunset. The mountain winds generally associate with a horizontally convergent flow, whereas the valley winds associate with a horizontally divergent flow. The distributions of the kinetic energy of the mean and turbulent motions show a maximum occurring in the central part of the valley and two minimums, one in the northern and one in the southern part of the valley. In the afternoon, both the mean and turbulent motions increase their intensities, particularly in the western part of the valley. An analysis of the dispersion characteristics indicates that the rate of diffusion in the valley changes with time and space, with a maximum occurring in the early afternoon and minimum in the early morning. 相似文献
12.
The turbulent flow in and above plant canopies is of fundamental importance to the understanding oftransport processes of momentum,heat and mass between plant canopies and atmosphere,and to microme-teorology.The Reynolds stress equation model(RSM)has been applied to calculate the turbulence in cano-pies in this paper.The calculated mean wind velocity profiles,Reynolds stress,turbulent kinetic energy andviscous dissipation rate in a corn canopy and a spruce forest are compared with field observed data and withWilson's and Shaw's model.The velocity profiles and Rynolds stress calculated by both models are in goodagreement,and the length scale of turbulence appears to be similar. 相似文献
13.
The processes influencing turbulence in a deciduous forest and the relevant length and time scales are investigated with spectral and cross-correlation analysis. Wind velocity power spectra were computed from three-dimensional wind velocity measurements made at six levels inside the plant canopy and at one level above the canopy. Velocity spectra measured within the plant canopy differ from those measured in the surface boundary layer. Noted features associated with the within-canopy turbulence spectra are: (a) power spectra measured in the canopy crown peak at higher wavenumbers than do those measured in the subcanopy trunkspace and above the canopy; (b) peak spectral values collapse to a relatively universal value when scaled according to a non-dimensional frequency comprised of the product of the natural frequency and the Eulerian time scale for vertical velocity; (c) at wavenumbers exceeding the spectral peak, the slopes of the power spectra are more negative than those observed in the surface boundary layer; (d) Eulerian length scales decrease with depth into the canopy crown, then increase with further depth into the canopy; (e) turbulent events below crown closure are more correlated with turbulent events above the canopy than are those occurring in the canopy crown; and (f) Taylor's frozen eddy hypothesis is not valid in a plant canopy. Interactions between plant elements and the mean wind and turbulence alter the processes that produce, transport and remove turbulent kinetic energy and account for the noted observations. 相似文献
14.
H. G. Ouwersloot A. F. Moene J. J. Attema J. Vilà-Guerau de Arellano 《Boundary-Layer Meteorology》2017,162(1):71-89
The representation of a neutral atmospheric flow over roughness elements simulating a vegetation canopy is compared between two large-eddy simulation models, wind-tunnel data and recently updated empirical flux-gradient relationships. Special attention is devoted to the dynamics in the roughness sublayer above the canopy layer, where turbulence is most intense. By demonstrating that the flow properties are consistent across these different approaches, confidence in the individual independent representations is bolstered. Systematic sensitivity analyses with the Dutch Atmospheric Large-Eddy Simulation model show that the transition in the one-sided plant-area density from the canopy layer to unobstructed air potentially alters the flow in the canopy and roughness sublayer. Anomalously induced fluctuations can be fully suppressed by spreading the transition over four steps. Finer vertical resolutions only serve to reduce the magnitude of these fluctuations, but do not prevent them. To capture the general dynamics of the flow, a resolution of 10 % of the canopy height is found to suffice, while a finer resolution still improves the representation of the turbulent kinetic energy. Finally, quadrant analyses indicate that momentum transport is dominated by the mean velocity components within each quadrant. Consequently, a mass-flux approach can be applied to represent the momentum flux. 相似文献
15.
Albert F. Kurbatskiy Lyudmila I. Kurbatskaya 《Meteorology and Atmospheric Physics》2009,104(1-2):63-81
A modified three-parameter model of turbulence for a thermally stratified atmospheric boundary layer (ABL) is presented. The model is based on tensor-invariant parameterizations for the pressure–strain and pressure–temperature correlations that are more complete than the parameterizations used in the Mellor–Yamada model of level 3.0. The turbulent momentum and heat fluxes are calculated with explicit algebraic models obtained with the aid of symbol algebra from the transport equations for momentum and heat fluxes in the approximation of weakly equilibrium turbulence. The turbulent transport of heat and momentum fluxes is assumed to be negligibly small in this approximation. The three-parameter $E - \varepsilon - {\left\langle {\theta ^{2} } \right\rangle }$ model of thermally stratified turbulence is employed to obtain closed-form algebraic expressions for the fluxes. A computational test of a 24-h ABL evolution is implemented for an idealized two-dimensional region. Comparison of the computed results with the available observational data and other numerical models shows that the proposed model is able to reproduce both the most important structural features of the turbulence in an urban canopy layer near the urbanized ABL surface and the effect of urban roughness on a global structure of the fields of wind and temperature over a city. The results of the computational test for the new model indicate that the motion of air in the urban canopy layer is strongly influenced by mechanical factors (buildings) and thermal stratification. 相似文献
16.
Organised Motion and Radiative Perturbations in the Nocturnal Canopy Sublayer above an Even-Aged Pine Forest 总被引：2，自引：3，他引：2
Using time series measurements of velocity, carbon dioxideand water vapour concentration, and temperature collected justabove a 15 m tall even-aged pine forest, we quantify the roleof organized motion on scalar and momentum transport withinthe nocturnal canopy sublayer (CSL). We propose a frameworkin which the nocturnal CSL has two end-members, bothdominated by organised motion. These end-members representfully developed turbulent flows at near-neutral or slightly stablestratification and no turbulence for very stable stratification.Our analysis suggests that ramps dominate scalar transport fornear-neutral and slightly stable conditions, while linear canopywaves dominate the flow dynamics for very stable conditions.For intermediate stability, the turbulence is highly damped andoften dominated by fine scale motions. Co-spectral analysissuggests that ramps are the most efficient net scalar mass-transportingagent while linear canopy waves contribute little to net scalartransport between the canopy and atmosphere for averagingintervals that include complete wave cycles. However, canopywaves significantly contribute to the spectral properties of thescalar time series. Ramps are the most frequently occurringorganised motion in the nocturnal CSL for this site.Numerous night-time runs, however, resided between thesetwo end-members. Our analysis suggests that whenradiative perturbations are sufficient large (>20 W m-2 innet radiation), the flow can switch from being highly dampedfine-scale turbulence to being organized with ramp-like properties. We also found that when ramps are already the dominant eddymotion in the nocturnal CSL, radiative perturbations have aminor impact on scalar transport. Finally, in agreement withprevious studies, we found that ramps and canopy waves havecomparable length scales of about 30–60 metres. Consequencesto night-time flux averaging are also discussed. 相似文献
17.
2009年“莫拉克”台风登陆过程阵风特征分析 总被引：3，自引：2，他引：1
利用上海台风研究所移动观测车获取的“莫拉克”台风登陆过程中超声风、温等观测资料对地面阵风特性进行了诊断分析.结果表明,在风速时间序列中叠加有周期为3-7 min的阵风扰动,显现出明显的相干结构,即沿顺风方向阵风风速峰期有下沉运动,谷期有上升运动;阵风扰动的各向异性特征明显,沿顺风方向的阵风扰动能量最大,其次是沿侧向和垂直方向的扰动能量;沿顺风方向的阵风垂直动量通量向下传播,而沿侧风方向阵风扰动动量垂直通量总体贡献接近于0.阵风扰动沿顺风方向的积分空间尺度和时间尺度最大,沿侧风方向和垂直方向其次,均明显大于湍流的积分空间和时间尺度.此外,阵风扰动的其他特征还包括:感热垂直通量极小;当平均风速较大时阵风风向变化幅度较小,而风速较小时阵风风向变化幅度则较大;动力学分析表明,阵风扰动主要表现出重力内波的一些特性. 相似文献
18.
Turbulent transport processes for momentum and scalar quantities are examined by a joint probability distribution analysis using data observed within and above a deciduous forest. Characteristics of transport processes in the frequency domain were also analyzed using Tukey's procedure. The results confirm that sweep phenomena prevail within and at the top of a tall plant canopy and that downdrafts are more effective for vertical transport of momentum and scalar quantities. On the other hand, updrafts become more efficient for vertical transport in the daytime at levels about twice treetop height. The results show that within the forest, the sweep phenomenon prevails over a wide frequency range, while above the forest, prevalence of the ejection phenomenon is limited to low frequencies. It is again noted that the plant canopy plays an important role in the sweep-ejection cycle as well as in turbulent transport processes. 相似文献
19.
Particle image velocimetry (PIV) data obtained in a wind-tunnel model of a canopy boundary layer is used to examine the characteristics
of mean flow and turbulence. The vector spacing varies between 1.7 and 2.5 times the Kolmogorov scales. Conditional sampling
based on quadrants, i.e. based on the signs of velocity fluctuations, reveals fundamental differences in flow structure, especially
between sweep and ejection events, which dominate the flow. During sweeps, the downward flow generates a narrow, highly turbulent,
shear layer containing multiple small-scale vortices just below canopy height. During ejections, the upward flow expands this
shear layer and the associated small-scale flow structures to a broad region located above the canopy. Consequently, during
sweeps the turbulent kinetic energy (TKE), Reynolds stresses, as well as production and dissipation rates, have distinct narrow
peaks just below canopy height, whereas during ejections these variables have broad maxima well above the canopy. Three methods
to estimate the dissipation rate are compared, including spectral fits, measured subgrid-scale (SGS) energy fluxes at different
scales, and direct measurements of slightly underresolved instantaneous velocity gradients. The SGS energy flux is 40–60%
of the gradient-based (direct) estimates for filter sizes inside the inertial range, while decreasing with scale, as expected,
within the dissipation range. The spectral fits are within 5–30% of the direct estimates. The spectral fits exceed the direct
estimates near canopy height, but are lower well above and below canopy height. The dissipation rate below canopy height increases
with velocity magnitude, i.e. it has the highest values during sweep and quadrant 1 events, and is significantly lower during
ejection and quadrant 3 events. Well above the canopy, ejections are the most dissipative. Turbulent transport during sweep
events acts as a source below the narrow shear layer within the canopy and as a sink above it. Transport during ejection events
is a source only well above the canopy. The residual term in the TKE transport equation, representing mostly the effect of
pressure–velocity correlations, is substantial only within the canopy, and is dominated by sweeps. 相似文献
20.
In southern China,cold air is a common weather process during the winter season;it can cause strong wind,sharp temperature decreases,and even the snow or freezing rain events.However,the features of the atmospheric boundary layer during cold air passage are not clearly understood due to the lack of comprehensive observation data,especially regarding turbulence.In this study,four-layer gradient meteorological observation data and one-layer,10-Hz ultrasonic anemometer-thermometer monitoring data from the northern side of Poyang Lake were employed to study the main features of the surface boundary layer during a strong cold-air passage over southern China.The results show that,with the passage of a cold air front,the wind speed exhibits low-frequency variations and that the wind systematically descends.During the strong wind period,the wind speed increases with height in the surface layer.Regular gust packets are superimposed on the basic strong wind flow.Before the passage of cold air,the wind gusts exhibit a coherent structure.The wind and turbulent momentum fluxes are small,although the gusty wind momentum flux is slightly larger than the turbulent momentum flux.However,during the invasion of cold air,both the gusty wind and turbulent momentum fluxes increase rapidly with wind speed,and the turbulent momentum flux is larger than the gusty wind momentum flux during the strong wind period.After the cold air invasion,this structure almost disappears. 相似文献