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1.
Probability distributions of concentration fluctuations of a weakly diffusive passive plume in a turbulent boundary layer 总被引：4，自引：0，他引：4
Results are presented from an experimental investigation of turbulent dispersion of a saline plume of large Schmidt number (Sc=830) in a turbulent boundary-layer shear flow simulated in a laboratory water channel. The dispersion measurements are obtained in a neutrally buoyant plume from an elevated point source over a range of downstream distances, where both plume meandering and fine-structure variations in the instantaneous plume are important. High-resolution measurements of the scalar fluctuations in the plume are made with a rake of conductivity probes from which probability distributions of concentration at various points throught the plume are extracted from the time series.Seven candidate probability distributions were tested, namely, the exponential, lognormal, clipped normal, gamma, Weibull, conjugate beta, andK-distributions. Using the measured values of the conditional mean concentration,
, and the conditional fluctuation intensity,i
p
, the Weibull distribution provided the best match to the skewness and kurtosis over all downstream fetches. The skewness and kurtosis were always overpredicted by the lognormal probability density function (pdf), and underpredicted by the gamma pdf. The conjugate beta distribution for which the model parameters are determined using a method of moments based on the fluctuation intensity,i
p
, and skewness,S
p
, was capable of modeling the distribution of scalar concentration over a wide range of positions in the plume. 相似文献
2.
Eugene Yee R. Chan P. R. Kosteniuk G. M. Chandler C. A. Biltoft J. F. Bowers 《Boundary-Layer Meteorology》1994,67(1-2):11-39
A meandering plume model that explicitly incorporates the effects of small-scale structure in the instantaneous plume has been formulated. The model requires the specification of two physically based input parameters; namely, the meander ratio,M, which is dependent on the ratio of the meandering plume dispersion to the instantaneous relative plume dispersion and, a relative in-plume fluctuation measure,k, that is related inversely to the fluctuation intensity in relative coordinates. Simple analytical expressions for crosswind profiles of the higher moments (including the important shape parameters such as fluctuation intensity, skewness, and kurtosis) and for the concentration pdf have been derived from the model. The model has been tested against some field data sets, indicating that it can reproduce many key aspects of the observed behavior of concentration fluctuations, particularly with respect to modeling the change in shape of the concentration pdf in the crosswind direction.List of Symbols
C
Mean concentration in absolute coordinates
-
C
r
Mean concentration in relative coordinates
- C0
Centerline mean concentration in absolute coordinates
- C
r,0
Centerline mean concentration in relative coordinates
-
f
Probability density function of concentration in absolute coordinates
-
f
c
Probability density function of plume centroid position
-
f
r
Probability density function of concentration in relative coordinates
-
i
Absolute concentration fluctuation intensity (standard deviation to mean ratio)
-
i
r
Relative concentration fluctuation intensity (standard deviation to mean ratio)
-
k
Relative in-plume fluctuation measure:k=1/i
r
2
-
K
Concentration fluctuation kurtosis
-
M
Meander ratio of meandering plume variance to relative plume variance
-
S
Concentration fluctuation skewness
-
x
Downwind distance from source
-
y
Crosswind distance from mean-plume centerline
-
z
Vertical distance above ground
-
Instantaneous (random) concentration
-
Crosswind dispersion ofnth concentration moment about zero
-
ny
Mean-plume crosswind (absolute) dispersion
-
y
Plume centroid (meandering) dispersion in crosswind direction
-
y,c
Instantaneous plume crosswind (relative) dispersion
-
Normalized mean concentration in absolute coordinates:C/C
0
-
Particular value taken on by instantaneous concentration, 相似文献
3.
Observations of the dispersion of a contaminant plume in the atmospheric boundary layer, obtained using a Lidar, are analysed in the coordinate frame relative to the instantaneous centre of mass of the plume, as well as the absolute (or fixed) coordinate frame. The study extends the work presented in a previous article, which analysed the structure of the probability density function (pdf) of concentration within the relative coordinate frame. Firstly, the plume displacement component, or plume meander, is analysed and a simple parametric form for the pdf of the plume centreline position is suggested. This is then used to analyse the accuracy and applicability of absolute framework statistical quantities obtained by a convolution of the relative frame statistical quantity with the plume centreline pdf. 相似文献
4.
An analysis of wind tunnel data of dispersion from elevated point sources over a flat floor and gently sloping, 2-D hills is performed. (The data were obtained by Khurshudyan et al., 1981.) Formulas for the mixing lengths and the Lagrangian time scales are tested, suitable for use in various dispersion models. Some expressions for the vertical first moments of the plume concentration distribution suitable for shear flow (Hunt, 1985) are also tested.Then, a normalization is suggested, based on the source mean flow and turbulence parameters, for the ground-level concentration maximum value and position along the plume centerline. Using this normalization, the maximum position is almost constant, regardless of source height variation and of whether the hill is present or not, at least for source positions upstream or over the top.The maximum values allow the determination of normalized terrain amplification factors TAFN, which are shown to be in most cases closer to one than the corresponding TAF obtained without normalization. 相似文献
5.
The effects of a pressure jump and a following internal gravity wave on turbulence and plume diffusion in the stable planetary
boundary layer are examined. The pressure jump was accompanied by a sudden increase in turbulence and plume dispersion. The
effects of wave perturbations on turbulence statistics are analysed by calculating fluxes and variances with and without the
wave signal for averaging times ranging from 1 to 30 min. The wave signals are obtained using a band-pass filter. It is shown
that second-order turbulence quantities calculated without first subtracting the wave perturbations from the time are greater
than those calculated when the wave signal is separated from the turbulence. Estimates of the vertical dispersion of an elevated
tracer plume in the stable boundary layer are made using an elastic backscatter lidar. Plume dispersion observed 25 m downwind
of the source increases rapidly with the arrival of the flow disturbances. Measured plume dispersion and plume centreline
height correlate with the standard deviation of the vertical velocity but not with the wave signal. 相似文献
6.
The average dispersion of a plume in the atmospheric boundary layer is strongly influenced by atmospheric turbulence. Atmospheric turbulence determines also concentration fluctuations due to turbulent meandering by large scale turbulent eddies and in-plume fluctuations, due to smaller scale eddies. Conversion of NO to NO2 in a plume is influenced by micro-scale mixing, due to the concentration fluctuation correlation % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaa0aaaeaaca% qGobGaae4tamaaCaaaleqabaGaaeymaaaakiaab+eadaqhaaWcbaGa% ae4maaqaaiaabgdaaaaaaaaa!3AF4!\[\overline {{\rm{NO}}^{\rm{1}} {\rm{O}}_{\rm{3}}^{\rm{1}} } \] and macro-scale mixing, the mixing in of ambient air containing O3 into the plume.The study of turbulent meandering, in-plume fluctuations, microscale and macro-scale mixing will contribute to a better understanding of concentration fluctuations in general. 相似文献
7.
When the wind speed decreases below a certain value (1–2 m s-1) meandering (low frequency horizontal wind oscillations) starts to prevail. In these conditions it becomes difficult to define a precise mean wind direction and to estimate the airborne dispersion. To study the wind and turbulence characteristics during meandering, two sonic anemometer datasets, containing hourly wind observations, were analysed: the first one, lasting 1 year, was recorded in complex terrain (Graz, Austria) and the second one, lasting about 1month, was recorded in a rather flat area (Tisby, Sweden). It was found that meandering seems to exist under all meteorological conditions regardless of the stability or wind speed and it was confirmed that meandering sets a lower limit for the horizontal wind component variances. Further, it was found that the autocorrelation functions of the horizontal wind components, computed for the low wind cases, show an oscillating behaviour with the presence of large negative lobes. Two different relationships from the literature, and relevant to these oscillatory aspects, were fitted to the data. They contain two parameters: one associated and relevant to the classical integral time scale and the second with meandering occurrence. Based on these relationships, expressions for the mean square displacement of particles y2(t) were also derived. 相似文献
8.
A Forest SO2 Absorption Model (ForSAM) was developed to simulate (1) SO2 plume dispersion from an emission source, (2) subsequent SO2 absorption by coniferous forests growing downwind from the source. There are three modules: (1) a buoyancy module, (2) a dispersion module, and (3) a foliar absorption module. These modules were used to calculate hourly abovecanopy SO2 concentrations and in-canopy deposition velocities, as well as daily amounts of SO2 absorbed by the forest canopy for downwind distances to 42 km. Model performance testing was done with meteorological data (including ambient SO2 concentrations) collected at various locations downwind from a coal-burning power generator at Grand Lake in central New Brunswick, Canada. Annual SO2 emissions from this facility amounted to about 30,000 tonnes. Calculated SO2 concentrations were similar to those obtained in the field. Calculated SO2 deposition velocities generally agreed with published values.Notation
c
air parcel cooling parameter (non-dimensional)
-
E
foliar absorption quotient (non-dimensional)
-
f
areal fraction of foliage free from water (non-dimensional)
-
f
w
SO2 content of air parcel
-
h
height of the surface layer (m)
-
H
height of the convective mixing layer (m)
-
H
stack
stack height (m)
-
k
time level
-
k
drag
coefficient of drag on the air parcel (non-dimensional)
-
K
z
eddy viscosity coefficient for SO2 (m2·s–1)
-
L
Monin-Obukhov length scale (m)
-
L
A
single-sided leaf area index (LAI)
-
n
degree-of-sky cloudiness (non-dimensional)
-
N
number of parcels released with every puff (non-dimensional)
- PAR
photosynthetically active radiation (W m–2)
-
Q
emission rate (kg s–2)
-
r
b
diffusive boundary-layer resistance (s m–1)
-
r
c
canopy resistance (s m–1)
-
r
cuticle
cuticular resistance (s m–1)
-
r
m
mesophyllic resistance (s m–1)
-
r
s
stomatal resistance (s m–1)
-
r
exit
smokestack exit radius (m)
-
R
normally distributed random variable with mean of zero and variance of t (s)
-
u
*
frictional velocity scale,
(m s–1)
-
v
lateral wind vector (m s–1)
-
v
d
SO2 dry deposition velocity (m s–1)
- VCD
water vapour deficit (mb)
-
z
can
mean tree height (m)
-
Z
zenith position of the sun (deg)
-
environmental lapse rate (°C m–1)
-
dry adiabatic lapse rate (0.00986°C m–1)
-
von Kármán's constant (0.04)
-
B
vertical velocities initiated by buoyancy (m s–1)
-
canopy extinction coefficient (non-dimensional)
- ()a
denotes ambient conditions
- ()can
denotes conditions at the top of the forest canopy
- ()h
denotes conditions at the top of the surface layer
- ()H
denotes conditions at the top of the mixed layer
- ()s
denotes conditions at the canopy surface
- ()p
denotes conditions of the air parcels 相似文献
9.
Dispersion of a Point-Source Release of a Passive Scalar Through an Urban-Like Array for Different Wind Directions 总被引：1，自引：1，他引：0
The dispersion of a point-source release of a passive scalar in a regular array of cubical, urban-like, obstacles is investigated
by means of direct numerical simulations. The simulations are conducted under conditions of neutral stability and fully rough
turbulent flow, at a roughness Reynolds number of Re
τ
= 500. The Navier–Stokes and scalar equations are integrated assuming a constant rate release from a point source close to
the ground within the array. We focus on short-range dispersion, when most of the material is still within the building canopy.
Mean and fluctuating concentrations are computed for three different pressure gradient directions (0°, 30°, 45°). The results
agree well with available experimental data measured in a water channel for a flow angle of 0°. Profiles of mean concentration
and the three-dimensional structure of the dispersion pattern are compared for the different forcing angles. A number of processes
affecting the plume structure are identified and discussed, including: (i) advection or channelling of scalar down ‘streets’,
(ii) lateral dispersion by turbulent fluctuations and topological dispersion induced by dividing streamlines around buildings,
(iii) skewing of the plume due to flow turning with height, (iv) detrainment by turbulent dispersion or mean recirculation,
(v) entrainment and release of scalar in building wakes, giving rise to ‘secondary sources’, (vi) plume meandering due to
unsteady turbulent fluctuations. Finally, results on relative concentration fluctuations are presented and compared with the
literature for point source dispersion over flat terrain and urban arrays. 相似文献
10.
James M. Godowitch 《Boundary-Layer Meteorology》1990,50(1-4):375-404
The spatial variability and temporal behavior of the vertical flux of ozone have been investigated from turbulence measurements collected on aircraft flight legs in the daytime period during two consecutive summer experimental field programs. The data were obtained during horizontal flight legs conducted over agricultural crops and forested land in three different regions of the eastern United States.Results from individual experimental cases and statistics derived from all cases in each region are presented. Ozone flux generally exhibited a significant height dependency. The strongest negative (downward) fluxes in the lowest-level flight legs were primarily attributed to the uptake of ozone by the surface and vegetative cover. Fluxes were near-zero in the middle of the convective boundary layer (CBL) in the afternoon period. As ozone flux was proportional to concentration, slightly stronger fluxes were found in low-level urban plume segments where ozone concentrations were 10–20 ppb higher than in the surrounding area. The derived deposition velocity showed no such bias as a function of position across the urban plume. Ozone flux differences were not apparent between the more heavily forested sections and the primarily agricultural cropland areas in these regions. During the afternoon period when no clear temporal trend was evident, means from values obtained below 0.15Z
i
(Z
i
being the CBL height) were -0.421 and -0.431 ppb m-2 s-1 for ozone flux and 0.81 and 0.82 cm s-1 for the derived mean deposition velocity in the southeastern Pennsylvania and central Ohio areas, respectively. These experimental results for ozone provide support to a dry deposition parameterization module which computes grid-area averaged deposition velocities for use in regional-scale models.On assignment from the National Oceanic and Atmospheric Administration, U.S. Department of Commerce. 相似文献
11.
The higher-order correlation functions for the concentrationfluctuations arising from a two-point-source configuration have beencalculated analytically within the context of the phenomenology of afluctuating plume model (viz., a meandering plume model that explicitlyincorporates internal fluctuations). Explicit expressions for thesecond-, third-, and fourth-order correlationfunctions between the concentrationfluctuations produced by two point sources are given in terms of the sourceseparation d and the five physically based parameters that define thegeneralized fluctuating plume model: namely, the absolute plume dispersion,a, which determines the outer plume length scale; the relative plume dispersion, r, which determines the inner plume length scale; the fluctuation intensity, ir, in relative coordinates, which determines the internal concentration fluctuation level; the correlation coefficient, r,between the positions of the centroids of the two interfering plumes; and,the correlation coefficient, r*, between the concentration fluctuationsof the two plumes in relative coordinates, which determines the degree ofinternal mixing of the two scalars. Furthermore, the form of the totalconcentration probability density function arising from the interferenceproduced by two point sources is presented. Predictions for the second-ordercorrelation function, , and for the total concentration probabilitydensity function have been compared with some new experimental data fora two-point-source configuration in grid turbulence generated in awater-channel simulation. These results are in good agreement with the dataand suggest that the analytical model for the second-order correlationfunction and the total concentration probability density function canreproduce many qualitative trends in the interaction of plumes from twosources. 相似文献
12.
Steven R. Hanna 《Boundary-Layer Meteorology》1984,29(4):361-375
Observations of 1-s average concentration fluctuations during two trials of a U.S. Army diffusion experiment are presented and compared with model predictions based on an exponential probability density function (pdf). The source is near the surface and concentration monitors are on lines about 30 to 100 m downwind of the source. The observed ratio of the standard deviation to the mean of the concentration fluctuations is about 1.3 on the mean plume axis and 4 to 5 on the mean plume edges. Plume intermittency (fraction of non-zero readings) is about 50%; on the mean plume axis and 10%; on the mean plume edges. A meandering plume model is combined with an exponential pdf assumption to produce predictions of the intermittency and the standard deviation of the concentration fluctuations that are within 20%; of the observations. 相似文献
13.
14.
Florence Bocquet Ben Balsley Michael Tjernström Gunilla Svensson 《Boundary-Layer Meteorology》2011,138(1):43-60
Tethered Lifting System (TLS) estimates of the dissipation rate of turbulent kinetic energy (e){(\varepsilon)} are reasonably well correlated with concurrent measurements of vertical velocity variance (sw2){(\sigma_{w}^{2})} obtained from sonic anemometers located on a nearby 60-m tower during the CASES-99 field experiment. Additional results in
the first 100 m of the nocturnal stable boundary layer confirm our earlier claim that the presence of weak but persistent
background turbulence exists even during the most stable atmospheric conditions, where e{\varepsilon} can exhibit values as low as 10−7 m2 s−3. We also present a set of empirical equations that incorporates TLS measurements of temperature, horizontal wind speed, and
e{\varepsilon} to provide a proxy measurement for sw2{\sigma_{w}^{2}} at altitudes higher than tower heights. 相似文献
15.
Rainfall is triggered and mainly dominated by atmospheric thermo-dynamics and rich water vapor.Nonetheless, turbulence is also considered as an important factor influencing the evolution of rainfall microphysical parameters. To study such an influence, the present study utilized boundary layer wind profiler radar measurements. The separation point of the radar power spectral density data was carefully selected to classify rainfall and turbulence signals;the turbulent dissipation rate ε and rainfall microphysical parameters can be retrieved to analyze the relationship betweenε and microphysical parameters. According to the retrievals of two rainfall periods in Beijing 2016, it was observed that(1) ε in the precipitation area ranged from 10~(-3.5) to 10~(-1) m~2 s~(-3) and was positively correlated with the falling velocity spectrum width;(2) interactions between turbulence and raindrops showed that small raindrops got enlarge through collision and coalescence in weak turbulence, but large raindrops broke up into small drops under strong turbulence, and the separation value of ε being weak or strong varied with rainfall attributes;(3) the variation of rainfall microphysical parameters(characteristic diameters, number concentration, rainfall intensity, and water content) in the middle stage were stronger than those in the early and the later stages of rainfall event;(4) unlike the obvious impacts on raindrop size and number concentration, turbulence impacts on rain rate and LWC were not significant because turbulence did not cause too much water vapor and heat exchange. 相似文献
16.
A numerical model of convective heat transfer due to isolated thermals in the atmospheric boundary layer is used to describe the temperature profile transformation in undisturbed conditions as a result of intensive dry free convection. Based on some assumptions, the heat transfer Equation (2) is transformed to the form (14) in which the coefficients and the function F are expressed by (d/dz)(ln ) and by parameters of thermals. Equation (14) has been solved numerically with the help of Equation (15) obtained from the statics equation because of Equation (8). The size distribution function f(z, r, t) of the thermals is discrete (Table I), according to Vulf'son (1961). On Figures 1 and 2 are plotted successive temperature profiles for a ground inversion, transformed due to free convection and turbulence (Figures 1a and 2a), and due to turbulence only (Figures 1b and 2b). The profiles are computed from Equation 14 (Figures 1a and 2a) and Equation 16 (Figures 1b and 2b) for k
z= 1 m2 s–1 (Figure 1) and k
z= 10 m2 s–1 (Figure 2). On Figure 3 the real temperature profiles in Sofia for June 22nd 1976 are compared with the profiles computed using the real initial profile for 4.30 h local time. Good qualitative agreement can be seen. 相似文献
17.
Water-tunnel measurements of velocity, turbulence and scalar concentration for three model urban canopies with aspect ratios A r of building height-to-width of 0.25, 1 and 3 are presented. The measurements for the canopies with A r = 1 and 3 are new, while the measurements for A r = 0.25 were previously published. A passive scalar was continuously released from a near-ground point source, and the concentration was measured at several distances from the source and at different heights above the ground. Plume spreads, concentration and distance from the source were non-dimensionalized using length, time and velocity scales reflecting the geometry of the buildings. The scaling collapses the data for all aspect ratios and is valid when the vertical extent of the plume is smaller than the canopy height. The observed plume spreads are compared with analytical relations, which predict linear growth in both transverse and vertical directions. The observed mean concentration is compared with a Gaussian dispersion model that predicts a ?2 power-law decay with distance from the source. 相似文献
18.
B. J. Devenish G. G. Rooney H. N. Webster D. J. Thomson 《Boundary-Layer Meteorology》2010,134(3):411-439
We consider large-eddy simulations (LES) of buoyant plumes from a circular source with initial buoyancy flux F
0 released into a stratified environment with constant buoyancy frequency N and a uniform crossflow with velocity U. We make a systematic comparison of the LES results with the mathematical theory of plumes in a crossflow. We pay particular
attention to the limits [(U)\tilde] << 1{\tilde{U}\ll1} and [(U)\tilde] >> 1{\tilde{U}\gg 1}, where [(U)\tilde]=U/(F0 N)1/4{\tilde{U}=U/(F_0 N)^{1/4}}, for which analytical results are possible. For [(U)\tilde] >> 1{\tilde{U}\gg 1}, the LES results show good agreement with the well-known two-thirds law for the rise in height of the plume. Sufficiently
far above the source, the centreline vertical velocity of the LES plumes is consistent with the analytical z
−1/3 and z
−1/2 scalings for respectively [(U)\tilde] << 1{\tilde{U}\ll 1} and [(U)\tilde] >> 1{\tilde{U}\gg 1}. In the general case, where the entrainment is assumed to be the sum of the contributions from the horizontal and vertical
velocity components, we find that the discrepancy between the LES data and numerical solutions of the plume equations is largest
for [(U)\tilde]=O(1){\tilde{U}=O(1)}. We propose a modified additive entrainment assumption in which the contributions from the horizontal and vertical velocity
components are not equally weighted. We test this against observations of the plume generated by the Buncefield fire in the
U.K. in December 2005 and find that the results compare favourably. We also show that the oscillations of the plume as it
settles down to its final rise height may be attenuated by the radiation of gravity waves. For [(U)\tilde] << 1{\tilde{U}\ll 1} the oscillations decay rapidly due to the transport of energy away from the plume by gravity waves. For ${\tilde{U}>rsim 1}${\tilde{U}>rsim 1} the gravity waves travel in the same direction and at the same speed as the flow. In this case, the oscillations of the plume
do not decay greatly by radiation of gravity waves. 相似文献
19.
Enrico Ferrero Luca Mortarini Stefano Alessandrini Carlo Lacagnina 《Boundary-Layer Meteorology》2013,147(1):123-137
The joint concentration probability density function of two reactive chemical species is modelled using a bivariate Gamma distribution coupled with a three-dimensional fluctuating plume model able to simulate the diffusion and mixing of turbulent plumes. A wind-tunnel experiment (Brown and Bilger, J Fluid Mech 312:373–407, 1996), carried out in homogeneous unbounded turbulence, in which nitrogen oxide is released from a point source in an ozone doped background and the chemical reactions take place in non-equilibrium conditions, is considered as a test case. The model is based on a stochastic Langevin equation reproducing the barycentre position distribution through a proper low-pass filter for the turbulence length scales. While the meandering large-scale motion of the plume is directly simulated, the internal mixing relative to the centroid is reproduced using a bivariate Gamma density function. The effect of turbulence on the chemical reaction (segregation), which in this case has not yet attained equilibrium, is directly evaluated through the covariance of the tracer concentration fields. The computed mean concentrations and the O3–NO concentration covariance are also compared with those obtained by the Alessandrini and Ferrero Lagrangian single particle model (Alessandrini and Ferrero, Physica A 388:1375–1387, 2009) that entails an ad hoc parametrization for the segregation coefficient. 相似文献
20.
Vertical turbulent fluxes of water vapour, carbon dioxide, and sensible heat were measured from 16 August to the 28 September
2006 near the city centre of Münster in north-west Germany. In comparison to results of measurements above homogeneous ecosystem
sites, the CO2 fluxes above the urban investigation area showed more peaks and higher variances during the course of a day, probably caused
by traffic and other varying, anthropogenic sources. The main goal of this study is the introduction and establishment of
a new gap filling procedure using radial basis function (RBF) neural networks, which is also applicable under complex environmental
conditions. We applied adapted RBF neural networks within a combined modular expert system of neural networks as an innovative
approach to fill data gaps in micrometeorological flux time series. We found that RBF networks are superior to multi-layer
perceptron (MLP) neural networks in the reproduction of the highly variable turbulent fluxes. In addition, we enhanced the
methodology in the field of quality assessment for eddy covariance data. An RBF neural network mapping system was used to
identify conditions of a turbulence regime that allows reliable quantification of turbulent fluxes through finding an acceptable
minimum of the friction velocity. For the data analysed in this study, the minimum acceptable friction velocity was found
to be 0.15 m s−1. The obtained CO2 fluxes, measured on a tower at 65 m a.g.l., reached average values of 12 μmol m−2 s−1 and fell to nighttime minimum values of 3 μmol m −2 s−1. Mean daily CO2 emissions of 21 g CO2 m−2d −1 were obtained during our 6-week experiment. Hence, the city centre of Münster appeared to be a significant source of CO2. The half-hourly average values of water vapour fluxes ranged between 0.062 and 0.989 mmol m−2 s−1and showed lower variances than the simultaneously measured fluxes of CO2. 相似文献