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1.
Spectra and Cospectra of Turbulence in an Internal Boundary Layer over a Heterogeneously Irrigated Cotton Field 
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下载免费PDF全文 During the Energy Balance Experiment, patch-to-patch irrigation generated gradients in soil moisture in a north-south oriented cotton field. An internal boundary layer (IBL) developed as a result of strong horizontal advection from relatively dry upstream patches to relatively wet downstream patches associated with the prevailing northerly winds. This generated large eddies of multiple sizes, which had significant influences on the structure of turbulence in the IBL. The power spectra and cospectra of wind speed, temperature, humidity, and energy fluxes measured at two heights within the IBL are presented and used to investigate the influence of the IBL on surface layer turbulence. The spectra and cospectra were greatly enhanced by external disturbances at low frequencies. The peak frequencies of these disturbances did not change with height. The spectra and cospectra typically converged and were parallel to the Kansas spectrum at high frequencies (in the inertial subrange). A clear gap in the spectra of horizontal wind velocity existed at intermediate frequencies when the surface layer was stable. The results indicate that large eddies that originated in the upstream convective boundary layer had considerable impacts on the spectra and cospectra of surface layer turbulence. The influence of these large eddies was greater (1) when the IBL was well-developed in the near surface layer than when the IBL did not exist, (2) at higher levels than at lower levels, and (3) when the atmospheric surface layer (ASL) was unstable than when the ASL was stable. The length scales of these large eddies were consistent with the dominant scales of surface heterogeneity at the experiment site. 相似文献
2.
Yu Zhang Heping Liu Thomas Foken Quinton L. Williams Matthias Mauder Christoph Thomas 《Theoretical and Applied Climatology》2011,103(1-2):119-131
The turbulence data measured at two levels (i.e., 8.7 and 2.7?m) in the Energy Balance Experiment (EBEX), which was conducted in San Joaquin Valley in California during the period from July 20 to August 24, 2000, are used to study the characteristics of coherent structures over an irrigated cotton field. Patch-to-patch irrigation in the field generated the dry-to-wet horizontal advection and the oasis effects, leading to the development of a stably internal boundary layer (SIBL) in the late mornings or the early afternoons. The SIBL persisted in the rest of the afternoons. Under this circumstance, a near-neutral atmospheric surface layer (ASL) developed during the period with a stratification transition from the unstable to stable conditions during the daytime. Therefore, EBEX provides us with unique datasets to investigate the features of coherent structures that were generated over the patches upstream and passed by our site in the unstable ASL, the near-neutral ASL, and the SIBL. We use an objective detection technique and the conditional average method that is developed based on the wavelet analysis. Our data reveal some consistencies and inconsistencies in the characteristics of coherent structures as compared with previous studies. Ramp-like structures and sweep?Cejection cycles under the daytime SIBL have similar patterns to those under the nocturnal stable ASL. However, some features (i.e., intermittence) are different from those under the nocturnal stable ASL. Under the three stratifications, thermal and mechanical factors in the ASL perform differently in affecting the ramp intensity for different quantities (i.e., velocity components, temperature, and specific humidity), leading to coherent structures that modulate turbulence flow and alter turbulent transfer differently. It is also found that coherent structures contribute about 10?C20% to the total fluxes in our case with different flux contributions under three stratifications and with higher transporting efficiency in sensible heat flux than latent heat and momentum fluxes. 相似文献
3.
Temperature variance and temperature power spectra in the unstable surface layer have always presented a problem to the standard
Monin-Obukhov similarity model. Recently that problem has intensified with the demonstration by Smedman et al. (2007, Q J
Roy Meteorol Soc 133: 37–51) that temperature spectra and heat-flux cospectra can have two distinct peaks in slightly unstable
conditions, and by McNaughton et al. (2007, Nonlinear Process Geophys 14: 257–271) who showed that the wavenumber of the peak
of temperature spectra in a convective boundary layer (CBL), closely above the surface friction layer (SFL), can be sensitive
to the CBL depth, z
i. Neither the two-peak form at slight instability nor the dependence of peak position on z
i at large instability is compatible with the Monin-Obukhov model. Here we examine the properties of temperature spectra and
heat-flux cospectra from between these extremes, i.e. from within the unstable SFL, in two experiments. The analysis is based
on McNaughton’s model of the turbulence structure in the SFL. According to this model, heat is transported through most of
the SFL by sheet plumes, created by the action of impinging outer eddies. The smallest and most effective of these outer eddies
have sizes that scale on SFL depth, z
s. The z
s-scale eddies and plumes are organised within the overall convection pattern in the CBL, and in turn they organise the motion
of smaller eddies within the SFL, whose sizes scale on height, z. The main experimental results are: (1) the peak amplitudes of the temperature spectra in the SFL are collapsed with a scaling
factor (zsz)1/3eo2/3{(z_{\rm s}z)^{1/3}\varepsilon_{\rm o}^{2/3}} divided by the square of the surface temperature flux, where eo{\varepsilon_{\rm o}} is the dissipation rate of turbulent energy in the outer CBL (above the SFL); (2) the peak wavenumbers of the temperature
spectra are collapsed with the mixed length scale (z
i
z
s)1/2; (3) the peak wavenumbers of the heat-flux cospectra are collapsed with the doubly-mixed length scale (z
i
z
s)1/4
z
1/2; (4) for z/z
s < 0.03, the peak in the cospectrum is replaced by another peak at a wavenumber about a magnitude larger. This peak’s position
scales on z; (5) all these findings are consistent with the observations of Smedman et al. 相似文献
4.
The dependence on atmospheric stability of flow characteristics adjacent to a very rough surface was investigated in a larch
forest in Japan. Micrometeorological measurements of three-dimensional wind velocity and air temperature were taken at two
heights above the forest, namely 1.7 and 1.2 times the mean canopy height h. Under near-neutral and stable conditions, the observed turbulence statistics suggest that the flow was likely to be that
of the atmospheric surface layer (ASL) at 1.7h, and of the roughness sublayer (RSL) at 1.2h. However, in turbulence spectra, canopy-induced large coherent motions appeared clearly at both heights. Even under strongly
stable conditions, the large-scale motions were retained at 1.2h, whereas they were overwhelmed by small-scale motions at 1.7h. This phenomenon was probably due to the enhanced contribution of the ASL turbulence associated with nocturnal decay of the
RSL depth, because the small-scale motions appeared at frequencies close to the peak frequencies of well-known ASL spectra.
This result supports the relatively recent concept that canopy flow is a superimposition of coherent motions and the ASL turbulence.
The large-scale motions were retained in temperature spectra over a wider region of stability compared to streamwise wind
spectra, suggesting that a canopy effect extended higher up for temperature than wind. The streamwise spacing of dominant
eddies according to the plane mixing-layer analogy was only valid in a narrow range at near neutral, and it was stabilised
at nearly half its value under stable conditions. 相似文献
5.
Large-eddy simulations (LESs) are employed to investigate the turbulence characteristics in the shear-free convective boundary
layer (CBL) driven by heterogeneous surface heating. The patterns of surface heating are arranged as a chessboard with two
different surface heat fluxes in the neighbouring patches, and the heterogeneity scale Λ in four different cases is taken as 1.2, 2.5, 5.0 and 10.0 km, respectively. The results are compared with those for the
homogeneous case. The impact of the heterogeneity scale on the domain-averaged CBL characteristics, such as the profiles of
the potential temperature and the heat flux, is not significant. However, different turbulence characteristics are induced
by different heterogeneous surface heating. The greatest turbulent kinetic energy (TKE) is produced in the case with the largest
heterogeneity scale, whilst the TKE in the other heterogeneous cases is close to that for the homogeneous case. This result
indicates that the TKE is not enhanced unless the scale of the heterogeneous surface heating is large enough. The potential
temperature variance is enhanced more significantly by a larger surface heterogeneity scale. But this effect diminishes with
increasing CBL height, which implies that the turbulent eddy structures are changed during the CBL development. Analyses show
that there are two types of organized turbulent eddies: one relates to the thermal circulations induced by the heterogeneous
surface heating, whilst the other identifies with the inherent turbulent eddies (large eddies) induced by the free convection.
At the early stage of the CBL development, the dominant scale of the organized turbulent eddies is controlled by the scale
of the surface heterogeneity. With time increasing, the original pattern breaks up, and the vertical velocity eventually displays
horizontal structures similar to those for the homogeneous heating case. It is found that after this transition, the values
of λ/z
i
(λ is the dominant horizontal scale of the turbulent eddies, z
i
is the boundary-layer height) ≈1.6, which is just the aspect ratio of large eddies in the CBL. 相似文献
6.
7.
Peter Hurley 《Boundary-Layer Meteorology》2007,125(3):525-536
The treatment of turbulence closure in atmospheric models is examined in the context of the dry convective boundary layer
(CBL) and the eddy-diffusivity/mass-flux (EDMF) approach. The EDMF approach is implemented into a model called TAPM to use
a coupled two-equation prognostic turbulence closure and the mass-flux approach to represent turbulence in the CBL. This work
also extends the range of turbulence variables that can be derived from the mass-flux component of the model and uses these
along with their values from the prognostic scheme to provide total turbulence fields that can be used to compare to data
and/or to feed into other components of TAPM, including those needed to drive Eulerian and Lagrangian air pollution dispersion
modules. Model results are presented for the afternoon of a simulated summer day and are compared to both laboratory and field
observations in a mixed-layer scaled framework. The results show that the EDMF approach works well within TAPM and can provide
good predictions of mean and turbulence fields, including in the upper levels of the CBL. The EDMF approach has several attractive
features, including the potential to be one approach to unify the treatment of turbulence and dry and moist convection in
atmospheric models. 相似文献
8.
The convective boundary layer (CBL) with a wide range of stability is simulated experimentally using a thermally stratified wind tunnel, and numerically by direct numerical simulation (DNS). The turbulence structures and flow characteristics of various CBL flows, capped by a strong temperature inversion and affected by surface shear, are investigated. The various vertical profiles of turbulence statistics similar to those from the observed CBL in the field are successfully simulated in both the wind-tunnel experiment and in DNS. The comparison of the wind-tunnel data and DNS results with those of atmospheric observations and water-tank studies shows the crucial dependence of the turbulence statistics in the upper part of the layer on the strength of the inversion layer, as well as the modification of the CBL turbulence regime by the surface shear. 相似文献
9.
利用大涡模式模拟了对流边界层结构演变以及深对流触发过程。通过改变鲍恩比的敏感性试验研究不同大气初始状况下湿润和干旱下垫面湍流特征及其对深对流触发过程的影响。结果表明:干旱下垫面的混合层干而暖,厚度较大;湿润下垫面相反。由于地表感热通量对热力湍流形成的作用更大,干旱下垫面上湍流混合和夹卷作用更强,使得水汽和相当位温在边界层内分布更均一,而在边界层顶有较大的负扰动;干旱下垫面上对流强度较湿润下垫面大,但均表现为泡状对流,水平方向上呈网状结构。不同下垫面上深对流的发生与大气初始状况有关,当初始时刻1—3 km的逆温强度较弱时（0.15 K/（100 m））,边界层内湍流迅速发展,深对流首先在干旱下垫面发生,但因对流有效位能较小,云层厚度小于湿润下垫面。当1—3 km的逆温强度增加到0.55 K/（100 m）时,云层形成时间较晚,云层厚度明显减小,仅当边界层顶的比湿较大时,有深对流发生,但仍首先发生在干旱下垫面,考虑贯穿对流在边界层顶引起的较强冷却作用,云层厚度大于湿润下垫面。 相似文献
10.
Spectra of CO<Subscript>2</Subscript> and Water Vapour in the Marine Atmospheric Surface Layer 总被引：1，自引：1，他引：0
Erik Sahlée Ann-Sofi Smedman Anna Rutgersson Ulf Högström 《Boundary-Layer Meteorology》2008,126(2):279-295
Spectra of CO2 and water vapour fluctuations from measurements made in the marine atmospheric surface layer have been analyzed. A normalization
of spectra based on Monin–Obukhov similarity theory, originally developed for wind speed and temperature, has been successfully
extended also to CO2 and humidity spectra. The normalized CO2 spectra were observed to have somewhat larger contributions from low frequencies compared to humidity spectra during unstable
stratification. However, overall, the CO2 and humidity spectra showed good agreement as did the cospectra of vertical velocity with water vapour and CO2 respectively. During stable stratification the spectra and cospectra displayed a well-defined spectral gap separating the
mesoscale and small-scale turbulent fluctuations. Two-dimensional turbulence was suggested as a possible source for the mesoscale
fluctuations, which in combination with wave activity in the vertical wind is likely to explain the increase in the cospectral
energy for the corresponding frequency range. Prior to the analysis the turbulence time series of the density measurements
were converted to time series of mixing ratios relative to dry air. Some differences were observed when the spectra based
on the original density measurements were compared to the spectra based on the mixing ratio time series. It is thus recommended
to always convert the density time series to mixing ratio before performing spectral analysis. 相似文献
11.
K. G. Mcnaughton 《Boundary-Layer Meteorology》2006,118(1):83-107
We present a new account of the kinetic energy budget within an unstable atmospheric surface layer (ASL) beneath a convective
outer layer. It is based on the structural model of turbulence introduced by McNaughton (Boundary-Layer Meteorology, 112:
199–221, 2004). In this model the turbulence is described as a self-organizing system with a highly organized structure that
resists change by instability. This system is driven from above, with both the mean motion and the large-scale convective
motions of the outer layer creating shear across the surface layer. The outer convective motions thus modulate the turbulence
processes in the surface layer, causing variable downwards fluxes of momentum and kinetic energy. The variable components
of the momentum flux sum to zero, but the associated energy divergence is cumulative, increasing both the average kinetic
energy of the turbulence in the surface layer and the rate at which that energy is dissipated. The tendency of buoyancy to
preferentially enhance the vertical motions is opposed by pressure reaction forces, so pressure production, which is the work
done against these reaction forces, exactly equals buoyant production of kinetic energy. The pressure potential energy that
is produced is then redistributed throughout the layer through many conversions, back and forth, between pressure potential
and kinetic energy with zero sums. These exchanges generally increase the kinetic energy of the turbulence, the rate at which
turbulence transfers momentum and the rate at which it dissipates energy, but does not alter its overall structure. In this
model the velocity scale for turbulent transport processes in the surface layer is (kzɛ)1/3 rather than the friction velocity, u*. Here k is the von Kármán constant, z is observation height, ɛ is the dissipation rate. The model agrees very well with published experimental results, and provides
the foundation for the new similarity model of the unstable ASL, replacing the older Monin–Obukhov similarity theory, whose
assumptions are no longer tenable. 相似文献
12.
为研究黄河源区边界层湍流特征及其对物质和能量输送的影响,本文首次采用大涡模拟的方法,对比分析了黄河源区两种不同下垫面上（鄂陵湖和湖边草地）对流边界层（CBL）中精细的湍流结构特征。使用资料为2012年夏季黄河源区鄂陵湖流域野外观测实验的GPS探空资料、涡动相关观测资料。分析表明,模拟的黄河源区草地和湖上CBL的平均结构与实测结果吻合较好,但草地和湖上CBL的湍流结构特征差异较明显。模拟结果显示,草地CBL内湍能收支、湍流特征量的时空分布和湍涡结构特征均与陆地上热力驱动CBL的研究结果一致;湖上CBL顶部存在明显的对流卷特征,且夹卷层的湍流强度比草地的强,而草地近地面湍强则更大。通过改变水平分辨率的模拟试验,发现两个不同下垫面上模拟结果对模式分辨率的敏感性不同,湖面CBL的模拟要选择较高的水平分辨率（50～100 m）,以提高近湖面和夹卷层对湍流动能和湍流通量模拟的精度,也充分模拟出各种尺度的波对湍流通量的累积贡献。考虑到计算时间等影响,模拟草地边界层精细的湍流结构时建议选择网格距为100～200 m。 相似文献
13.
Erik Sahlée Anna Rutgersson Eva Podgrajsek Hans Bergström 《Boundary-Layer Meteorology》2014,150(2):235-258
Turbulence measurements taken at a Swedish lake are analyzed. Although the measurements took place over a relatively large lake with several km of undisturbed fetch, the turbulence structure was found to be highly influenced by the surrounding land during daytime. Variance spectra of both horizontal velocity and scalars during both unstable and stable stratification displayed a low frequency peak. The energy at lower frequencies showed a daily variation, increasing in the morning and decreasing in the afternoon. This behaviour is explained by spectral lag, where the low frequency energy due to large eddies that originate from the convective boundary layer above the surrounding land. When the air is advected over the lake the small eddies rapidly equilibrate with the new surface forcing. However, the large eddies remain for an appreciable distance and influence the turbulence in the developing lake boundary layer. The variances of the horizontal velocity and scalars are increased by these large eddies, while the turbulent fluxes are mainly unaffected. The drag coefficient, Stanton number and Dalton number used to parametrize the momentum flux, heat flux and latent heat flux respectively all compare well with current parametrizations developed for open sea conditions. The diurnal cycle of the partial pressure of methane, $p\mathrm{CH}_{4}$ , observed at this site is closely related to the diurnal cycle of the lake-air methane flux. An idealized two-dimensional model simulation of the boundary layer at a lake site indicates that the strong response of $p\mathrm{CH}_{4}$ to the surface methane flux is due to the shallow internal boundary layer that develops above the lake, allowing methane to accumulate in a relatively small volume. 相似文献
14.
An atmospheric surface-layer (ASL) experiment conducted at a meteorological site in the Oostelijk-Flevoland polder of the Netherlands is described. Turbulent fluctuations of wind velocity, air temperature and static pressure were measured, using three 10 m towers.Simultaneous turbulent signals at several heights on the towers were used to investigate the properties of the turbulent structures which contribute most significantly to the turbulent vertical transports in the unstable ASL. These turbulent structures produce between 30 and 50% of the mean turbulent vertical transport of horizontal alongwind momentum and they contribute to between 40 and 50% of the mean turbulent vertical heat transport; in both cases this occurs during 15 to 20% of the total observation time.The translation speed of the turbulent structures equals the wind speed averaged over the depth of the ASL, which scales on the surface friction velocity. The inclination angle of the temperature interface at the upstream edge of the turbulent structures to the surface is significantly smaller than that of the internal shear layer, which is associated with the temperature interface. The turbulent structures in the unstable ASL are determined by a large-scale temperature field: Convective motions, which encompass the whole depth of the planetary boundary layer (PBL), penetrate into the ASL. The curvature of the vertical profile of mean horizontal alongwind velocity forces the alignment of the convective cells in the flow direction (Kuettner, 1971), which have an average length of several hundreds of metres and an average width of a few tens of metres. This mechanism leads to the formation of turbulent structures, which extend throughout the depth of the ASL. 相似文献
15.
涡旋相关法测定湍流通量偏低的研究 总被引：15，自引：3，他引：12
针对野外实验所发现的不同观测法测定地表能通量不平衡问题,进行了均匀加热大气边界层的大涡模拟实验.用模拟的湍流风、温度和湿度涨落的时间序列证实,对流边界层低频涡普遍存在,并经常以一簇一簇热泡的形式出现.风速较小时,有限时长的取样不足以捕捉低频涡的贡献,可造成涡旋相关法测量的统计量异常偏低.仿照涡旋相关法的步骤进行数据处理发现,经去除平均或趋势计算的温度和湿度通量偏低程度在边界层下部随观测高度的增高而显著,其中尤以湿度通量为甚.其结果在一定程度上可以解释低风速条件下地表能通量测量的不闭合问题,但是尚不能完全解释诸如青藏高原实验出现的严重不闭合.文中对此作了探讨性的讨论. 相似文献
16.
A large set of tower data was used to identify the gap that separates small-scale turbulence and mesoscale structures in the
cospectra of surface fluxes. The cospectra were obtained using a multi-resolution decomposition algorithm. The gap time scale
τ
g
was found by fitting a fifth-order polynomial to the cospectra and identifying special points occurring after the peak at
small scales. In unstable conditions (day) τ
g
was found to fall as the mean wind speed increased, while no such dependence was observed in stable conditions (night). The
gap scale was found to change very weakly with stability both in moderately stable and moderately unstable conditions, with
a sharp drop from about 1100 to 250 s occurring in near-neutral conditions. The vertical fluxes computed at different averaging
intervals were found to correlate exceptionally well with each other, the scatter being somewhat larger during the night.
Although considerable discrepancy may occur for individual records, when averaged over 10 months, the difference in the flux
estimated at 7 to 109 min intervals never exceeded 4%, which is comparable or less than the instrumental error. 相似文献
17.
A laboratory study of the turbulent velocity characteristics in the convective boundary layer 总被引：4，自引：0，他引：4
Based on the measurement of the velocity field in the convective boundary layer （CBL） in a convection water tank with the particle image velocimetry （PIV） technique, this paper studies the characteristics of the CBL turbulent velocity in a modified convection tank. The experiment results show that the velocity distribution in the mixed layer clearly possesses the characteristics of the CBL thermals, and the turbulent eddies can be seen obviously. The comparison of the vertical distribution of the turbulent velocity variables indicates that the modeling in the new tank is better than in the old one. The experiment data show that the thermal＇s motion in the entrainment zone sometimes fluctuates obviously due to the intermittence of turbulence. Analyses show that this fluctuation can influence the agreement of the measurement data with the parameterization scheme, in which the convective Richardson number is used to characterize the entrainment zone depth. The normalized square velocity wi^2/w＊^2. at the top of the mixed layer seems to be time-dependent, and has a decreasing trend during the experiments. This implies that the vertical turbulent velocity at the top of the mixed layer may not be proportional to the convective velocity （w＊）. 相似文献
18.
黄土高原陆-气相互作用预试验及其晴天地表能量特征分析 总被引：23，自引：8，他引：15
详细介绍了在2004年8月24日～9月11日在甘肃平凉进行的陆-气相互作用野外预试验,对观测仪器进行了对比和标定,并利用观测资料,初步分析了黄土高原塬上裸地、塬上和塬下玉米地的地表净辐射各分量、感热、潜热和地热流的特征。由于作物（玉米）的存在,降低了地表反射率,使白天地面向上短波减少,同时向上长波也减少,增大了地表净辐射,地表潜热明显增大,玉米地白天向下的地热流和夜晚向上的地热流都比裸地小。与塬上相比,塬下玉米地的向下短波略小,向上短波明显偏小,向上长波基本一致;净辐射只在中午附近略小,地热流上午偏小,下午偏大,夜晚基本一致;感热和潜热在白天基本一致;塬下玉米地在凌晨明显存在负感热通量。 相似文献
19.