共查询到20条相似文献，搜索用时 31 毫秒
1.
Wind field simulation in the surface layer is often used to manage natural resources in terms of air quality,gene flow(through pollen drift),and plant disease transmission(spore dispersion).Although Lagrangian stochastic(LS)models describe stochastic wind behaviors,such models assume that wind velocities follow Gaussian distributions.However,measured surface-layer wind velocities show a strong skewness and kurtosis.This paper presents an improved model,a non-Gaussian LS model,which incorporates controllable non-Gaussian random variables to simulate the targeted non-Gaussian velocity distribution with more accurate skewness and kurtosis.Wind velocity statistics generated by the non-Gaussian model are evaluated by using the field data from the Cooperative Atmospheric Surface Exchange Study,October 1999 experimental dataset and comparing the data with statistics from the original Gaussian model.Results show that the non-Gaussian model improves the wind trajectory simulation by stably producing precise skewness and kurtosis in simulated wind velocities without sacrificing other features of the traditional Gaussian LS model,such as the accuracy in the mean and variance of simulated velocities.This improvement also leads to better accuracy in friction velocity(i.e.,a coupling of three-dimensional velocities).The model can also accommodate various non-Gaussian wind fields and a wide range of skewness–kurtosis combinations.Moreover,improved skewness and kurtosis in the simulated velocity will result in a significantly different dispersion for wind/particle simulations.Thus,the non-Gaussian model is worth applying to wind field simulation in the surface layer. 相似文献
2.
Xiaoying Cao Gilles Roy William J. Hurley William S. Andrews 《Boundary-Layer Meteorology》2011,139(3):487-500
The Gaussian distribution is a good approximation for transient (instantaneously released) puff concentration distributions
within a short period of time after release. Artificial neural network (ANN) models for puff dispersion coefficients were
developed, based on observations from field experiments covering a wide range of meteorological conditions (in March, May,
August and November). Their average predictions were in very good agreement with measurements, having high correlation coefficients
(r > 0.99). A non-linear multi-variable regression model for dispersion coefficients was also developed, under the assumption
that puff dispersion coefficients increase with time, and follow power laws. Both ANN-based and multi-regression non-linear
models were able to use easily measured atmospheric parameters directly, without the necessity of predefining the Pasquill
stability category. Predictions of ANN-based and multi-regression-based Gaussian puff models were compared with those of Gaussian
puff models using Slade’s dispersion coefficients and COMBIC, a sophisticated model based on Gaussian distributions. Predictions
from our two new models showed better agreement with concentration measurements than the other Gaussian puff models, by having
a much higher fraction within a factor of two of measured values, and lower normalized mean square errors. 相似文献
3.
B.L. Sawford 《Boundary-Layer Meteorology》1999,93(3):411-424
We present a new measure for the rotation of Lagrangian trajectories in turbulence that simplifies and generalises that suggested by Wilson and Flesch ( Boundary-Layer Meteorol. 84, 411–426). The new measure is the cross product of the velocity and acceleration and is directly related to the area, rather than the angle, swept out by the velocity vector. It makes it possible to derive a simple but exact kinematic expression for the mean rotation of the velocity vector and to partition this expression into terms that are closed in terms of Eulerian velocity moments up to second order and unclosed terms. The unclosed terms arise from the interaction of the fluctuating part of the velocity and the rate of change of the fluctuating velocity.We examine the mean rotation of a class of Lagrangian stochastic models that are quadratic in velocity for Gaussian inhomogeneous turbulence. For some of these models, including that of Thomson ( J. Fluid Mech. 180, 113–153), the unclosed part of the mean rotation vanishes identically, while for other models it is non-zero. Thus the mean rotation criterion clearly separates the class of models into two sets, but still does not provide a criterion for choosing a single model.We also show that models for which = 0 are independent of whether the model is derived on the assumption that total Lagrangian velocity is Markovian or whether the fluctuating part is Markovian. 相似文献
4.
A number of authors have reported the problem of unrealistic velocities (“rogue trajectories”) when computing the paths of
particles in a turbulent flow using modern Lagrangian stochastic (LS) models, and have resorted to ad hoc interventions. We
suggest that this problem stems from two causes: (1) unstable modes that are intrinsic to the dynamical system constituted
by the generalized Langevin equations, and whose actual triggering (expression) is conditional on the fields of the mean velocity
and Reynolds stress tensor and is liable to occur in complex, disturbed flows (which, if computational, will also be imperfect
and discontinuous); and, (2) the “stiffness” of the generalized Langevin equations, which implies that the simple stochastic
generalization of the Euler scheme usually used to integrate these equations is not sufficient to keep round-off errors under
control. These two causes are connected, with the first cause (dynamical instability) exacerbating the second (numerical instability);
removing the first cause does not necessarily correct the second, and vice versa. To overcome this problem, we introduce a
fractional-step integration scheme that splits the velocity increment into contributions that are linear (U
i
) and nonlinear (U
i
U
j
) in the Lagrangian velocity fluctuation vector U, the nonlinear contribution being further split into its diagonal and off-diagonal parts. The linear contribution and the
diagonal part of the nonlinear contribution to the solution are computed exactly (analytically) over a finite timestep Δt, allowing any dynamical instabilities in the system to be diagnosed and removed, and circumventing the numerical instability
that can potentially result in integrating stiff equations using the commonly applied explicit Euler scheme. We contrast results
using this and the primitive Euler integration scheme for computed trajectories in a drastically inhomogeneous urban canopy
flow. 相似文献
5.
Frank Quintarelli 《Boundary-Layer Meteorology》1990,52(3):209-219
Until recently, pollution dispersion models have made predictions on the basis that the pollutant concentration is Gaussian. Such is not the case for convective conditions where the observed vertical velocity distribution is skewed towards the updraught portion of the distribution. One recent dispersion model assumes that the observed distribution can be synthesized by superimposing two Gaussians of appropriate means, variances and amplitudes.In the current paper, two techniques for deriving the constituent distributions are investigated. The first technique is based on conditionally sampling the vertical velocity time series and partitioning the vertical velocity samples into two sets — one set recorded when the sensor was experiencing an updraught and the other when the sensor was experiencing a downdraught. The second method consists of fitting two Gaussian distributions to the observed data and adjusting these using an iterative procedure until a specified tolerance is achieved.Both techniques give similar results which compare favourably with results obtained by other researchers. Assumptions, as well as advantages and disadvantages of each technique are also discussed. 相似文献
6.
A three-dimensional Lagrangian stochastic (LS) model to evaluate pollutant dispersion in the atmospheric boundary layer has been developed. The model satisfies the well-mixed criterion of Thomson and allows for inhomogeneous, skew turbulence. Making use of the spherical reference frame, one of the possible solutions has been obtained. A skewed joint probability density function (PDF), which reproduces the given velocity moments (means, variances, skewness and covariances), has been built-up by a linear combination of eight Gaussian PDFs. In order to verify consistency with the well-mixed criterion, the long term results have been compared with the theoretical behaviour. A comparison between our model and Thomson's published algorithms was also carried out. By comparing wind-tunnel data and numerical predictions, a further validation of our LS model has been obtained. From an analysis of the numerical results, we can state that our model is able to evaluate dispersion in the case of complex flows where the application of previous models is unsuccessful. 相似文献
7.
A study of the neutrally-stratified flow within and over an array of three-dimensional buildings (cubes) was undertaken using
simple Reynolds-averaged Navier—Stokes (RANS) flow models. These models consist of a general solution of the ensemble-averaged,
steady-state, three-dimensional Navier—Stokes equations, where the k-ε turbulence model (k is turbulence kinetic energy and ε is viscous dissipation rate) has been used to close the system of equations. Two turbulence
closure models were tested, namely, the standard and Kato—Launder k-ε models. The latter model is a modified k-ε model designed specifically to overcome the stagnation point anomaly in flows past a bluff body where the standard k-ε model overpredicts the production of turbulence kinetic energy near the stagnation point. Results of a detailed comparison
between a wind-tunnel experiment and the RANS flow model predictions are presented. More specifically, vertical profiles of
the predicted mean streamwise velocity, mean vertical velocity, and turbulence kinetic energy at a number of streamwise locations
that extend from the impingement zone upstream of the array, through the array interior, to the exit region downstream of
the array are presented and compared to those measured in the wind-tunnel experiment. Generally, the numerical predictions
show good agreement for the mean flow velocities. The turbulence kinetic energy was underestimated by the two different closure
models. After validation, the results of the high-resolution RANS flow model predictions were used to diagnose the dispersive
stress, within and above the building array. The importance of dispersive stresses, which arise from point-to-point variations
in the mean flow field, relative to the spatially-averaged Reynolds stresses are assessed for the building array. 相似文献
8.
A Lagrangian Stochastic Model for Heavy Particle Dispersion in the Atmospheric Marine Boundary Layer
The dispersion of heavy particles and pollutants is often simulated with Lagrangian stochastic (LS) models. Although these
models have been employed successfully over land, the free surface at the air-sea interface complicates the implementation
of traditional LS models. We present an adaptation of traditional LS models to the atmospheric marine boundary layer (MBL),
where the bottom boundary is represented by a realistic wavy surface that moves and deforms. In addition, the correlation
function for the turbulent flow following a particle is extended to the anisotropic, unsteady case. Our new model reproduces
behaviour for Lagrangian turbulence in a stratified air flow that departs only slightly from the expected behaviour in isotropic
turbulence. When solving for the trajectory of a heavy particle in the air flow, the modelled turbulent forcing on the particle
also behaves remarkably well. For example, the spectrum of the turbulence at the particle location follows that of a massless
particle for time scales approximately larger than the Stokes’ particle response time. We anticipate that this model will
prove especially useful in the context of sea-spray dispersion and its associated momentum, sensible and latent heat, and
gas fluxes between spray droplets and the atmosphere. 相似文献
9.
The probability density functions (pdf’s) of the wind increments are measured under different weather conditions in the atmospheric
boundary layer, including the extreme weather of a typhoon and sand storm. It is found that in each case the measured pdf’s
with respect to different time lags coincide by suitable scaling transformation. This property is similar to that of the stable
distributions. However, fitting results show that the tails of the stable distributions are generally heavier than that of
the measured ones. Beside, the stable distributions (except for the Gaussian distribution) have infinite variance, which implies
infinite average kinetic energy. In fact, it can be proved that if the tails of the pdf’s are heavy enough, the variance will
be infinite. Therefore, the tail-truncated stable distributions with finite variances are introduced to fit the data and the
fitting results are excellent. 相似文献
10.
Chenning Tong 《Boundary-Layer Meteorology》1996,81(3-4):399-410
Experimentally obtained time coherence has traditionally been interpreted as streamwise one-dimensional spatial coherence
through Taylor’s hypothesis. We calculate corrections to the highwavenumber part of the coherence to account for the errors
caused by the deviation from Taylor’s hypothesis in high-intensity turbulent flows. The small-scale turbulence is assumed
to be frozen and convected by a fluctuating convection velocity. Both Lumley’s two-term approximation and the Gaussian approximation
are used in the calculations. In general, we find that the coherence for crossstream separations is significantly overestimated
by the direct use of Taylor’s hypothesis, the error increasing with wavenumber; that for streamwise separations is underestimated.
The analyses are compared with cross-stream coherence measurements in the atmospheric surface layer. Our results indicate
that predictions from Lumley’s approximation yield better agreement with experimental data for cross-stream separations than
those from the Gaussian model. Our study suggests that reliable measurement of two-point spatial coherence can be achieved
only for scales not too small compared to the sensor separation. 相似文献
11.
S. Parey 《Climate Dynamics》2008,30(1):99-112
Power plant construction requires anticipation to achieve a liable dimensioning on the long functioning time of the installation.
In the present climate change context, dimensioning towards extremely high temperature for installations intended to run until
the 2070s or later implies an evaluation of plausible extreme values at this time scale. This study is devoted to such an
estimation for France, using both observation series and climate model simulation results. The climate model results are taken
from the European PRUDENCE (Prediction of Regional scenarios and Uncertainties for Defining European Climate change risks
and Effects) project database of regional climate change scenarios for Europe. Comparison of high summer temperature distributions
given by observations and climate models under current climate conditions, conducted using Generalized Extreme Value distribution,
reveals that only a few models are able to correctly reproduce it. For these models, climate change under IPCC A2 and B2 scenarios
leads to differences in the variability of high values, whose proportion has an important impact on future 100-year return
levels.
This study was first presented at the EGU General Assembly in Vienna, 2–7 April 2006. 相似文献
12.
A. M. Reynolds 《Boundary-Layer Meteorology》1998,86(2):333-344
Lagrangian stochastic models, quadratic in velocity and satisfying the well-mixed condition for two-dimensional Gaussian turbulence, are used to make predictions of scalar dispersion within a model plant canopy. The non-uniqueness associated with satisfaction of the well-mixed condition is shown to be non-trivial (i.e. different models produce different predictions for scalar dispersion). The best agreement between measured and predicted mean concentrations of scalars is shown to be obtained with a small sub-class of optimal models. This sub-class of optimal models includes Thomson's model (J. Fluid Mech. 180, 529–556, 1987), the simplest model that satisfies the well-mixed condition for Gaussian turbulence, but does not include two other models identified recently as being in optimal agreement with the measured spread of tracers in a neutral boundary layer. It is therefore demonstrated that such models are not universal, i.e. applicable to a wide range of flows without readjustment of model parameters. Predictions for scalar dispersion in the model plant canopy are also obtained using the model of Flesch and Wilson (Boundary-Layer Meteorol. 61, 349–374, 1992). It is shown that, when used with a Gaussian velocity distribution or a maximum-missing-information velocity distribution, which accounts for the measured skewness and kurtosis of velocity statistics, the agreement between predictions obtained using the model of Flesch and Wilson and measurements is as good as that obtained using Thomson's model. 相似文献
13.
14.
Matthew C. Wyant Christopher S. Bretherton Julio T. Bacmeister Jeffrey T. Kiehl Isaac M. Held Ming Zhao Stephen A. Klein Brian J. Soden 《Climate Dynamics》2006,27(2-3):261-279
Low-latitude cloud distributions and cloud responses to climate perturbations are compared in near-current versions of three leading U.S. AGCMs, the NCAR CAM 3.0, the GFDL AM2.12b, and the NASA GMAO NSIPP-2 model. The analysis technique of Bony et al. (Clim Dyn 22:71–86, 2004) is used to sort cloud variables by dynamical regime using the monthly mean pressure velocity ω at 500 hPa from 30S to 30N. All models simulate the climatological monthly mean top-of-atmosphere longwave and shortwave cloud radiative forcing (CRF) adequately in all ω-regimes. However, they disagree with each other and with ISCCP satellite observations in regime-sorted cloud fraction, condensate amount, and cloud-top height. All models have too little cloud with tops in the middle troposphere and too much thin cirrus in ascent regimes. In subsidence regimes one model simulates cloud condensate to be too near the surface, while another generates condensate over an excessively deep layer of the lower troposphere. Standardized climate perturbation experiments of the three models are also compared, including uniform SST increase, patterned SST increase, and doubled CO2 over a mixed layer ocean. The regime-sorted cloud and CRF perturbations are very different between models, and show lesser, but still significant, differences between the same model simulating different types of imposed climate perturbation. There is a negative correlation across all general circulation models (GCMs) and climate perturbations between changes in tropical low cloud cover and changes in net CRF, suggesting a dominant role for boundary layer cloud in these changes. For some of the cases presented, upper-level clouds in deep convection regimes are also important, and changes in such regimes can either reinforce or partially cancel the net CRF response from the boundary layer cloud in subsidence regimes. This study highlights the continuing uncertainty in both low and high cloud feedbacks simulated by GCMs. 相似文献
15.
Qian Yongfu 《大气科学进展》1988,5(2):159-170
In numerical weather prediction (NWP), the accuracy of vertical interpolation of the initial data is a problem which is greatly concerned by people. In this paper, we specify vertical distributions of the temperature and the geopotential height fields and examine three interpolation methods, i.e. the Lagrangian polynomial inter-polation method (hereafter abbreviated to LP method), the linear interpolation method (LN method) and the local spline interpolation method (LS method) proposed by the author. The examination shows that when the vertical resolution of the initial data is high enough, for example, the number of the given data levels N is 10 or more, all the three methods get good accuracy of interpolation, especially, the LP and the LS methods have very little errors almost tending to zero, while the LN method has a little larger errors than the two formers and the errors at various levels have the same sign. When N is reduced to 5, the LP and the LS methods still have quite good accuracy and similar error distributions, while the LN method has less accuracy. If the geopo-tential height field needs to be adjusted in order to satisfy the hydrostatic equilibrium with the temperature field which is assumed fixed, then the LS method has minimum errors. The examination also indicates that the vertical resolution with at least 5 levels of initial data can keep the interpolation accuracy. Otherwise the accuracy will not be guaranteed no matter which method is used.It is also pointed out in this paper that the temperature and the geopotential height fields can be given inde-pendently in numerical prediction models in order to keep higher interpolation accuracy. However, the hydro-static equation should be finite differenced in other way which is somewhat different from the conventional one. In other words, the time dependent difference form of the equation should be used, so that the initial interpola-tion accuracy could have influence on the time integration. 相似文献
16.
We examine the performance of two steady-state models, a numerical solution of the advection-diffusion equation and the Gaussian
plume-model-based AERMOD (the American Meteorological Society/Environmental Protection Agency Regulatory Model), to predict
dispersion for surface releases under low wind-speed conditions. A comparison of model estimates with observations from two
tracer studies, the Prairie Grass experiment and the Idaho Falls experiment indicates that about 50% of the concentration
estimates are within a factor of two of the observations, but the scatter is large: the 95% confidence interval of the ratio
of the observed to estimated concentrations is about 4. The model based on the numerical solution of the diffusion equation
in combination with the model of Eckman (1994, Atmos Environ 28:265–272) for horizontal spread performs better than AERMOD in explaining the observations. Accounting for
meandering of the wind reduces some of the overestimation of concentrations at low wind speeds. The results deteriorate when
routine one-level observations are used to construct model inputs. An empirical modification to the similarity estimate of
the surface friction velocity reduces the underestimation at low wind speeds. 相似文献
17.
A Lagrangian stochastic (LS) micromixing model is used for estimating concentration fluctuations in plumes of a passive, non-reactive
tracer dispersing from elevated and ground-level compact sources into a neutral wall shear-layer flow. SPMMM (for sequential
particle micromixing model) implements the familiar IECM (interaction by exchange with the conditional mean) micromixing scheme.
The parametrization of the scalar micromixing time scale is identical to that proposed in a previously reported LS–IECM model
(Cassiani et al., Atmos Environ 39:1457–1469, 2005a). However, while SPMMM is mathematically equivalent to the previously reported model, it differs in its numerical implementation:
SPMMM releases N independent particles sequentially, whereas the previously reported model releases N independent particles simultaneously. In both implementations, the trajectories of the N particles are governed by single-point velocity statistics. The sequential particle implementation is computationally efficient,
but cannot be applied to the case of reacting species. Results from both implementations are compared to experimental wind-tunnel
dispersion data and to each other. 相似文献
18.
Among well-mixed multi-dimensional Lagrangian stochastic (LS) dispersion models, we observe that those in poorest agreement with observations produce spiralling trajectories, with an associated reduction in dispersion. We therefore investigate statistics of increments d ' to the orientation '= arctan(W'/U') of the Lagrangian velocity-fluctuation vector – as a possible means to distinguish the better LS models within the well-mixed class. Zero-spin models, having d' = 0, are found to provide best agreement with observations. It is not clear however, whether imposition of the zero-spin property selects (in conjunction with the well-mixed condition) a unique model. 相似文献
19.
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
20.
A theoretical requirement of the Interaction by Exchange with the Conditional Mean (IECM) micromixing model is that the mean concentration field produced by it must be consistent with the mean concentration field produced by a traditional Lagrangian stochastic (LS) marked particle model. We examine the violation of this requirement that occurs in a coupled LS–IECM model when unrealistically high particle velocities occur. No successful strategy was found to mitigate the effects of these rogue trajectories. It is our hope that this work will provide renewed impetus for investigation into rogue trajectories and methods to eliminate them from LS models. 相似文献