共查询到20条相似文献，搜索用时 31 毫秒
1.
Harlan D. Shannon George S. Young Michael A. Yates Mark R. Fuller William S. Seegar 《Boundary-Layer Meteorology》2002,104(2):167-199
An examination of boundary-layer meteorological and avian aerodynamic theories suggests that soaring birds can be used to measure the magnitude of vertical air motions within the boundary layer. These theories are applied to obtain mixed-layer normalized thermal updraft intensity over both flat and complex terrain from the climb rates of soaring American white pelicans and from diagnostic boundary-layer model-produced estimates of the boundary-layer depth zi and the convective velocity scale w*. Comparison of the flatland data with the profiles of normalized updraft velocity obtained from previous studies reveals that the pelican-derived measurements of thermal updraft intensity are in close agreement with those obtained using traditional research aircraft and large eddy simulation (LES) in the height range of 0.2 to 0.8 zi. Given the success of this method, the profiles of thermal vertical velocity over the flatland and the nearby mountains are compared. This comparison shows that these profiles are statistically indistinguishable over this height range, indicating that the profile for thermal updraft intensity varies little over this sample of complex terrain. These observations support the findings of a recent LES study that explored the turbulent structure of the boundary layer using a range of terrain specifications. For terrain similar in scale to that encountered in this study, results of the LES suggest that the terrain caused less than an 11% variation in the standard deviation of vertical velocity. 相似文献
2.
The derivation of the Panofsky–Dutton internal boundary-layer(IBL) height formula has been revisited. We propose that the upwindroughness length (rather than downwind) should be used in theformula and that a turbulent vertical velocity (w) ratherthan the surface friction velocity (u*) should be considered asthe appropriate scaling for the rate of propagation ofdisturbances into the turbulent flow. A published set ofwind-tunnel and atmospheric data for neutral stratification hasbeen used to investigate the influence of the magnitude ofroughness change on the IBL height. 相似文献
3.
Spectra and co-spectra of the streamwise (u) and normal or vertical (w) velocity fluctuations have been measured in the inner region of a large Reynolds number laboratory boundary layer over a rough wall. There is reasonable evidence of ak
1
–1
range in theu spectrum (wherek
1 is the streamwise wavenumber). Such a range results from an overlap between a spectral region dominated by largescale, inactive motion, which scales on the boundary-layer thickness, and a region dominated by smaller-scale, active motion which scales on the distance from the wall. Spectra ofw, anduw cospectra, scale in a manner consistent with the dominance by active motion. The present spectral data do not support local isotropy over the inertial subrange. A comparison between measuredw spectra and calculations based on isotropy indicates that the inertial subrange anisotropy is only slightly affected by the magnitude of the non-dimensional mean shear. 相似文献
4.
Relaxed Eddy Accumulation Simulations of Aerosol Number Fluxes and Potential Proxy Scalars 总被引：1，自引：1，他引：0
Andreas Held Edward Patton Luciana Rizzo Jim Smith Andrew Turnipseed Alex Guenther 《Boundary-Layer Meteorology》2008,129(3):451-468
Direct eddy-covariance measurements of aerosol number fluxes obtained during the 2007 CHATS field experiment in Dixon, California,
USA are compared with relaxed eddy accumulation simulations using temperature and water vapour concentration as proxy scalars.
After a brief discussion of the limited time response of the aerosol measurement, the applicability of temperature and water
vapour concentration as proxy scalars for aerosol number concentration is investigated by evaluating scalar and spectral correlation
coefficients as simple measures of scalar similarity. In addition, the proportionality factor b, which compensates for the use of a constant sampling flow in relaxed eddy accumulation, is derived from the time series
of aerosol number, temperature and water vapour, and its variability is analyzed. The reduction of the b factor due to application of a deadband, i.e. the rejection of data when the vertical wind speed is close to zero, is evaluated
for all three studied scalars, and compared with published functional relationships. In this study, using temperature or water
vapour as proxy scalars for aerosol number shows no advantage over the use of a constant b factor. Thus, it is suggested to apply a deadband H
REA
= w′/σ
w
= 0.6 to 0.8 (where w′ is the vertical velocity fluctuation and σ
w
is its standard deviation), to use a theoretical b factor based on a parameterization that includes a stability dependence, and to calculate the deadband effect according to
a derived relation for aerosol relaxed eddy accumulation. 相似文献
5.
Characteristics of vertical turbulent velocities in the urban convective boundary layer 总被引：3，自引：0，他引：3
James M. Godowitch 《Boundary-Layer Meteorology》1986,35(4):387-407
Turbulence measurements of the vertical velocity component were obtained by an instrumented aircraft under fair weather conditions in the St. Louis, Missouri, metropolitan area. Time series of vertical velocity fluctuations from horizontal flight segments made in the lower part of and near the middle of the convective boundary layer (CBL) over the urban area and surrounding region were subjected to various statistical and objective analyses. Higher order vertical velocity moments, and positive and negative velocity statistics, were computed. The horizontal dimensions of updrafts and downdrafts, and related properties of these turbulent eddies were derived by conditional sampling analysis. Emphasis is on a comparison of the results from urban and selected rural measurements from the lower part of the CBL.The vertical velocity probability density distribution for each segment was positively skewed and the mode was negative. The means and standard deviations of positive and negative velocity fluctuations were greater over the urban area. The urban vertical velocity variance was 50% greater than rural values, and power spectra revealed greater production of vertical turbulent energy in the urban area over a wide frequency range.The mean and maximum widths of downdrafts were generally larger than the corresponding values for updrafts. Differences between urban and rural eddy sizes were not statistically significant. The widths of the largest updraft and downdraft are comparable to the boundary-layer depth, Z
i, and the mean value of the ratio of spectral peak wavelength to Z
iwas about 1.3 and 1.1 for urban and rural areas, respectively. Convective similarity scaling parameters appeared to order both the urban and rural measurements.On assignment from the National Oceanic and Atmospheric Administration, U.S. Dept. of Commerce. 相似文献
6.
A nocturnal gravity wave was detected over a south-western Amazon forest during the Large-Scale Biosphere–Atmosphere experiment
in Amazonia (LBA) in the course of the dry-to-wet season campaign on October 2002. The atmospheric surface layer was stably
stratified and had low turbulence activity, based on friction velocity values. However, the passage of the wave, an event
with a period of about 180–300 s, caused negative turbulent fluxes of carbon dioxide (CO2) and positive sensible heat fluxes, as measured by the eddy-covariance system at 60 m (≈30 m above the tree tops). The evolution
of vertical profiles of air temperature, specific humidity and wind speed during the wave movement revealed that cold and
drier air occupied the sub-canopy space while high wind speeds were measured above the vegetation. The analysis of wind speed
and scalars high frequency data was performed using the wavelet technique, which enables the decomposition of signals in several
frequencies allowed by the data sampling conditions. The results showed that the time series of vertical velocity and air
temperature were −90° out of phase during the passage of the wave, implying no direct vertical transport of heat. Similarly,
the time series of vertical velocity and CO2 concentration were 90° out of phase. The wave was not directly associated with vertical fluxes of this variable but the mixing
induced by its passage resulted in significant exchanges in smaller scales as measured by the eddy-covariance system. The
phase differences between horizontal velocity and both air temperature and CO2 concentration were, respectively, zero and 180°, implying phase and anti-phase relationships. As a result, the wave contributed
to positive horizontal fluxes of heat and negative horizontal fluxes of carbon dioxide. Such results have to be considered
in nocturnal boundary-layer surface-atmosphere exchange schemes for modelling purposes. 相似文献
7.
J. D. Wilson 《Boundary-Layer Meteorology》2012,145(1):165-184
This paper explores the utility of specifying the eddy viscosity for the horizontally uniform boundary layer as the product of the variance of vertical velocity and an empirical time scale τ w , as opposed to the more usual formulation where k is the turbulent kinetic energy (TKE), λ k is a length scale and α is a dimensionless coefficient. Simulations were compared with the observations on Day 33 of the Wangara experiment, and with a plausible specification of τ w (or λ k ) each model simulated convective boundary-layer development reasonably well, although the closure produced a more realistic width for the entrainment layer. Under the light winds of Day 33, and with the onset of evening cooling, an excessively shallow and strongly-stratified nocturnal inversion developed, and limited its own further deepening. Boundary-layer models that neglect radiative heat transport and parametrize convective transport by eddy viscosity closure are prone to this runaway (unstable) feedback when forced by a negative (i.e. downward) surface flux of sensible heat. 相似文献
8.
A model of the planetary boundary layer is used to determine the field of vertical motion over large-scale orography. This model represents Ekman boundary-layer dynamics modified by the inclusion of accelerations of the geostrophic wind under the geostrophic momentum approximation. The orography is represented by a circular mountain. The inviscid solution is provided by the sum of a constant translation and a steady, uniform potential vorticity, anticyclonic vortex. The boundary-layer solution vanishes on the mountain, but is matched to the inviscid solution as the top of the boundary layer is approached. The vertical velocity field at the top of the boundary layer is determined by integration of the continuity equation. The field of motion is largely determined by descent from above into the anticyclonic circulation, as in the classical Ekman model. Contributions that arise from the inclusion of accelerations are associated with boundary-layer advection and ageostrophic divergence that produce vorticity tendencies. Finally, the boundary-layer vertical motion is shown to be comparable in magnitude to the vertical motion forced by inviscid flow over the orography, although the distributions of each are significantly different. Effects of mountain asymmetry and a changing pressure field, that can be treated more fully by numerical model simulations, are not considered in the present study.On leave at the University of Colorado, 1990. 相似文献
9.
Dispersion of Heavy Particles Emitted from Area Sources in the Unstable Atmospheric Boundary Layer 总被引：2，自引：2，他引：0
Reliable predictions of the daytime dispersal of heavy particles in the unstable atmospheric boundary layer are important in a variety of disciplines. For many applications, particles disperse from area sources near the ground, and corresponding theoretical solutions are desired to reveal insight into the physical processes. Here, theoretical solutions recently developed for neutral conditions are modified to include the effects of atmospheric instability. The Obukhov length L O and convection velocity w ? are introduced to characterize the patterns of particle dispersion, in additional to friction velocity u ? and settling velocity w s used in the neutral case. The major effects of atmospheric instability are accounted for by modifying the vertical velocity variance profile and considering the ratio of velocity scales w ?/u ?. Theoretical predictions including the mean concentration profile, plume height, and horizontal transport above the source, and ground deposition flux downwind from the source agree well with large-eddy simulation results while the particle plume is within the atmospheric surface layer. The deposition curve is characterized by a power-law decay whose exponent depends on u ?, w s, and w ?. A second steeper power-law develops once the plume extends into the mixed layer. This effect is enhanced with increasing atmospheric instability, implying that particles disperse farther from the source. 相似文献
10.
11.
P. K. Misra 《Boundary-Layer Meteorology》1979,17(1):93-99
The paper presents a simple method to compute the stability parameter Z/L and boundary-layer parameters such as friction velocity U
* and surface heat flux Q
0 on an operational basis. The method is based on the autocorrelation function of the vertical velocity which is relatively insensitive to averaging times except for very large lag times. Eddy correlation techniques on the other hand are very sensitive to averaging times. 相似文献
12.
Osamu Chiba 《Boundary-Layer Meteorology》1984,29(4):313-319
Several experimental results indicate that S
w (the skewness of the vertical turbulent velocity) increases with height, and K
w (the kurtosis) decreases with height in the neutral surface layer. The measured behavior of S
w and K
w with height under neutral conditions is shown to be consistent with available models for the gradient-diffusion and the pressure-velocity correlation. In addition, an estimate of the mode of the probability density distribution turns out to be helpful in interpreting the height dependence of K
w. 相似文献
13.
Transport of pollution and heatout of streets into the boundary layer above is not currently understood and so fluxes cannot be quantified. Scalar concentration within the street is determined by the flux out of it and so quantifying fluxes for turbulent flow over a rough urban surface is essential. We have developed a naphthalene sublimation technique to measure transfer from a two-dimensional street canyon in a wind tunnel for the case of flow perpendicular to the street. The street was coated with naphthalene, which sublimes at room temperature, so that the vapour represented the scalar source. The transfer velocity wT relates the flux out of the canyon to the concentration within it and is shown to be linearly related to windspeed above the street. The dimensionless transfer coefficient wT/U represents the ventilation efficiency of the canyon (here, wT is a transfer velocity,U is the wind speed at the boundary-layer top). Observed values are between 1.5 and 2.7 ×10-3 and, for the case where H/W0 (ratio of buildingheight to street width), values are in the same range as estimates of transfer from a flat plate, giving confidence that the technique yields accurate values for street canyon scalar transfer. wT/U varies with aspect ratio (H/W), reaching a maximum in the wake interference regime (0.3 < H/W < 0.65). However, when upstream roughness is increased, the maximum in wT/U reduces, suggesting that street ventilation is less sensitive to H/W when the flow is in equilibrium with the urban surface. The results suggest that using naphthalene sublimation with wind-tunnel models of urban surfaces can provide a direct measure of area-averaged scalar fluxes. 相似文献
14.
The effect of an injection of specific momentum at the bottom boundary of an axisymmetric, quasisteady maintained vortex is studied by extending the earlier investigation of Kuo (1971). Non-zero vertical velocities at the top of the surface layer representing the lower extremity of a vortex are prescribed. Positive values of the superimposed vertical velocity signified pumping; negative, sucking. The two second-order ordinary differential equations governing the tangential and radial velocities of the vortex are solved by employing Newton's iterative method.The result, viz., that pumping produces a deeper inflow layer and destabilizes the motion while suction depresses the inflow layer and produces stability confirmed an earlier finding of certain fluid dynamicists. Modifications of the boundary-layer structure produced by spatially varying the angular momentum distribution of a vortex are analogous to those caused by the imposition of the Taylor boundary condition at the lower extremity of the vortex. They are also similar to those rendered by varying pumping or suction. The latter result is believed to be new while the former simply agrees with an earlier theoretical deduction. 相似文献
15.
Data from National Centre for Atmospheric Research (NCAR) Queen Air boundary-layer flights over the Nebraska Sandhills are analyzed to investigate the effects of these low hills on boundary-layer turbulence. The Sandhills are an area of anisotropic rolling terrain with characteristic wavelengths of order 2km and rms height variations of order 25m. The biggest impact is found in early morning flight data where horizontal velocity perturbations appear at the same wavelengths as the terrain and variances (normalised by u
2
, where u is the local friction velocity) are significantly enhanced relative to standard flat terrain values. By contrast the vertical velocity variance seems less affected and terrain effects are much less evident in data from the afternoon convective boundary layer. 相似文献
16.
Zbigniew Sorbjan 《Boundary-Layer Meteorology》2007,123(3):365-383
We examine daily (morning–afternoon) transitions in the atmospheric boundary layer based on large-eddy simulations. Under
consideration are the effects of the stratification at the top of the mixed layer and of the wind shear. The results describe
the transitory behaviour of temperature and wind velocity, their second moments, the boundary-layer height Z
m
(defined by the maximum of the potential temperature gradient) and its standard deviation σ
m
, the mixed-layer height z
i
(defined by the minimum of the potential temperature flux), entrainment velocity W
e, and the entrainment flux H
i
. The entrainment flux and the entrainment velocity are found to lag slightly in time with respect to the surface temperature
flux. The simulations imply that the atmospheric values of velocity variances, measured at various instants during the daytime,
and normalized in terms of the actual convective scale w*, are not expected to collapse to a single curve, but to produce a significant scatter of observational points. The measured
values of the temperature variance, normalized in terms of the actual convective scale Θ*, are expected to form a single curve in the mixed layer, and to exhibit a considerable scatter in the interfacial layer. 相似文献
17.
Summary The effects of atmospheric boundary-layer stability on urban heat island-induced circulation are numerically and theoretically
investigated using a nonlinear numerical model (ARPS) and a two-layer linear analytical model. Numerical model simulations
show that as the boundary layer becomes less stable, a downwind updraft cell induced by the urban heat island strengthens.
It is also shown that as the boundary layer becomes less stable, both the height of the maximum updraft velocity and the vertical
extent of the downwind updraft cell increase. Hence, in the daytime with a nearly neutral or less stable boundary layer the
urban heat island-induced circulation can become strong, even though the urban heat island is weak. It is suggested that these
findings can be a mechanism for urban-induced thunderstorms observed in the late afternoon or evening with a nearly neutral
or less stable boundary layer. The boundary-layer stability affects the spatial distribution of scalar concentration through
its influencing urban heat island-induced circulation. Analytical results from a two-layer model with different boundary-layer
stabilities in the lower and upper layers are in general qualitatively consistent with the numerical simulation results, although
the low-level maximum vertical velocity does not change monotonically with lower-layer stability. 相似文献
18.
The Langevin equation is used to derive the Markov equation for the vertical velocity of a fluid particle moving in turbulent flow. It is shown that if the Eulerian velocity variance
wE is not constant with height, there is an associated vertical pressure gradient which appears as a force-like term in the Markov equation. The correct form of the Markov equation is: w(t + t) = aw(t) + b
wE + (1 – a)T
L
(
wE
2)/z, where w(t) is the vertical velocity at time t, a random number from a Gaussian distribution with zero mean and unit variance, T
L the Lagrangian integral time scale for vertical velocity, a = exp(–t/T
L), and b = (1 – a
2)1/2. This equation can be used for inhomogeneous turbulence in which the mean wind speed,
wE and T
L vary with height. A two-dimensional numerical simulation shows that when this equation is used, an initially uniform distribution of tracer remains uniform. 相似文献
19.
Two three-dimensional split-film anemometers were used to measure turbulence statistics within and above a corn canopy. Normalised profiles of mean windspeed, root-mean-square velocity, momentum flux, and heat flux were constructed from half-hourly averages by dividing within-canopy measurements by the simultaneous canopy-top measurement. With the exception of the heat flux, these profiles showed consistent shape from day to day. Time series of the three velocity components were recorded on magnetic tape and subsequently analysed to obtain Eulerian time and length scales and the power spectrum of each component at several heights. The timescale was found to have a local minimum value at the top of the canopy. However the length scale L
wformed from the timescale and the root-mean-square vertical velocity varied with height as L
w 0.1 z. The power-spectra were non-dimensionalised to facilitate comparison of spectra at different heights and times. All spectra had -5/3 regions spanning at least two decades in frequency. 相似文献
20.
The density correction theory of Webb et al. (1980, Q J Roy Meteorol Soc 106: 85–100, hereafter WPL) is a principle underpinning the experimental investigation of surface fluxes
of energy and masses in the atmospheric boundary layer. It has a long-lasting influence in boundary-layer meteorology and
micrometeorology, and the year 2010 marks the 30th anniversary of the publication of the WPL theory. We provide here a critique
of the theory and review the research it has spurred over the last 30 years. In the authors’ opinion, the assumption of zero
air source at the surface is a fundamental novelty that gives the WPL theory its enduring vitality. Considerations of mass
conservation show that, in a non-steady state, the WPL mean vertical velocity and the thermal expansion velocity are two distinctly
different quantities of the flow. Furthermore, the integrated flux will suffer a systematic bias if the expansion velocity
is omitted or if the storage term is computed from time changes in the CO2 density. A discussion is provided on recent efforts to address several important practical issues omitted by the original
theory, including pressure correction, unintentional alternation of the sampled air, and error propagation. These refinement
efforts are motivated by the need for an unbiased assessment of the annual carbon budget in terrestrial ecosystems in the
global eddy flux network (FluxNet). 相似文献