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1.
The relationship between fluxes and variances has been widely studied in the frame of the similarity theory. However, the dynamics of urban morphology should be highlighted in the urban canopy. Therefore, the relationship between fluxes and variances of turbulent quantities (wind velocity, temperature, carbon dioxide, and water vapor concentration) without stability parameters has been investigated with data collected from the Beijing 325-m meteorological tower in the urban canopy. Though the similarity theory is available for the statistics of wind velocity and temperature, urban land use plays an important role in the linear relationship between momentum fluxes and velocity variances, while the linear relationship between sensible heat fluxes and temperature variances is influenced at large by stratification. As for carbon dioxide and water vapor, the linear relationship between their fluxes and variances is a better way compared to the similarity theory to describe their motion, though the plots are a little scattered owing to their fugitive source. 相似文献
2.
In many atmospheric flows, a dispersed phase is actively suspended by turbulence, whose competition with gravitational settling ultimately dictates its vertical distribution. Examples of dispersed phases include snow, sea-spray droplets, dust, or sand, where individual elements of much larger density than the surrounding air are carried by turbulent motions after emission from the surface. In cases where the particle is assumed to deviate from local fluid motions only by its gravitational settling (i.e., they are inertialess), traditional flux balances predict a power-law dependence of particle concentration with height. It is unclear, however, how particle inertia influences this relationship, and this question is the focus of this work. Direct numerical simulations are conducted of turbulent open-channel flow, laden with Lagrangian particles of specified inertia; in this way the study focuses on the turbulent transport which occurs in the lowest few meters of the planetary boundary layer, in regions critical for connecting emission fluxes to the fluxes felt by the full-scale boundary layer. Simulations over a wide range of particle Stokes number, while holding the dimensionless settling velocity constant, are performed to understand the role of particle inertia on vertical dispersion. It is found that particles deviate from their inertialess behaviour in ways that are not easily captured by traditional theory; concentrations are reduced with increasing Stokes number. Furthermore, a similarity-based eddy diffusivity for particle concentration fails as particles experience inertial acceleration, precluding a closed-form solution for particle concentration as in the case of inertialess particles. The primary consequence of this result is that typical flux parametrizations connecting surface emission models (e.g., saltation models or sea-spray generation functions) to elevated boundary conditions may overestimate particle concentrations due to the reduced vertical transport caused by inertia in between; likewise particle emission may be underestimated if inferred from concentration measurements aloft. 相似文献
3.
The Effect of the Vertical Source Distribution on Scalar Statistics within and above a Forest Canopy 总被引：1，自引：1，他引：0
Little is known about in-canopy processes that may alter forest–atmosphere exchanges of trace gases and aerosols. To improve
our understanding of in-canopy mixing, we use large-eddy simulation to study the effect of scalar source/sink distributions
on scalar concentration moments, fluxes, and correlation coefficients within and above an ideal forest canopy. Scalars are
emitted from: (1) the ground, (2) the canopy, and (3) both the ground and the canopy; a scalar is also deposited onto the
canopy. All scalar concentration moments, fluxes, and correlation coefficients are affected by the source location/distribution,
as is the scalar segregation intensity. We conclude that vertical source/sink distribution has a profound impact on scalar
concentration profiles, fluxes, correlation coefficient, and scalar segregation. 相似文献
4.
The effects of source size on plume behaviour have been examined in a 1.2 m wind tunnel boundary layer for isokinetic sources with diameters from 3 to 35 mm at source heights of 230 mm and at ground level. Experimental measurements of mean concentration and the variance, intermittency and probability density functions of the concentration fluctuations were obtained. In addition, a fluctuating Gaussian plume model is presented which reproduces many of the observed features of the elevated emission. The mean plume width becomes independent of source size much more rapidly than the instantaneous plume width. Since it is the meandering of the instantaneous plume which generates most of the concentration fluctuations near the source, these are also dependent on source size. The flux of variance in the plume reaches a maximum, whose value is greatest for the smallest source size, close to the source and thereafter is monotonically decreasing. The intermittency factor reaches a minimum, whose value is lowest for the smallest source, and increases back towards one. Concentration fluctuations for the ground-level source are much less dependent on source size due to the effects of the surface. 相似文献
5.
Based on the surface energy budget, the sea surface temperature (SST) variance is related to the product of three factors: the sum of the variances of surface radiative and turbulent energy fluxes and of ocean heat transport, an efficiency factor depending on the covariances among them, and a transfer factor involving the persistence of surface temperature via its lagged autocorrelation. These quantities are analyzed for current climate conditions based on results from the NCEP/NCAR reanalyses and a simulation with the CCCma coupled climate model. Potential changes with climate change are considered based on two quasi-equilibrium climate change integrations for which the forcing has been stabilized at years 2050 and 2100 values of the IS92a forcing scenario. The surface energy fluxes, which contribute to the variance of SST, are similar in the modelled and reanalyzed atmosphere but modelled temperature variance is conditioned on the thickness of the upper ocean model layer. Changes of SST variance with global warming show broad scale patterns with decreases in the tropical central-eastern Pacific and the northern extra-tropical Pacific, and increases in both the sub-tropical Pacific and mid-latitudes of the North Atlantic. The changes in SST variance are not associated only with changes in the variances of surface energy fluxes/transports but also with changes in the covariances among them and by changes in the temperature autocorrelation structure. 相似文献
6.
We present a numerical study aimed at quantifying the effects of concentration-dependent density on the spread of a seeping
plume of CO2 into the atmosphere such as could arise from a leaking geologic carbon sequestration site. Results of numerical models can
be used to supplement field monitoring estimates of CO2 seepage flux by modelling transport and dispersion between the source emission and concentration-measurement points. We focus
on modelling CO2 seepage dispersion over relatively short distances where density effects are likely to be important. We model dense gas dispersion
using the steady-state Reynolds-averaged Navier-Stokes equations with density dependence in the gravity term. Results for
a two-dimensional system show that a density dependence emerges at higher fluxes than prior estimates. A universal scaling
relation is derived that allows estimation of the flux from concentrations measured downwind and vice versa. 相似文献
7.
Turbulence measurements in the lower half of the convective boundary layer (CBL), which includes both mixed layer and surface layer, were carried out with five sonic anemometers mounted on a 213-m tower over a complex flat suburban area with patches of forest, agricultural land, houses and buildings. Also made were radiosoundings of temperature, humidity and wind speed, to determine the CBL height. The sonic anemometer data of wind speed and temperature were processed to derive the second-moment turbulent statistics and were analyzed to investigate the applicability of variance methods to estimate regional surface fluxes of sensible heat. It was found that the temperature variances in the lower mixed layer, coupled with universal functions, produced sensible heat fluxes H over the area with an rms error of the order of 40 Wm-2 when compared with H values derived from the eddy correlation method. The variance of the vertical wind speed did not produce as good a result. In contrast, the surface-layer temperature variances yielded H values with rms error of the order of 20 Wm-2, even though the underlying surface was non-uniform and highly non-isothermal, above which enhanced temperature variances could be suspected. 相似文献
8.
Large-eddy Simulation of Turbulent Flow across a Forest Edge. Part II: Momentum and Turbulent Kinetic Energy Budgets 总被引：1，自引：0，他引：1
Bai Yang Andrew P. Morse Roger H. Shaw Kyaw Tha Paw U 《Boundary-Layer Meteorology》2006,121(3):433-457
Momentum and turbulent kinetic energy (TKE) budgets across a forest edge have been investigated using large-eddy simulation (LES). Edge effects are observed in the rapid variation of a number of budget terms across this vegetation transition. The enhanced drag force at the forest edge is largely balanced by the pressure gradient force and by streamwise advection of upstream momentum, while vertical turbulent diffusion is relatively insignificant. For variance and TKE budgets, the most important processes at the forest edge are production due to the convergence (or divergence) of the mean flow, streamwise advection, pressure diffusion and enhanced dissipation by canopy drag. Turbulent diffusion, pressure redistribution and vertical shear production, which are characteristic processes in homogeneous canopy flow, are less important at the forest transition. We demonstrate that, in the equilibrated canopy flow, a substantial amount of TKE produced in the streamwise direction by the vertical shear of the mean flow is redistributed in the vertical direction by pressure fluctuations. This redistribution process occurs in the upper canopy layers. Part of the TKE in the vertical velocity component is transferred by turbulent and pressure diffusion to the lower canopy levels, where pressure redistribution takes place again and feeds TKE back to the streamwise direction. In this TKE cycle, the primary source terms are vertical shear production for streamwise velocity variance and pressure redistribution for vertical velocity variance. The evolution of these primary source terms downwind of the forest edge largely controls the adjustment rates of velocity variances. 相似文献
9.
Field measurements of emission of nitric oxide from fertilized and unfertilized forest soils in Sweden 总被引：3，自引：0，他引：3
C. Johansson 《Journal of Atmospheric Chemistry》1984,1(4):429-442
Application of nitrate fertilizers on two types of forest soils led to a marked increase in the NO emission rate indicating a large potential for NO production in these soils. The largest fluxes on the fertilized plots were up to 60 ng NO–N m–2 s–1. About 0.35% of the applied nitrogen was lost as NO within about 14 days after fertilization. The fluxes from the unfertilized forest soils were in the range 0.1 to 0.8 ng NO–N m–2 s–1 with a median value of 0.3 ng NO–N m–2 s–1. If this value, obtained during June and August to September, is representative for the growing season (150 days), it corresponds to an annual emission of 0.04 kg NO–N ha–1. This is about 30% of the value obtained for an unfertilized agricultural soil. Because of the large areas occupied by forests in Sweden the flux of NO from forest soils represents a significant contribution to the total flux of NO from soils in Sweden.Earlier observations of equilibrium concentrations for NO have been verified. These were found to range from 0.2 to 2 ppbv for an unfertilized forest soil and up to 170 ppbv for a fertilized soil. At the rural site in Sweden where these measurements were performed the ambient concentrations where found to be less than this equilibrium concentration, and consequently there was generally a net emission of NO.There are still large uncertainties about the global flux of NO from soils. Using direct measurements on three different types of ecosystems and estimates based on a qualitative discussion for the remaining land areas, a global natural source for NO of the order of 1 Tg N a–1 was obtained. If 0.35% of the total annual production of fertilizer nitrogen is lost as NO, fertilization of soils may contribute with 20% to the natural flux from soils. 相似文献
10.
We report a two-dimensional (alongwind u, vertical w) trajectory-simulation model, consistent with Thomson's (1987) well-mixed criteria, that allows for the non-Gaussian turbulence typical of flow within a plant canopy. The effect of non-Gaussian turbulence was examined by formulating a non-Gaussian u, w joint probability density function (PDF) as the sum of two Gaussian joint-PDFs. The resultant PDF reproduced the desired means, variances, skewnesses, and kurtoses, and the correct covariance. In prediction of the location of maximum concentration downwind of a line source in homogeneous, slightly non-Gaussian turbulence, it proved advantageous to incorporate skewness and kurtosis. However, in the case of inhomogeneous, highly non-Gaussian turbulence, the addition of skewness and kurtosis in the model resulted in substantially worse agreement with measurements than the results of the model using Gaussian PDFs. This may be due to inaccuracy in our PDF formulation. Dispersion predictions from the model with Gaussian PDFs were generally not statistically different from measurements. These results indicate that a two-dimensional Gaussian trajectory-simulation approach is adequate to predict mean concentrations and fluxes resulting from sources within plant canopies. 相似文献
11.
We describe the results of an experiment focusing on wind speed and momentum fluxes in the atmospheric boundary layer up to
200 m. The measurements were conducted in 1996 at the Cabauw site in the Netherlands. Momentum fluxes are measured using the
K-Gill Propeller Vane. Estimates of the roughness length are derived using various techniques from the wind speed and flux
measurements, and the observed differences are explained by considering the source area of the meteorological parameters.
A clear rough-to-smooth transition is found in the wind speed profiles at Cabauw. The internal boundary layer reaches the
lowest k-vane (20 m) only in the south-west direction where the obstacle-free fetch is about 2 km. The internal boundary layer
is also reflected in the roughness lengths derived from the wind speed profiles. The lower part of the profile (< 40 m) is
not in equilibrium and no reliable roughness analysis can be given. The upper part of the profile can be linked to a large-scale
roughness length. Roughness lengths derived from the horizontal wind speed variance and gustiness have large footprints and
therefore represent a large-scale average roughness. The drag coefficient is more locally determined but still represents
a large-scale roughness length when it is measured above the local internal boundary layer. The roughness length at inhomogeneous
sites can therefore be determined best from drag coefficient measurements just above the local internal boundary layers directly,
or indirectly from horizontal wind speed variance or gustiness. In addition, the momentum and heat fluxes along the tower
are analysed and these show significant variation with height related to stability and possibly surface heterogeneity. It
appears that the dimensionless wind speed gradients scale well with local fluxes for the variety of conditions considered,
including the unstable cases. 相似文献
12.
Nobuyuki Kinoshita 《Boundary-Layer Meteorology》2003,106(1):61-91
Turbulent flow data of wind velocity and temperature in the unstably stratifiedatmospheric boundary layer, derived from steel tower observations in the field and wind-tunnel experiments were used to study the relationship between the plumes and the small-scale eddies in the inertial subrange. Flow visualisation experiments in the wind tunnel were also conducted to observe the structure of the flow in the plumes, and time series data were analysed by using wavelet transforms. The results show that variances of velocity and temperature due to the small-scale eddies are large in the plumes and small outside of the plumes, and that the momentum and heat fluxes due to the small-scale eddies follow the same tendency as found in the variances. The ratios of the variances caused by the small-scale eddies in the plumes to the whole of the variances caused by the small-scale eddies in and out the plumes increase with non-dimensional height -z/L in which L is the local Obukhov length. Similar ratios of the fluxes caused by the small-scale eddies also show the same tendency. These ratios can be expressed as functions of -z/L for results based on field observationand the wind tunnel experiments. This relation hardly changes even if the wavelet function is changed. The flow visualisation experiments show that the plumes have a complicated structure in which mushroom type flows are stacked on top of each other. This characteristic structure seems to increase the energy of the small-scale eddies in the plumes. 相似文献
13.
Richard S. J. Tol 《Climatic change》2003,56(3):265-289
Cost-benefit analysis is only applicable if the variances of both costs and benefits are finite. In the case of climate change, the variances of the net present marginal costs and benefits of greenhouse gas emission reduction need to be finite. Finiteness is hard, if not impossible to prove. The opposite is easier to establish as one only needs to show that there is one, not impossible representation of the climate change with infinite variance. The paper shows that all relevant current variables of theFUND model have finite variances. However, there is a small chance that climate change reverses economic growth in some regions. In that case, the discount rate becomes negative and the net present marginal benefits of greenhouse gas emission reduction becomes very large. So large, that its variance is unbounded. One could interpret this as an indication that cost-benefit analysis is invalid. Alternatively, one could argue that the infinity is present in both the base case and the policy scenario, and therefore irrelevant; in that interpretation, cost-benefit analysis is a valid tool. 相似文献
14.
将INTEX-B排放源应用到空气质量模型Model3-CMAQ中,对中国地区对流层NO2的浓度分布进行了数值模拟,并与OMI卫星对流层NO2资料进行了对比和验证。结果表明：将INTEX-B排放源应用到Model3-CMAQ模式,模拟的NO2浓度在中国地区的分布、季节变化规律与卫星资料所得结果一致。敏感性试验表明,工业及电厂排放对NO2的浓度贡献最大,而交通排放的贡献相对较小,两种排放均主要集中在京津、长江三角洲等经济发达地区。 相似文献
15.
16.
Large-eddy simulations of an evolving cloud field are used to investigate the contribution of microphysical processes to the evolution of the variance of total water and liquid water potential temperature in the boundary layer. While the first hours of such simulations show a transient behaviour and have to be analyzed with caution, the final portion of the simulation provides a quasi-equilibrium situation. This allows investigation of the budgets of the variances of total water and liquid water potential temperature and quantification of the contribution of several source and sink terms. Accretion is found to act as a strong sink for the variances, while the contributions from the processes of evaporation and autoconversion are small. A simple parametrization for the sink term connected to accretion is suggested and tested with a different set of simulations. 相似文献
17.
A large-eddy simulation (LES) of a cumulus-topped boundary layer (CTBL) has been made, based on observations gathered on 10 September 1974 during the GARP (Global Atmospheric Research Program) Atlantic Tropical Experiment (GATE). On this day there were light winds, making buoyancy production the only important mechanism for generating turbulent kinetic energy. Cumulus clouds with a depth of a few hundred meters covered about 10% of the sky. A comparison between LES results and observed variances and fluxes will be made.In order to understand the dynamics of the cumulus, we will investigate the physical mechanisms by which variances and fluxes are produced in CTBL's. This will be done by analyzing the budget equations which describe the time evolution of the variances and fluxes. The production, transport and destruction terms, that appear in these equations, will be calculated explicitly with the LES model.Budgets are calculated from the GATE simulation and two slightly modified GATE simulations, in which only the initial water vapor specific humidity is increased. As a result in these two modified GATE simulations the vertical extent of the clouds and the cloud cover is larger. In this way, the influence of the cumulus clouds on the budgets can be investigated more thoroughly. Finally, the budgets are compared with the results obtained from an earlier simulation based on the Puerto Rico Field Experiment and other results found in the literature. 相似文献
18.
Atmospheric concentrations and deposition of heavy metals over the North Sea: A literature review 总被引：2，自引：0，他引：2
A literature review of the atmospheric concentration rates and dry and wet deposition fluxes of particulate Cd, Cr, Cu, Pb and Zn to the North Sea and adjacent areas is given. The results of direct measurements of dry and wet deposition fluxes are compared to indirect estimates and to modelling values. This work points out the large uncertainties in results of different studies on atmospheric input of trace elements into the North Sea. The current knowledge about the dependence of the deposition velocity upon the particle size and about the processes controlling wet deposition fluxes, and the quality and completeness of the emission data are still inadequate for describing the environmental cycle and impact of heavy metals in the North Sea. 相似文献
19.
Abstract Fluxes of temperature, water vapour, O3, SO2 and CO2 were estimated from the measurement of their variances, taken over a wetland region in northern Ontario (Canada) during the summer of 1990 and over a deciduous forest when it was fully leafed during the summer of 1988 and when it was leafless during the winter of 1990. A set of flux‐variance relations was employed, including empirical forms of universal functions that could be adjusted with some constants. Results from the present study show that these constants needed to be adjusted with site‐specific data in order to achieve a closer agreement between estimated and observed fluxes. Best estimates were obtained for the fluxes of temperature and water vapour and it was found that the flux estimates of O3, SO2 and CO2 correlated better with water vapour than with temperature. For these trace gases the flux‐variance method yielded estimates of dry deposition velocities that were either comparable with or larger than those obtained from a resistance analogue model. Both methods yielded values that overestimated the observed dry deposition velocities. The employment of the flux‐variance method in an operational network would require the use of fast‐response sensors and a practical method for reducing the noise level of the measured variances. 相似文献
20.
Local similarity theory, an analogy to the Monin-Obukhov similarity theory, is successfully applied to airborne observations in a coastal area of South Australia. The boundary layer over this highly non-uniform surface is characterized by extensive variations in its thermal stratification and turbulence characteristics. However, the behaviour of some statistical parameters of second- and higher moments seems to be determined mainly by local forcing, while horizontal advection plays a less important role. For these parameters, local scaling is effective. It is shown that the dimensionless variances of vertical velocity and potential temperature are functions of z/ only, where is the local Monin-Obukhov length and z is the height above ground. The dimensionless variance of horizontal velocity components is found to depend on h/, where h is the height of the oundary layer. Similarity relationships for some triple correlations are also discussed. The empirically determined local similarity relationships are found to agree with those obtained from surface-layer similarity. Finally, to illustrate the complexity of the local forcing, distributions of vertical energy and momentum fluxes, from which the local scaling parameters are derived, are shown. 相似文献