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1.
Spectral characteristics of surface layer turbulence in an urban atmosphere are investigated. The observations used for this purpose represent low wind conditions in the tropics. The normalized power spectral shapes exhibit the usual characteristics in the inertial subrange and obey Monin-Obukhov scaling. However, the low-frequency behaviours do not conform to the previous observed relations. For horizontal components, large energy is contained in the low frequencies in contrast to the vertical component where roll-off to zero frequency is faster.The turbulent kinetic energy dissipation rate estimated from the spectra using Kolmogorov's inertial subrange law is found to be isotropic unlike the velocity variances. The expressions for the dimensionless dissipation rate do not seem to work well in low winds in an urban atmosphere. For the data considered, the dissipation rate exhibits a power law relationship with the mean windspeed and the friction velocity. 相似文献
2.
Spectral Maxima In A Perturbed Stable Boundary Layer 总被引：4，自引：1，他引：4
Wind velocity data have been collected on Nansen Ice Sheet, Antarctica, close to the base of a steeply sloping glacier along which frequently flow katabatic winds. The aim of this study is to investigate how turbulent energy and momentum flux are perturbed by the flow interaction with topography and by the strong mechanical mixing produced by downslope flows. Spectral and cospectral analyses, performed on the wind velocity components, provide evidence that such a perturbation, at any stability, is restricted to frequencies lower than the inertial subrange. Longitudinal spectra display an energy increment, due to turbulence generated by topography and by mechanical forcing related to the katabatic wind structure. The energy, supplied by the topographic forcing, displaces the turbulent energy maximum toward lower frequencies. In near-neutral stratification the spectral maximum occurs at a reduced frequency, which seems to be consistent with the height of the steepest part of the slope, and seems to shift toward higher frequencies as a linear ,function of the local stability parameter,Ll. The parameterisation of the orographic perturbation by means of a similarity relationship allows us to scale u spectra in the same way as over uniform terrain. The scaled, perturbed spectra collapse onto a unique curve in the mid-frequency as well in the inertial subrange, while maxima are grouped in a cluster. Lateral and vertical velocity spectra exhibit shapes independent of stability, suggesting a topographic perturbation that is predominantly over stability effects. 相似文献
3.
4.
Katja Henjes 《Boundary-Layer Meteorology》1997,84(1):149-167
The correlations in turbulent wind speed data from the open ocean are studied. They are calculated as Fourier transforms of the spectraldensities, which are computed from Fourier transforms of the measured windspeed components. All correlations are found to decay within a few seconds.It is shown that the spectra are most efficiently calculated using data sections whose length does not exceed the observed correlation decay time.Longer runs include more random noise without containing more information.The ratio of the along-wind to transverse spectral levels is investigatedand found to be in excellent agreement with the assumption of isotropy inthe inertial wave number range. 相似文献
5.
Turbulence statistics were measured in a natural black-spruce forest canopy in southeastern Manitoba, Canada. Sonic anemometers were used to measure time series of vertical wind velocity (w), and cup anemometers to measure horizontal wind speed (s), above the canopy and at seven different heights within the canopy. Vertical profiles were measured during 25 runs on eight different days when conditions above the canopy were near-neutral.Profiles of s and of the standard deviation (
w
) of w show relatively little scatter and suggest that, for this canopy and these stability conditions, profiles can be predicted from simple measurements made above the canopy. Within the canopy, a negative skewness and a high kurtosis of the w-frequency distributions indicate asymmetry and the persistence of large, high-velocity eddies. The Eulerian time scale is only a weak function of height within the canopy.Although w-power spectra above the canopy are similar to those in the free atmosphere, we did not observe an extensive inertial subrange in the spectra within the canopy. Also, a second peak is present that is especially prominent near the ground. The lack of the inertial subrange is likely caused by the presence of sources and sinks for turbulent kinetic energy within our canopy. The secondary spectral peak is probably generated by wake turbulence caused by form drag on the wide, horizontal spruce branches. 相似文献
6.
An investigation into high Reynolds number turbulent flow over a ridge top in New Zealand is described based on high-resolution
in-situ measurements, using ultrasonic anemometers for two separate locations on the same ridge with differing upwind terrain
complexity. Twelve 5-h periods during neutrally stratified and weakly stable atmospheric conditions with strong wind speeds
were sampled at 20 Hz. Large (and small) turbulent length scales were recorded for both vertical and longitudinal velocity
components in the range of 7–23 m (0.7–3.3 m) for the vertical direction and 628–1111 m (10.5–14.5 m) for the longitudinal
direction. Large-scale eddy sizes scaled to the WRF (Weather Research and Forecasting) numerical model simulated boundary-layer
thickness for both sites, while small-scale turbulent features were a function of the complexity of the upwind terrain. Evidence
of a multi-scale turbulent structure was obtained at the more complex terrain site, while an assessment of the three-dimensional
isotropy assumption in the inertial subrange of the spectrum showed anisotropic turbulence at the less complex site and evidence
of isotropic turbulence at the more complex site, with a spectral ratio convergence deviating from the 4/3 or unity values
suggested by previous theory and practice. Existing neutral spectral models can represent locations along the ridge top with
simple upwind complexity, especially for the vertical wind spectra, but sites with more orographic complexity and strong vertical
wind speeds are often poorly represented using these models. Measured spectra for the two sites exhibited no significant diurnal
variation and very similar large-scale and small-scale turbulent length scales for each site, but the turbulence energy measured
by the variances revealed a strong diurnal difference. 相似文献
7.
8.
Ann-Sofi Smedman-Högström 《Boundary-Layer Meteorology》1973,3(3):329-347
Temperature and humidity spectra have been measured at 3 and 12 m above the ground, together with profiles of wind, temperature and humidity, and flux measurements. Both temperature and humidity spectra appear to follow Monin-Obukov similarity as well as Kolmogorov's prediction for the inertial subrange. The standard deviations of temperature and humidity fluctuations support Monin-Obukov similarity and the predictions of local free convection. The spectral constants for the inertial subrange have been estimated as 0.8 for temperature and 0.6 for humidity. 相似文献
9.
We present the power spectra of wind velocity and the cospectra of momentum and heat fluxes observed for different wind directions
over flat terrain and a large valley on the Loess Plateau. The power spectra of longitudinal (u) and lateral (v) wind speeds satisfy the −5/3 power law in the inertial subrange, but do not vary as observed in previous studies within
the low frequency range. The u spectrum measured at 32 m height for flow from the valley shows a power deficit at intermediate frequencies, while the v spectrum at 32 m downwind of the valley reaches another peak in the low frequency range at the same frequency as the u spectrum. The corresponding peak wavelength is consistent with the observed length scale of the convective outer layer at
the site. The v spectrum for flat terrain shows a spectral gap at mid frequencies while obeying inner layer scaling in its inertial subrange,
suggesting two sources of turbulence in the surface layer. All the spectra and cospectra from the valley direction show a
height dependency over the three levels. 相似文献
10.
本文利用100m~3系留气艇携带超声风温仪在复杂的盆地地形上空对1000m以下大气进行观测所获得的资料,研究小风稳定条件下大气边界层湍流结构特征。结果表明,在双对数坐标中,纵向速度u谱,垂直速度w谱,温度T谱在惯性区均遵循Kolmogorov的-2/3次律;横向速度V谱有其特殊的情形;协谱uw,wT及近地层的uT协谱在惯性区服从-4/3次律。和平坦、均一、开阔下垫面不同的是谱的峰值频率向高频移动,且没有发现近地层具有的谱峰随高度的明显变化关系。 相似文献
11.
Ship-board measurements and estimations of air-sea fluxes in the western tropical pacific during TOGA COARE 总被引：1，自引：0，他引：1
Direct air-sea flux measurements were made on R/V Kexue #1 at 4 ° S, 156 ° E during the Tropical Ocean Global Atmosphere (TOGA)
Coupled Ocean-Atmospheric Response Experiment (COARE) Intensive Observation Period (IOP). An array of six accelerometers was
used to measure the motion of the anchored ship, and a sonic anemometer and Lyman-α hygrometer were used to measure the turbulent
wind vector and specific humidity. The contamination of the turbulent wind components by ship motion was largely removed by
an improvement of a procedure due to Shao based on the acceleration signals. The scheme of the wind correction for ship motion
is briefly outlined. Results are presented from data for the best wind direction relative to the ship to minimize flow distortion
effects. Both the time series and the power spectra of the sonic-measured wind components show swell-induced ship motion contamination,
which is largely removed by the accelerometer correction scheme. There was less contamination in the longitudinal wind component
than in the vertical and transverse components. The spectral characteristics of the surface-layer turbulence properties are
compared with those from previous land and ocean results. Momentum and latent heat fluxes were calculated by eddy correlation
and compared to those estimated by the inertial dissipation method and the TOGA COARE bulk formula. The estimations of wind
stress determined by eddy correlation are smaller than those from the TOGA COARE bulk formula, especially for higher wind
speeds, while those from the bulk formula and inertial dissipation technique are generally in agreement. The estimations of
latent heat flux from the three different methods are in reasonable agreement. The effect of the correction for ship motion
on latent heat fluxes is not as large as on momentum fluxes. 相似文献
12.
Rainer Roth 《Boundary-Layer Meteorology》1970,1(2):131-136
A model is described, in which the mean vertical wind profile and turbulence spectra at different heights are calculated for a turbulent boundary layer without thermal stratification. The model makes use of Heisenberg's formula for the transfer of turbulent energy and is based on the assumption of a constant shearing stress in that boundary layer. As a result, a logarithmic wind profile follows with 0.39 as the value of von Kármán's constant, which is — in this model — strongly related to the inertial subrange of the turbulent energy spectra and therefore to the Kolmogoroff constant.This paper is based on studies done by the author during a one-year visit to CSIRO Division of Meteorological Physics, Aspendale, Australia, and was presented at the AGARD Specialists Meeting on The Aerodynamics of Atmospheric Shear Flows sponsored by the Fluid Dynamics Panel at Munich, Germany, during 15–17 Sept. 1969. 相似文献
13.
The optical refractive index fluctuation has been determined from measurements of turbulent temperature and humidity fluctuations in the atmospheric surface layer over the ocean. Probability density, spectral density and even- and odd-order structure functions of the refractive index fluctuation are presented. The spectral density exhibits a significant -5/3 inertial subrange as a result of the existence of a -5/3 subrange in the spectra of temperature and humidity fluctuations and in the temperature-humidity co-spectrum. The behaviour in the inertial subrange of fourth- and sixth-order structure functions of the refractive index is in reasonable agreement with that predicted by the analysis of Antonia and Van Atta (1975, 1978). The third-order structure functions of the refractive index exhibit an approximately linear dependence on separation in the inertial subrange, in agreement with a ramp model for the large coherent structure of the motion. 相似文献
14.
B. D. Amiro 《Boundary-Layer Meteorology》1990,52(3):227-246
Atmospheric turbulence was measured within a black spruce forest, a jack pine forest, and a trembling aspen forest, located in southeastern Manitoba, Canada. Drag coefficients (C
d
) varied little with height within the pine and aspen canopies, but showed some height dependence within the dense spruce canopy. A constant C
d
of 0.15, with the measured momentum flux and velocity profiles, gave good estimates of leaf-area-index (LAI) profiles for the pine and aspen canopies, but underestimated LAI for the spruce canopy.Velocity spectra were scaled using the Eulerian integral time scales and showed a substantial inertial subrange above the canopies. In the bottom part of the canopies, the streamwise and cross-stream spectra showed rapid energy loss whereas the vertical spectra showed an apparent energy gain, in the region where the inertial subrange is expected. The temperature spectra showed an inertial subrange with the expected -2/3 slope at all heights. Cospectra of momentum and heat flux had slopes of about -1 in much of the inertial subrange. Possible mechanisms to explain some of the spectral features are discussed. 相似文献
15.
Turbulent Structures in a Pine Forest with a Deep and Sparse Trunk Space: Stand and Edge Regions 总被引：1，自引：0，他引：1
Sylvain Dupont Mark R. Irvine Jean-Marc Bonnefond Eric Lamaud Yves Brunet 《Boundary-Layer Meteorology》2012,143(2):309-336
Forested landscapes often exhibit large spatial variability in vertical and horizontal foliage distributions. This variability
may affect canopy-atmosphere exchanges through its action on the development of turbulent structures. Here we investigate
in neutral stratification the turbulent structures encountered in a maritime pine forest characterized by a high, dense foliated
layer associated with a deep and sparse trunk space. Both stand and edge regions are considered. In situ measurements and
the results of large-eddy simulations are used and analyzed together. In stand conditions, far from the edge, canopy-top structures
appear strongly damped by the dense crown layer. Turbulent wind fluctuations within the trunk space, where the momentum flux
vanishes, are closely related to these canopy-top structures through pressure diffusion. Consequently, autocorrelation and
spectral analyses are not quite appropriate to characterize the vertical scale of coherent structures in this type of canopy,
as pressure diffusion enhances the actual scale of structures. At frequencies higher than those associated with canopy-top
structures, wind fluctuations related to wake structures developing behind tree stems are observed within the trunk space.
They manifest themselves in wind velocity spectra as secondary peaks in the inertial subrange region, confirming the hypothesis
of spectral short-cuts in vegetation canopies. In the edge region specific turbulent structures develop just below the crown
layer, in addition to canopy-top structures. They are generated by the wind shear induced by the sub-canopy wind jet that
forms at the edge. These structures provide a momentum exchange mechanism similar to that observed at the canopy top but in
the opposite direction and with a lower magnitude. They may develop as in plane mixing-layer flows, with some perturbations
induced by canopy-top structures. Wake structures are also observed within the trunk space in the edge region. 相似文献
16.
Different flux estimation techniques are compared here in order to evaluate air–sea exchange measurement methods used on moving
platforms. Techniques using power spectra and cospectra to estimate fluxes are presented and applied to measurements of wind
speed and sensible heat, latent heat and CO2 fluxes. Momentum and scalar fluxes are calculated from the dissipation technique utilizing the inertial subrange of the power
spectra and from estimation of the cospectral amplitude, and both flux estimates are compared to covariance derived fluxes.
It is shown how even data having a poor signal-to-noise ratio can be used for flux estimations. 相似文献
17.
C. A. Biltoft 《Boundary-Layer Meteorology》2001,100(3):393-404
Applications of Kolmogorov's universal equilibrium hypothesis and the Taylor transform to velocity spectra derived from measurements within the low frequency portion of the atmospheric surface layer (ASL) inertial subrange are examined. The measured ratios of lateral to longitudinal velocity component spectra exhibit considerable scatter, but suggest convergence towards 1.0 rather than towards the 4/3 expected from theory. Shear and buoyancy introduce anisotropy to the inertial subrange, thereby contributing to the observed scatter. The apparent discrepancy between the 4/3 velocity component spectrum ratio expected from theory and the measurements could arise as a consequence of the processing used to produce spectra. These processing effects must be considered from the perspective of the propagating eddy. Spectral averaging used with sonic anemometer data is done over time periods that are large with respect to inertial subrange eddy correlation decay times. This averaging causes energy from larger scale eddies to appear as `local convection' that dominates the Taylor transform. Spectrum ratio convergence and cospectra approaching zero are necessary, but not sufficient, conditions for onset of local isotropy. Measurements of spectrum ratios and cospectra over the entire inertial subrange are needed to determine whether or not local isotropy might occur within the ASL. 相似文献
18.
Günther Heinemann 《Boundary-Layer Meteorology》2002,103(1):49-81
Turbulence structures in the katabatic flow in the stable boundary layer (SBL) over the ice sheet are studied for two case studies with high wind speeds during the aircraft-based experiment KABEG (Katabatic wind and boundary layer front experiment around Greenland) in the area of southern Greenland. The aircraft data allow the direct determination of turbulence structures in the katabatic flow. For the first time, this allows the study of the turbulence structure in the katabatic wind system over the whole boundary layer and over a horizontal scale of 80 km.The katabatic flow is associated with a low-level jet (LLJ), with maximum wind speeds up to 25 m s-1. Turbulent kinetic energy (TKE) and the magnitude of the turbulent fluxes show a strong decrease below the LLJ. Sensible heat fluxes at the lowest level have values down to -25 W m-2. Latent heat fluxes are small in general, but evaporation values of up to +13 W m-2 are also measured. Turbulence spectra show a well-defined inertial subrange and a clear spectral gap around 250-m wavelength. While turbulence intensity decreases monotonously with height above the LLJ for the upper part of the slope, high spectral intensities are also present at upper levels close to the ice edge. Normalized fluxes and variances generally follow power-law profiles in the SBL.Terms of the TKE budget are computed from the aircraft data. The TKE destruction by the negative buoyancy is found to be very small, and the dissipation rate exceeds the dynamical production. 相似文献
19.
Experimental observations on the temperature and wind fields above flat grassy terrain have been obtained with an instrumented 92-m tower during intervals of strong insolation about midday. The turbulence characteristics of the air confirm that free convection prevailed at heights between 16 and 48 m, with some tendency for departure at higher levels. The spectra of temperature and vertical velocity contain gaps at wave numbers in the range 0.01–0.025 m–1. These are attributed to natural thermal plumes that act as sources of extra energy input to the Kolmogorov-Obukhov-Corrsin scheme of turbulence in or at the low-wave number limit of the inertial subrange. Modified forms of the K-O-C spectral laws for thermally unstable air are derived which agree with the observed spectra over the whole range of wave numbers examined, and which contain the spectral gap at wave numbers corresponding to the thermal plume diameters. 相似文献
20.
Doppler Lidar Measurements of Vertical Velocity Spectra in the Convective Planetary Boundary Layer 总被引：1，自引：1，他引：0
We utilized a Doppler lidar to measure spectra of vertical velocity w from 390m above the surface to the top of the daytime convective boundary layer (CBL). The high resolution 2μm wavelength
Doppler lidar developed by the NOAA Environmental Technology Laboratory was used to detect the mean radial velocity of aerosol
particles. It operated continuously during the daytime in the zenith-pointing mode for several days in summer 1996 during
the Lidars-in-Flat-Terrain experiment over level farmland in central Illinois, U.S.A. The temporal resolution of the lidar
was about 1 s, and the range-gate resolution was about 30m. The vertical cross-sections were used to calculate spectra as
a function of height with unprecedented vertical resolution throughout much of the CBL, and, in general, we find continuity
of the spectral peaks throughout the depth of the CBL. We compare the observed spectra with previous formulations based on
both measurements and numerical simulations, and discuss the considerable differences, both on an averaged and a case-by-case
basis. We fit the observed spectra to a model that takes into account the wavelength of the spectral peak and the curvature
of the spectra across the transition from low wavenumbers to the inertial subrange. The curvature generally is as large or
larger than the von Kármán spectra. There is large case-to-case variability, some of which can be linked to the mean structure
of the CBL, especially the mean wind and the convective instability. We also find a large case-to-case variability in our
estimates of normalized turbulent kinetic energy dissipation deduced from the spectra, likely due for the most part to a varying
ratio of entrainment flux to surface flux. Finally, we find a relatively larger contribution to the low wavenumber region
of the spectra in cases with smaller shear across the capping inversion, and suggest that this may be due partly to gravity
waves in the inversion and overlying free atmosphere. 相似文献