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1.
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
2.
A large-eddy simulation (LES) model, using the one-equation subgrid-scale (SGS) parametrization, was developed to study the
flow and pollutant transport in and above urban street canyons. Three identical two-dimensional (2D) street canyons of unity
aspect ratio, each consisting of a ground-level area source of constant pollutant concentration, are evenly aligned in a cross-flow
in the streamwise direction x. The flow falls into the skimming flow regime. A larger computational domain is adopted to accurately resolve the turbulence
above roof level and its influence on the flow characteristics in the street canyons. The LES calculated statistics of wind
and pollutant transports agree well with other field, laboratory and modelling results available in the literature. The maximum
wind velocity standard deviations σ
i
in the streamwise (σ
u
), spanwise (σ
v
) and vertical (σ
w
) directions are located near the roof-level windward corners. Moreover, a second σ
w
peak is found at z ≈ 1.5h (h is the building height) over the street canyons. Normalizing σ
i
by the local friction velocity u
*, it is found that σ
u
/u
* ≈ 1.8, σ
v
/u
* ≈ 1.3 and σ
w
/u
* ≈ 1.25 exhibiting rather uniform values in the urban roughness sublayer. Quadrant analysis of the vertical momentum flux
u′′w′′ shows that, while the inward and outward interactions are small, the sweeps and ejections dominate the momentum transport
over the street canyons. In the x direction, the two-point correlations of velocity R
v,x
and R
w,x
drop to zero at a separation larger than h but R
u,x
(= 0.2) persists even at a separation of half the domain size. Partitioning the convective transfer coefficient Ω
T
of pollutant into its removal and re-entry components, an increasing pollutant re-entrainment from 26.3 to 43.3% in the x direction is revealed, suggesting the impact of background pollutant on the air quality in street canyons. 相似文献
3.
Observational evidence for the Monin-Obukhov similarity under all stability conditions 总被引:2,自引:0,他引:2
Data collected in the surface layer in a northern suburban area of Nanjing from 15 November to 29 December 2007 were analyzed to examine the Monin-Obukhov similarity for describing the turbulent fluctu- ations of 3D winds under all stability conditions and to obtain the turbulence characteristics under different weather conditions. The results show that the dimensionless standard deviations of turbulent velocity com- ponents (σ u /u* , σ v /u* , σ w /u * ) and dimensionless turbulent kinetic energy (TKE) can be well described by "1/3" power law relationships under stable, neutral, and unstable conditions, with σ u /u * > σ v /u * > σ w /u* . Land use and land cover changes mainly impact dimensionless standard deviations of horizontal component fluctuations, but they have very little on those of the vertical component. The dimensionless standard devi- ations of wind components and dimensionless TKE are remarkably affected by different weather conditions; the deviations of horizontal wind component and dimensionless TKE present fog day > clear sky > overcast > cloudy; the trend of the vertical wind component is the reverse. The surface drag coefficient at a Nan- jing suburban measurement site during the observation period was obviously higher than at other reported plains and plateau areas, and was approximately one order larger in magnitude than the reported plains areas. Dimensionless standard deviation of temperature declined with increasing |z /L| with an approximate "-1/3" slope in unstable stratification and "-2/3" slope in stable stratification. 相似文献
4.
The effect of topographical slope angle and atmospheric stratification on turbulence intensities in the unstably stratified surface layer have been parameterized using observations obtained from a three-dimensional sonic anemometer installed at 8 m height above the ground at the Seoul National University (SNU) campus site in Korea for the years 1999–2001. Winds obtained from the sonic anemometer are analyzed according to the mean wind direction, since the topographical slope angle changes significantly along the azimuthal direction. The effects of the topographical slope angle and atmospheric stratification on surface-layer turbulence intensity are examined with these data. It is found that both the friction velocity and the variance for each component of wind normalized by the mean wind speed decrease with increase of the topographical slope angle, having a maximum decreasing rate at very unstable stratification. The decreasing rate of the normalized friction velocity (u
* /U) is found to be much larger than that of the turbulence intensity of each wind component due to the reduction of wind shear with increase in slope angle under unstable stratification. The decreasing rate of the w component of turbulence intensity (σ
w
/U) is the smallest over the downslope surface whereas that of the u component (σ
u
/U) has a minimum over the upslope surface. Consequently, σ
w
/u
* has a maximum increasing rate with increase in slope angle for the downslope wind, whereas σ
u
/u
* has its maximum for the upslope wind. The sloping terrain is found to reduce both the friction velocity and turbulence intensity compared with those on a flat surface. However, the reduction of the friction velocity over the sloping terrain is larger than that of the turbulence intensity, thereby enhancing the turbulence intensity normalized by the friction velocity over sloping terrain compared with that over a flat surface. 相似文献
5.
The SF6 gas tracer observations for puffs released near the ground during the Joint Urban 2003 (JU2003) urban dispersion experiment
in Oklahoma City have been analysed. The JU2003 observations, at distances of about 100–1,100 m from the source, show that,
at small times, when the puff is still within the built-up downtown domain, the standard deviation of the concentration time
series, σt, is influenced by the initial puff spread due to buildings near the source and by hold-up in the wakes of large buildings
at the sampler locations. This effect is parameterised by assuming an initial σto of about 42 s, leading to a comprehensive similarity formula: σt = 42 + 0.1t. The second term, 0.1t, is consistent with an earlier similarity relation, σt = 0.1t, derived from puff observations in many experiments over rural terrain. The along-wind dispersion coefficient, σx, is assumed to equal σt
u, in which u is the puff speed calculated as the distance from the source to the sampler, x, divided by the time after the release that the maximum concentration is observed at the sampler. σx can be expressed as σx = σxo + 0.14x, with the initial σxo of 45 m. This initial σxo agrees with the suggestion of an initial plume spread of about 40 m, made by McElroy and Pooler from analysis of the 1960s’
St. Louis urban dispersion experiment. The puff speeds, u, are initially only about 20% of the observed wind speed, averaged over about 80 street-level and rooftop anemometers in
the city, but approach the mean observed wind speed as the puffs grow vertically. The scatter in the σt data is about ± a factor of two or three at any given travel time. The maximum σt is about 250 s, and the maximum duration of the puff over the sampler, Dt, sometimes called the retention time, is about
1,100 s or 18 min for these puffs and distances. 相似文献
6.
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
7.
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
8.
Fluctuations of the horizontal wind under unstable conditions 总被引:1,自引:0,他引:1
The similarity relations for u/u* proposed by Panofskyet al. (1977) and Højstrup (1982) have been verified using eddy-correlation data collected during the EFEDA-experiment, conducted over the extensive plain of La Mancha (Spain), where vine plants form a primary crop. Also, the standard Monin-Obukhov relation is considered. It is found that the expressions by Panofskyet al. and Højstrup both yield almost identical results, and are better than the Monin-Obukhov expression. Also, u measured with a cup anemometer obeys the similarity expressions well, provided that the effect of the variation of wind direction on propellor wind speed is accounted for. The relationship of Panofskyet al. works rather well even when the boundary-layer height scaleh is replaced by a fixed height,h
c. Best results were obtained forh
c=1800 m. This height scale is possibly associated with the horizontal variability in the surface sensible heat flux pattern. 相似文献
9.
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
10.
Many applied dispersion models require the knowledge of boundary-layer parameters such as sensible heat flux,Q
H
, friction velocity,u
*, and turbulent energy components, w and
v
. Formulas are suggested for calculating these parameters over a wide variety of types of ground surfaces, based on simple observations of wind speed near the ground and fractional cloud cover, and specification of constants such as roughness length, albedo, and soil moisture availability. Observations ofu
*,Q
H
,
w
, and
v
during field experiments in St. Louis and Indianapolis are used to test the formulas for urban sites. Relative errors of about ±20% in the predictions are seen to occur whenu
*,Q
H
,
w
, and
v
are large. However, when these quantities are small (e.g.,u
* < 0.2 m/s), the errors in the predictions are as large as the mean value of the quantity itself.In addition, it is concluded from studies of available field data and theories that the magnitude of
w
is not well-known at elevations above about 100m during the late afternoon and night. Some simple parameterizations for
w
. are suggested that are consistent with the observed steady decrease in ground-level concentration in the afternoon and the sudden increase in concentration that can occur a few hours after sunset due to wind shears associated with a low-level jet, for continuous plumes emitted from moderate to tall stacks. 相似文献
11.
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
12.
Diurnal variations of the standard deviation σ
g
of the refractive index vertical gradient g
n
in the lower 300-meter atmospheric layer are first analyzed for the vast area of Eastern Siberia and Far East by using observation
data obtained from 30 aerological stations for 10 years. This statistics is most variable in the Far North areas in the spring-summer
period. The diurnal cycle of σ
g
is most diverse (in terms of both its character and amplitude) in the moderate climatic belt of the territory under study.
Here, the maximum diurnal variability of σ
g
is observed in the summer and then, in the winter time. Diurnal variations of σ
g
in this region have significant amplitudes that are in many regions comparable with the amplitudes of its interseasonal variations. 相似文献
13.
A significant non-alignment between the mean horizontal wind vector and the stress vector was observed for turbulence measurements
both above the water surface of a large lake, and over a land surface (soybean crop). Possible causes for this discrepancy
such as flow distortion, averaging times and the procedure used for extracting the turbulent fluctuations (low-pass filtering
and filter widths etc.), were dismissed after a detailed analysis. Minimum averaging times always less than 30 min were established
by calculating ogives, and error bounds for the turbulent stresses were derived with three different approaches, based on
integral time scales (first-crossing and lag-window estimates) and on a bootstrap technique. It was found that the mean absolute
value of the angle between the mean wind and stress vectors is highly related to atmospheric stability, with the non-alignment
increasing distinctively with increasing instability. Given a coordinate rotation that aligns the mean wind with the x direction, this behaviour can be explained by the growth of the relative error of the u–w component with instability. As a result, under more unstable conditions the u–w and the v–w components become of the same order of magnitude, and the local stress vector gives the impression of being non-aligned with
the mean wind vector. The relative error of the v–w component is large enough to make it undistinguishable from zero throughout the range of stabilities. Therefore, the standard
assumptions of Monin–Obukhov similarity theory hold: it is fair to assume that the v–w stress component is actually zero, and that the non-alignment is a purely statistical effect. An analysis of the dimensionless
budgets of the u–w and the v–w components confirms this interpretation, with both shear and buoyant production of u–w decreasing with increasing instability. In the v–w budget, shear production is zero by definition, while buoyancy displays very low-intensity fluctuations around zero. As local
free convection is approached, the turbulence becomes effectively axisymetrical, and a practical limit seems to exist beyond
which it is not possible to measure the u-w component accurately. 相似文献
14.
P Thomas 《Atmospheric Research》1986,20(2-4)
Two different Doppler acoustic sounders have been operated at the Kernforschungszentrum Karlsruhe (KfK) since 1982. It has been investigated whether meteorological data from these sounders can be used for dispersion modeling and monitoring in the environment of pollutant-emitting plants. Data from the sounders and from a 200 m high meteorological tower have been sampled continuously for intercomparison.Two schemes of stability classification are presented. They are based on 30-min mean values of the following meteorological data measured by the acoustic sounders: (a) standard deviation σw of the vertical wind speed and horizontal wind speed u, at a height of 100 m; and (b) standard deviation σφ of the vertical wind direction at a height of 100 m and vertical profile of the backscattered amplitude Aw.The class limits applied in these schemes are determined by “statistical equivalence” with a standard classification scheme. This standard scheme is based on σφ, measured by a vector vane at the 100 m level of the tower. Statistical equivalence in this context means that the frequency distributions of the classes are approximately equal at the same site and during the same period.The reliability of these schemes is investigated and compared to the standard scheme by correlation analysis. Finally, the schemes are compared with other commonly applied classification methods. 相似文献
15.
Edgar L Andreas Reginald J. Hill James R. Gosz Douglas I. Moore William D. Otto Achanta D. Sarma 《Boundary-Layer Meteorology》1998,86(3):379-408
Refuge has patchy vegetation in sandy soil. During midday and at night, the surface sources and sinks for heat and moisture may thus be different. Although the Sevilleta is broad and level, its metre-scale heterogeneity could therefore violate an assumption on which Monin-Obukhov similarity theory (MOST) relies. To test the applicability of MOST in such a setting, we measured the standard deviations of vertical (w) and longitudinal velocity (u), temperature (t), and humidity (q), the temperature-humidity covariance (¯tq), and the temperature skewness (St). Dividing the former five quantities by the appropriate flux scales (u*, *, and q*) yielded the nondimensional statistics w/u*, u/u*, t/|t*|, q/|q*|, and ¯tq/t*q*. w/u*, t/|t*|, and St have magnitudes and variations with stability similar to those reported in the literature and, thus, seem to obey MOST. Though u/u* is often presumed not to obey MOST, our u/u* data also agree with MOST scaling arguments. While q/|q*| has the same dependence on stability as t/|t*|, its magnitude is 28% larger. When we ignore ¯tq/t*q* values measured during sunrise and sunset transitions – when MOST is not expected to apply – this statistic has essentially the same magnitude and stability dependence as (t/t*)2. In a flow that truly obeys MOST, (t/t*)2, (q/q*)2, and ¯tq/t*q* should all have the same functional form. That (q/q*)2 differs from the other two suggests that the Sevilleta has an interesting surface not compatible with MOST. The sources of humidity reflect the patchiness while, despite the patchiness, the sources of heat seem uniformly distributed. 相似文献
16.
Variability of surface roughness and turbulence intensities at a coastal site in India 总被引:4,自引:0,他引:4
Radhika Ramachandran J. Winston Jeeva Prakash K. Sen Gupta K. Narayanan Nair P. K. Kunhikrishnan 《Boundary-Layer Meteorology》1994,70(4):385-400
Surface-layer features with different prevailing wind directions for two distinct seasons (Southwest Monsoon and Northeast Monsoon) on the west coast of India are studied using data obtained from tower-based sensors at a site located about 500 m from the coast. Only daytime runs have been used for the present analysis. The surface boundary-layer fluxes have been estimated using the eddy correlation method. The surface roughnessz
0 obtained using the stability-corrected wind profiles (Paulson, 1970) has been found to be low for the Southwest monsson season. For the other season,z
0 is relatively high. The drag coefficientC
D varies with height in the NE monsoon season but not in the season with lowz
0. This aspect is reflected in the wind profiles for the two seasons and is discussed in detail. The scaling behaviour of friction velocityu
* and the turbulence intensity of longitudinal, lateral and vertical winds u, v and w, respectively) are further examined to study their dependence on fetch. Our study shows that for the non-dimensional case, u/u* and v/u* do not show any surface roughness dependence in either season. On the other hand, for w/u* for the season with lowz
0, the values are seen to agree well with that of Panofskyet al. (1977) for homogeneous terrain whereas for the other season with highz
0, the results seem to conform more to the values observed by Smedman and Högström (1983) for coastal terrain. The results are discussed in the light of observations by other investigators. 相似文献
17.
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
18.
The system transfer function ¦H(v)¦2 at frequencyv (units of Hz) for a vertical velocity propeller anemometer in a statistically stationary and horizontally homogeneous turbulent flow is determined from: (1) experimental estimates of propeller velocity spectra; and (2) estimates of Eulerian vertical velocity spectra based on the hypothesis that degradation of the input vertical velocity Fourier components occurs in the inertial subrange. The experimental estimates of ¦H(v)¦2 were adequately summarized with the mathematical expression for the system transfer function of a first-order system with parameterT which has units of time and is analogous to the time constant of a horizontal velocity propeller anemometer. Dimensional analysis techniques and the Monin-Obukhov similarity hypothesis were used to construct a model for the system parameterT which yielded the result that
w
/D
1 (
w
/)1/3, where
w
, andD
1 denote the standard deviation of the input vertical velocity fluctuations, the horizontal mean wind speed, and the diameter of the propeller, respectively. The system parameterT is interpreted in terms of the time required for the propeller velocity statistics to become asymptotically independent of time upon being released from rest in a statistically stationary turbulent flow.Currently on leave of absence from the Indian Institute of Technology, New Delhi, India. 相似文献
19.
The characteristics of turbulent velocity components in the surface layer under convective conditions 总被引:19,自引:5,他引:19
H. A. Panofsky H. Tennekes D. H. Lenschow J. C. Wyngaard 《Boundary-Layer Meteorology》1977,11(3):355-361
It is proposed that the ratios of the standard deviations of the horizontal velocity components to the friction velocity in the surface layer under convective conditions depend only onz
i
/L wherez
i
is the height of the lowest inversion andL is the Monin-Obukhov length. This hypothesis is tested by using observations from several data sets over uniform surfaces and appears to fit the data well. Empirical curves are fitted to the observations which have the property that at largez
i
/-L, the standard deviations become proportional tow
*, the convective scaling velocity.Fluctuations of vertical velocity obtained from the same experiments scale withz/L, wherez is the height above the surface, in good agreement with Monin-Obukhov theory.The National Center for Atmospheric Research is sponsored by the National Science Foundation. 相似文献
20.
An analysis was performed of the turbulent data obtained from Yucheng experimental station in the Shandong Province in 1984. [t is shown that at variant wind speed, the spectra of streamwise velocity remain similar and the intensity of wind fluctuations is proportional to wind speed in the downwind area of shelter belt. Therefore, we may decide the similarity of wind fluctuations by a speed scale and a length scale which is not correlated with stability, σu /V0 = F(X / H). The -5/3 power range of temperature spectra extends to lower frequency. The variation of ratio σ0 /T. with stability becomes σ0 / T . = C(X / H)( - Z / L)-1/3 . There is not such an extension of -5 / 3 power range in the humidity spectra. 相似文献