共查询到20条相似文献,搜索用时 229 毫秒
1.
Tateki Mizuno 《Boundary-Layer Meteorology》1982,23(1):69-83
The statistics of turbulence, such as the standard deviation of fluctuating velocities, in an unstable atmospheric boundary layer are assumed to be characterized by the combination of three specific lengths, Monin-Obukhov length L, observation height z and the height of mixing layer h. Unlike Monin-Obukhov similarity, even near the ground the effect of h is taken into account. According to observation, the length scale of the vertical velocity is proportional to z at least near the ground, but the lateral component depends mostly on h alone. The length scale of the longitudinal component depends on z and h. 相似文献
2.
This study investigates the impact of soil moisture availability on dispersion-related characteristics: surface fric-tion velocity (u*), characteristic scales of temperature and humidity (T* and q*), the planetary boundary layer height (h) and atmospheric stability classified by Monin-Obukhov length (L), Kazanski-Monin parameter (μ) and convective velocity scale (w*) during daytime convective condition using a one-dimensional primitive equation with a refined soil model. 相似文献
3.
By means of a large-eddy simulation, the convective boundary layer is investigated for flows over wavy terrain. The lower surface varies sinusoidally in the downstream direction while remaining constant in the other. Several cases are considered with amplitude up to 0.15H and wavelength ofH to 8H, whereH is the mean fluid-layer height. At the lower surface, the vertical heat flux is prescribed to be constant and the momentum flux is determined locally from the Monin-Obukhov relationship with a roughness lengthz
o=10–4
H. The mean wind is varied between zero and 5w
*, wherew
* is the convective velocity scale. After rather long times, the flow structure shows horizontal scales up to 4H, with a pattern similar to that over flat surfaces at corresponding shear friction. Weak mean wind destroys regular spatial structures induced by the surface undulation at zero mean wind. The surface heating suppresses mean-flow recirculation-regions even for steep surface waves. Short surface waves cause strong drag due to hydrostatic and dynamic pressure forces in addition to frictional drag. The pressure drag increases slowly with the mean velocity, and strongly with /H. The turbulence variances increase mainly in the lower half of the mixed layer forU/w
*>2. 相似文献
4.
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
5.
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
6.
C. R. Wood A. Lacser J. F. Barlow A. Padhra S. E. Belcher E. Nemitz C. Helfter D. Famulari C. S. B. Grimmond 《Boundary-Layer Meteorology》2010,137(1):77-96
Flow and turbulence above urban terrain is more complex than above rural terrain, due to the different momentum and heat transfer
characteristics that are affected by the presence of buildings (e.g. pressure variations around buildings). The applicability
of similarity theory (as developed over rural terrain) is tested using observations of flow from a sonic anemometer located
at 190.3 m height in London, U.K. using about 6500 h of data. Turbulence statistics—dimensionless wind speed and temperature,
standard deviations and correlation coefficients for momentum and heat transfer—were analysed in three ways. First, turbulence
statistics were plotted as a function only of a local stability parameter z/Λ (where Λ is the local Obukhov length and z is the height above ground); the σ
i
/u
* values (i = u, v, w) for neutral conditions are 2.3, 1.85 and 1.35 respectively, similar to canonical values. Second, analysis of urban mixed-layer
formulations during daytime convective conditions over London was undertaken, showing that atmospheric turbulence at high
altitude over large cities might not behave dissimilarly from that over rural terrain. Third, correlation coefficients for
heat and momentum were analyzed with respect to local stability. The results give confidence in using the framework of local
similarity for turbulence measured over London, and perhaps other cities. However, the following caveats for our data are
worth noting: (i) the terrain is reasonably flat, (ii) building heights vary little over a large area, and (iii) the sensor
height is above the mean roughness sublayer depth. 相似文献
7.
Velocity and Surface Shear Stress Distributions Behind a Rough-to-Smooth Surface Transition: A Simple New Model 总被引:1,自引:1,他引:0
A simple new model is proposed to predict the distribution of wind velocity and surface shear stress downwind of a rough-to-smooth
surface transition. The wind velocity is estimated as a weighted average between two limiting logarithmic profiles: the first
log law, which is recovered above the internal boundary-layer height, corresponds to the upwind velocity profile; the second
log law is adjusted to the downwind aerodynamic roughness and local surface shear stress, and it is recovered near the surface,
in the equilibrium sublayer. The proposed non-linear form of the weighting factor is equal to ln(z/z
01)/ln(δ
i
/z
01), where z, δ
i
and z
01 are the elevation of the prediction location, the internal boundary-layer height at that downwind distance, and the upwind
surface roughness, respectively. Unlike other simple analytical models, the new model does not rely on the assumption of a
constant or linear distribution for the turbulent shear stress within the internal boundary layer. The performance of the
new model is tested with wind-tunnel measurements and also with the field data of Bradley. Compared with other existing analytical
models, the proposed model shows improved predictions of both surface shear stress and velocity distributions at different
positions downwind of the transition. 相似文献
8.
George H. Fichtl 《Boundary-Layer Meteorology》1971,2(2):137-151
The standard deviation of vertical two-point longitudinal velocity fluctuation differences is analyzed experimentally with eleven sets of turbulence measurements obtained at the NASA 150-m ground-winds tower site at Cape Kennedy, Florida. It is concluded that /u
*0 is proportional to (fz/u
*0)0.22, where the coefficient of proportionality is a function of fz/u
*0 and u
*0/fL
0. The quantities f and L0 denote the Coriolis parameter and the surface Monin-Obukhov stability length, respectively; u
*0 is the surface friction velocity; z is the vertical distance between the two points over which the velocity difference is calculated; and zz is the mean height of the mid-point of the interval z above natural grade. The results of the analysis are valid for 20<-u
*0/fL
0<2000. 相似文献
9.
Results from large-eddy simulations and field measurements have previously shown that the velocity field is influenced by
the boundary layer height, z
i
, during close to neutral, slightly unstable, atmospheric stratification. During such conditions the non-dimensional wind
profile, φ
m
, has been found to be a function of both z/L and z
i
/L. At constant z/L, φ
m
decreases with decreasing boundary layer height. Since φ
m
is directly related to the parameterizations of the air–sea surface fluxes, these results will have an influence when calculating
the surface fluxes in weather and climate models. The global impact of this was estimated using re-analysis data from 1979
to 2001 and bulk parameterizations. The results show that the sum of the global latent and sensible mean heat fluxes increase
by 0.77 W m−2 or about 1% and the mean surface stress increase by 1.4 mN m−2 or 1.8% when including the effects of the boundary layer height in the parameterizations. However, some regions show a larger
response. The greatest impact is found over the tropical oceans between 30°S and 30°N. In this region the boundary layer height
influences the non-dimensional wind profile during extended periods of time. In the mid Indian Ocean this results in an increase
of the mean annual heat fluxes by 2.0 W m−2 and an increase of the mean annual surface stress by 2.6 mN m−2. 相似文献
10.
Potential temperature, specific humidity and wind profiles measured by radiosondes under unstable but windy conditions during FIFE in northeastern Kansas were analyzed within the framework of Monin-Obukhov similarity. Around 86% of these profiles were found to have a height range over which the similarity, formulated in terms of the Businger-Dyer functions, is valid and for which the resulting surface fluxes are in good agreement with independent measurements at ground stations. When scaled with the surface roughness z
0
= 1.05 m and the displacement height d
0
= 26.9 m, for the potential temperature this height range was 45 (±31) (z – d
0
)/z
0
104 (±54) and the comparison of the profile-derived surface fluxes with the independent measurements gave a correlation coefficient of r = 0.96. For the specific humidity these values are 42 (±29) (z – d
0
)/z
0
96 (±38) and r = 0.94. In terms of the height of the bottom of the inversion H
i
, in the morning hours the upper limit of (z – d
0
) in the Monin-Obukhov layer is approximately 0.3H
i
, whereas for a fully developed ABL it is closer to 0.1H
i
. Probably, as a result of the short sampling times and perhaps also of the small gradients under the windy conditions, the exact height range of validity was difficult to establish from a mere inspection of these profiles. 相似文献
11.
Summary In this paper the results of an urban measurement campaign are presented. The experiment took place from July 1995 to February
1996 in Basel, Switzerland. A total of more than 2000 undisturbed 30-minute runs of simultaneous measurements of the fluctuations
of the wind vector u′, v′, w′ and the sonic temperature θ
s
′ at three different heights (z=36, 50 and 76 m a.g.l.) are analysed with respect to the integral statistics and their spectral behaviour. Estimates of the
zero plane displacement height d calculated by the temperature variance method yield a value of 22 m for the two lower levels, which corresponds to 0.92 h
(the mean height of the roughness elements). At all three measurement heights the dimensionless standard deviation σ
w
/u
* is systematically smaller than the Monin-Obukhov similarity function for the inertial sublayer, however, deviations are smaller
compared to other urban turbulence studies. The σθ/θ* values follow the inertial sublayer prediction very close for the two lowest levels, while at the uppermost level significant
deviations are observed. Profiles of normalized velocity and temperature variances show a clear dependence on stability. The
profile of friction velocity u
* is similar to the profiles reported in other urban studies with a maximum around z/h=2.1. Spectral characteristics of the wind components in general show a clear dependence on stability and dimensionless measurement
height z/h with a shift of the spectral peak to lower frequencies as thermal stability changes from stable to unstable conditions and
as z/h decreases. Velocity spectra follow the −2/3 slope in the inertial subrange region and the ratios of spectral energy densities
S
w
(f)/S
u
(f) approach the value of 4/3 required for local isotropy in the inertial subrange. Velocity spectra and spectral peaks fit
best to the well established surface layer spectra from Kaimal et al. (1972) at the uppermost level at z/h=3.2.
Received September 26, 1997 Revised February 15, 1998 相似文献
12.
Formation of horizontal convective rolls in urban areas 总被引:6,自引:0,他引:6
The formation of horizontal convective rolls (HCRs) in urban areas is investigated in this paper using observations and fine-scale numerical simulations. Cloud streets organized parallel to the mean boundary-layer wind (a manifestation of HCRs) are seen in the Fengyun-2C satellite imagery around local noon in Beijing. Observed vertical velocity and horizontal wind fields from an urban wind profiler suggest that the time scale for alternating updraft and downdraft in the boundary layer is about 30 min, and the length of the updraft/downdraft is about 9 km. Numerical simulations show that most HCRs occur in the urban areas with − zi / L < 25 (zi: the boundary-layer depth, L: the Monin–Obukhov length). Sensitivity tests reveal that HCRs are common in urban boundary layers, while rural areas are more conducive to forming cellular convection; the aspect ratio of HCRs in urban areas is smaller than the typical value over natural landscapes. 相似文献
13.
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
14.
This paper examines the practical importance of stability, baroclinicity, and acceleration in the bulk ABL similarity formulations, in light of the random errors inherent in field measurements. This is done by propagating the measurement uncertainties through a theoretical model for the bulk ABL similarity functionsA
0 andB
0, under a range of assumed (but always unstable) conditions. It is shown that random measurement errors and acceleration effects may overwhelm most effects of baroclinicity and stability, once conditions are at least slightly unstable. Because of this, it is hard to discern a clear functional dependence ofA
0 andB
0 on degree of instability. Thus, for a given value ofh
i/z0, whereh
i is the inversion height andz
0 is the surface roughness, the geostrophic drag coefficient, which depends onA
0 andB
0, and weakly onh
i/z0, may also be taken to be nearly independent of degree of instability. 相似文献
15.
A. G. Goulart B. E. J. Bodmann M. T. M. B. de Vilhena P. M. M. Soares D. M. Moreira 《Boundary-Layer Meteorology》2011,138(1):61-75
Our focus is the time evolution of the turbulent kinetic energy for decaying turbulence in the convective boundary layer.
The theoretical model with buoyancy and inertial transfer terms has been extended by a source term due to mechanical energy
and validated against large-eddy simulation data. The mechanical effects in a boundary layer of height z
i
at a convective surface-layer height z = 0.05z
i
are significant in the time evolution of the vertical component of the spectrum, i.e. they enhance the decay time scale by
more than an order of magnitude. Our findings suggest that shear effects seem to feedback to eddies with smaller wavenumbers,
preserving the original shape of the spectrum, and preventing the spectrum from shifting towards shorter wavelengths. This
occurs in the case where thermal effects only are considered. 相似文献
16.
It is shown that predictions of a numerical trajectory-simulation method agree closely with the Project Prairie Grass observations of the concentrations 100 m downwind of a continuous point source of sulphur dioxide if the height (z) dependence of the Lagrangian length scale Λ L is chosen as: whereL is the Monin-Obukhov length. The value of 0.5 for Λ L /z in neutral conditions is consistent with the findings of Reid (1979) for the Porton experiment, and is also shown to be the best choice for simulation of an experiment in which concentration profiles were measured a short distance (< 40 m) downwind of an elevated point source of glass beads (40 μn diameter). $$\begin{gathered} \Lambda _L = 0.5z\left( {1 - 6\frac{z}{L}} \right)^{{1 \mathord{\left/ {\vphantom {1 4}} \right. \kern-\nulldelimiterspace} 4}} L< 0 \hfill \\ \Lambda _L = 0.5z/\left( {1 + 5\frac{z}{L}} \right)L > 0 \hfill \\ \end{gathered} $$ 相似文献
17.
Variance Method to Determine Turbulent Fluxes of Momentum And Sensible Heat in The Stable Atmospheric Surface Layer 总被引:1,自引:0,他引:1
Evidence is presented that in the stable atmospheric surface layer turbulent fluxes of heat and momentum can be determined
from the standard deviations of longitudinal wind velocity and temperature, σ u and σ T respectively, measured at a single level. An attractive aspect of this method is that it yields fluxes from measurements
that can be obtained with two-dimensional sonic anemometers. These instruments are increasingly being used at official weather
stations, where they replace the standard cup anemometer–wind vane system. With methods such as the one described in this
note, a widespread, good quality, flux network can be established, which would greatly benefit the modelling community. It
is shown that a ‘variance’ dimensionless height (ζ σ) defined from σ u and σ T is highly related to the ‘conventional’ dimensionless stability parameter ζ=z/L, where z is height and L is the Obukhov length. Empirical functions for ζ σ are proposed that allow direct calculation of heat and momentum fluxes from σ u and σ T. The method performs fairly well also during a night of intermittent turbulence. 相似文献
18.
A Modelling Study of Flux Imbalance and the Influence of Entrainment in the Convective Boundary Layer 总被引:3,自引:2,他引:1
It is frequently observed in field experiments that the eddy covariance heat fluxes are systematically underestimated as compared
to the available energy. The flux imbalance problem is investigated using the NCAR’s large-eddy simulation (LES) model imbedded
with an online scheme to calculate Reynolds-averaged fluxes. A top–down and a bottom–up tracer are implemented into the LES
model to quantify the influence of entrainment and bottom–up diffusion processes on flux imbalance. The results show that
the flux imbalance follows a set of universal functions that capture the exponential decreasing dependence on u
*/w
*, where u
* and w
* are friction velocity and the convective velocity scale, respectively, and an elliptic relationship to z/z
i
, where z
i
is the mixing-layer height. The source location in the boundary layer is an important factor controlling the imbalance magnitude
and its horizontal and vertical distributions. The flux imbalance of heat and the bottom–up tracer is tightly related to turbulent
coherent structures, whereas for the top–down diffusion, such relations are weak to nonexistent. Our results are broadly consistent
with previous studies on the flux imbalance problem, suggesting that the published results are robust and are not artefacts
of numerical schemes. 相似文献
19.
A suggested refinement for O'Brien's convective boundary layer Eddy exchange coefficient formulation
With observational data collected and interpreted by Crane et al. (1977), the adequacy of the O'Brien polynomial to represent the exchange profile of heat and pollution in a convective boundary layer is examined and a refinement suggested. Also, it is shown that the height of the surface layer, h=0.04 z i , developed by Blackadar and Tennekes (1968) for a neutrally stratified boundary layer (with z z =0.25u */f) appears to be equally valid for the convective boundary layer where z i , defined as the top of the mixed layer, is used. 相似文献
20.
Flux Footprints in the Convective Boundary Layer: Large-Eddy Simulation and Lagrangian Stochastic Modelling 总被引:1,自引:1,他引:0
We investigated the flux footprints of receptors at different heights in the convective boundary layer (CBL). The footprints
were derived using a forward Lagrangian stochastic (LS) method coupled with the turbulent fields from a large-eddy simulation
model. Crosswind-integrated flux footprints shown as a function of upstream distances and sensor heights in the CBL were derived
and compared using two LS particle simulation methods: an instantaneous area release and a crosswind linear continuous release.
We found that for almost all sensor heights in the CBL, a major positive flux footprint zone was located close to the sensor
upstream, while a weak negative footprint zone was located further upstream, with the transition band in non-dimensional upwind
distances −X between approximately 1.5 and 2.0. Two-dimensional (2D) flux footprints for a point sensor were also simulated. For a sensor
height of 0.158 z
i, where z
i is the CBL depth, we found that a major positive flux footprint zone followed a weak negative zone in the upstream direction.
Two even weaker positive zones were also present on either side of the footprint axis, where the latter was rotated slightly
from the geostrophic wind direction. Using CBL scaling, the 2D footprint result was normalized to show the source areas and
was applied to real parameters obtained using aircraft-based measurements. With a mean wind speed in the CBL of U = 5.1 m s−1, convective velocity of w
* = 1.37 m s−1, CBL depth of z
i = 1,000 m, and flight track height of 159 m above the surface, the total flux footprint contribution zone was estimated to
range from about 0.1 to 4.5 km upstream, in the case where the wind was perpendicular to the flight track. When the wind was
parallel to the flight track, the total footprint contribution zone covered approximately 0.5 km on one side and 0.8 km on
the other side of the flight track. 相似文献