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1.
Using the new high-frequency measurement equipment of the research aircraft DO 128, which is described in detail, turbulent vertical fluxes of ozone and nitric oxide have been calculated from data sampled during the ESCOMPTE program in the south of France. Based on airborne turbulence measurements, radiosonde data and surface energy balance measurements, the convective boundary layer (CBL) is examined under two different aspects. The analysis covers boundary-layer convection with respect to (i) the control of CBL depth by surface heating and synoptic scale influences, and (ii) the structure of convective plumes and their vertical transport of ozone and nitric oxides. The orographic structure of the terrain causes significant differences between planetary boundary layer (PBL) heights, which are found to exceed those of terrain height variations on average. A comparison of boundary-layer flux profiles as well as mean quantities over flat and complex terrain and also under different pollution situations and weather conditions shows relationships between vertical gradients and corresponding turbulent fluxes. Generally, NOx transports are directed upward independent of the terrain, since primary emission sources are located near the ground. For ozone, negative fluxes are common in the lower CBL in accordance with the deposition of O3 at the surface.The detailed structure of thermals, which largely carry out vertical transports in the boundary layer, are examined with a conditional sampling technique. Updrafts mostly contain warm, moist and NOx loaded air, while the ozone transport by thermals alternates with the background ozone gradient. Evidence for handover processes of trace gases to the free atmosphere can be found in the case of existing gradients across the boundary-layer top. An analysis of the size of eddies suggests the possibility of some influence of the heterogeneous terrain in mountainous area on the length scales of eddies. 相似文献
2.
We present a Lagrangian stochastic model of vertical dispersion in the convective boundary layer (CBL). This model is based on a generalized Langevin equation that uses the simplifying assumption that the skewed vertical velocity probability distribution is spatially homogeneous. This approach has been shown to account for two key properties of CBL turbulence associated with large-scale coherent turbulent structures: skewed vertical velocity distributions and long velocity correlation time. A 'linear-skewed' form of the generalized Langevin equation is used, which has a linear (in velocity) deterministic acceleration and a skewed random acceleration. 'Reflection' boundary conditions for selecting a new velocity for a particle that encounters a boundary were investigated, including alternatives to the standard assumption that the magnitudes of the particle incident and reflected velocities are positively correlated. Model simulations were tested using cases for which exact, analytic statistical properties of particle velocity and position are known, i.e., well-mixed spatial and velocity distributions. Simulations of laboratory experiments of CBL dispersion show that (1) the homogeneous linear-skewed Langevin equation model (as well as an alternative 'nonlinear-Gaussian' Langevin equation model) can simulate the important aspects of dispersion in the CBL, and (2) a negatively-correlated-speed reflection boundary condition simulates the observed dispersion of material near the surface in the CBL significantly better than alternative reflection boundary conditions. The homogeneous linear-skewed Langevin equation model has the advantage that it is computationally more efficient than the homogeneous nonlinear-Gaussian Langevin equation model, and considerably more efficient than inhomogeneous Langevin equation models. 相似文献
3.
对流边界层（ＣＢＬ）中的污染扩散是非高斯型的。本文在下列三个假设下建立了双高斯型几率分布函数（ＰＤＦ）模式：１．对流边界层任一确定高度铅直速度Ｗ的几率分布函数ｐｗ由两个高斯分布迭加而成；２．从污染源释放的粒子具有源高的铅直速度几率分布，且其轨迹是线性的；３．粒子在地面的反射为全反射，在混合层高度Ｚｉ为全反射或有部分吸收。然后分析了三个高度上铅直速度Ｗ的一些统计特征量，比较了由ＰＤＦ模式计算的横风向积分浓度和Ｌａｍｂ的数值模拟［１－３］，Ｄｅａｒｄｏｒｆｆ的水槽模拟［４－６］结果，并用美国ＣＯＮＤＯＲＳ计划的外场试验资料［７］对ＰＤＦ模式进行了验证，结果均相当一致。 相似文献
4.
A Large-Eddy Simulation and Lagrangian Stochastic Study of Heavy Particle Dispersion in the Convective Boundary Layer 总被引：4，自引：1，他引：4
Large-eddy simulation and Lagrangian stochastic dispersion models were used to study heavy particle dispersion in the convective boundary layer (CBL). The effects of various geostrophic winds, particle diameters, and subgrid-scale (SGS) turbulence were investigated. Results showed an obvious depression in the vertical dispersion of heavy particles in the CBL and major vertical stratification in the distribution of particle concentrations, relative to the passive dispersion. Stronger geostrophic winds tended to increase the dispersion of heavy particles in the lower CBL. The SGS turbulence, particularly near the surface, markedly influenced the dispersion of heavy particles in the CBL. For reference, simulations using passive particles were also conducted; these simulation results agreed well with results from previous convective tank experiments and numerical simulations. 相似文献
5.
Spectral Structure of Small-Scale Turbulent and Mesoscale Fluxes in the Atmospheric Boundary Layer over a Thermally Inhomogeneous Land Surface 总被引：1，自引：0，他引：1
Spectral analysis was performed on aircraft observations of a convective boundary layer (CBL) that developed over a thermally
inhomogeneous, well-marked mesoscale land surface. The observations, part of the GAME-Siberia experiment, were recorded between
April and June 2000 over the Lena River near Yakutsk City. A special integral parameter termed the ‘reduced depth of the CBL’
was used to scale the height of the mixed layer with variable depth. Analysis of wavelet cospectra and spectra facilitated
the separation of fluxes and other variables into small-scale turbulent fluctuations (with scales less than the reduced depth
of the CBL, approximately 2 km) and mesoscale fluctuations (up to 20 km). This separation approach allows for independent
exploration of the scales. Analyses showed that vertical distributions obeyed different laws for small-scale fluxes and mesoscale
fluxes (of sensible heat, water vapour, momentum and carbon dioxide) and for other variables (wind speed and air temperature
fluctuations, coherence and degree of anisotropy). Vertical profiles of small-scale turbulent fluxes showed a strong decay
that differed from generally accepted similarity models for the CBL. Vertical profiles of mesoscale fluxes and other variables
clearly showed sharp inflections at the same relative (with respect to the reduced depth of the CBL) height of approximately
0.55 in the CBL. Conventional similarity models for sensible heat fluxes describe both small-scale turbulent and mesoscale
flows. The present results suggest that mesoscale motions that reach up to the relative level of 0.55 could be initiated by
thermal surface heterogeneity. Entrainment between the upper part of the CBL and the free atmosphere may cause mesoscale motions
in that region of the CBL. 相似文献
6.
Summary This work presents a numerical study of non-reactive pollutant dispersion in sea breeze conditions. Sea breeze circulation is investigated using a 3-D mesoscale meteorological model. Simulation was conducted for the area of Tarragona (Spain) which has an important petrochemical industry in the coastal region and complex terrain. Results from the meteorological model were used as input to a Lagrangian particle model in order to analyze the pollutant dispersion of an elevated plume emitting near the shoreline. The simulation was performed for 24 h and an analysis of the meteorological and concentration fields was untertaken for this time period. The results are compared with measured surface data. Good correlation exists between observed and simulated conditions indicating that the coupling of the meteorological and particle models provides a good tool for analyzing air pollution in complex situations.With 13 Figures 相似文献
7.
A Lagrangian stochastic (LS) model, which is embedded into a parallelised large-eddy simulation (LES) model, is used for dispersion
and footprint evaluations. For the first time an online coupling between LES and LS models is applied. The new model reproduces
concentration patterns, which were obtained in prior studies, provided that subgrid-scale turbulence is included in the LS
model. Comparisons with prior studies show that the model evaluates footprints successfully. Streamwise dispersion leads to
footprint maxima that are situated less far upstream than previously reported. Negative flux footprints are detected in the
convective boundary layer (CBL). The wide range of applicability of the model is shown by applying it under neutral and stable
stratification. It is pointed out that the turning of the wind direction with height leads to a considerable dependency of
source areas on height. First results of an application to a heterogeneously heated CBL are presented, which emphasize that
footprints are severely affected by the inhomogeneity. 相似文献
8.
Abstract The wind climate of the mountainous terrain in the southern Yukon is simulated using the Wind Energy Simulation Toolkit (WEST) developed by the Recherche en Prévision Numérique (RPN) group of Environment Canada and is compared to measurements in the field. WEST combines two models that operate at different spatial scales. The Mesoscale Compressible Community (MC2) model is a mesoscale numerical weather prediction model that produces simulations over large domains of the order of a thousand kilometres. The MC2 model uses long‐term synoptic scale wind climate data from the analysis of radiosonde and other observations to simulate mean wind fields at tens of metres above the ground using a horizontal resolution of a few kilometres. The mesoscale results are used as input to MS‐Micro/3 (Mason and Sykes (1979) version of the Jackson and Hunt (1975) model version for microcomputers/3‐dimensional; MS‐Micro hereafter), a more computer‐efficient, microscale model with simpler linearized momentum equations and a domain restricted to a few tens of kilometres with horizontal grid sizes of tens or hundreds of metres. MS‐Micro provides wind field results at specific wind generator hub heights (typically 30 to 50 m above ground level (AGL)) which are of interest to researchers and developers of wind farms. WEST shows relatively strong correlations between its simulated long‐term mean wind speed and the measurements from ten wind energy monitoring stations. However, in the mountainous terrain of the Yukon, WEST tends to predict wind speeds which are about 40% too high. The model also produces erroneous wind directions and some were perpendicular to valley orientations. The most likely cause of the wind speed and direction errors is the substantially modified 5‐km grid‐spaced mesoscale terrain used in MC2. The WEST simulation was also found to double the wind speeds observed at airport stations and there was poor correlation between the simulated and observed wind speeds. The bias in the model could be attributed to a number of factors, including the use of smoothed topography by the model, the discrepancy between the neutral atmosphere assumed in MS‐Micro and the normally observed stable atmosphere, the application of MS‐Micro to every third grid point of the MC2 output, abnormally high sea level wind speeds in the input climate data for MC2, and a certain degree of disagreement between the land surface characteristics used in the model and those found in the field. At comparatively low computer cost, WEST predicts a wind climate map that compares favourably to the wind measurements made in several locations in the Yukon. However, the problem of the modified terrain in the mountainous regions is the most pressing problem and needs to be addressed before WEST is used in the mountainous regions of Canada. 相似文献
9.
Observations of the Relation Between Upslope Flows and the Convective Boundary Layer in Steep Terrain 总被引：1，自引：1，他引：0
Slope flow mechanisms are crucial for the transport of air pollutants in complex terrain. Previous observations in sloping terrain showed upslope flows filling the entire convective boundary layer (CBL) and reducing air pollution concentrations by venting air pollutants out of the CBL into the free atmosphere. During the Pacific 2001 Air Quality Field Study in the Lower Fraser Valley, British Columbia, Canada, we observed slope flows during weak synoptic winds, clear skies, and strong daytime solar heating. With a Doppler sodar we measured the three wind components at the foot of a slope having an average angle of 19° and a ridge height of 780 m. We operated a scanning lidar system and a tethersonde at a nearby site on the adjacent plain to measure backscatter of particulate matter, temperature, wind speed, wind direction, and specific humidity. Strong daytime upslope flows of up to 6 m s−1 through a depth of up to 500 m occurred in the lower CBL, but with often equally strong and deep return flows in the upper part of the CBL. The mass transport of upslope flow and return flow approximately balanced over a 4-h morning period, suggesting a closed slope-flow circulation within the CBL. These observations showed that air pollutants can remain trapped within a CBL rather than being vented from the CBL into the free atmosphere. 相似文献
10.
本文运用随机游动模拟方法改进建立一种粒子—烟团模式，成功地模拟了对流边界层条件下污染物扩散，粒子—烟团模式的模拟结果与水槽试验的结果吻合较好。数值模拟试验结果表明粒子—烟团模式能在施放粒子数较少，从而运行时间很短的情况下，得到优于一般随机游动模式的模拟性能。用KNRC的资料对模式的模拟性能作一验证，表明模式也能很好地模拟实际大气中不同稳定度情况下的扩散。 相似文献
11.
四川地形复杂多样,暴雨频发,常诱发山洪、泥石流等灾害。在利用中尺度模式对复杂地形区域的暴雨进行研究时,模式水平分辨率的选取缺乏定量依据。为了揭示四川地形的复杂特征和给中尺度模式水平网格分辨率的选取提供定量依据,利用二维离散余弦变换,对四川地形高度场和暴雨分布场进行谱分解,根据暴雨分布特征分区讨论了四川盆地地形特征,同时利用地形谱方差和数值试验定量讨论了数值模式水平分辨率的选取问题,得到主要结论有:(1)二维离散余弦变换能较好地表现出研究区域各向异性的复杂特征;(2)雅安地区和四川盆地西北部的地形谱与降水谱有较好的同相关系,盆地东北部和盆地中部的地形谱与降水谱在波长较大处出现反相关系;(3)针对某个区域的地形特征,可以通过计算模式能分辨的地形方差与总地形方差的比值来确定合适的中尺度模式水平网格分辨率。 相似文献
12.
S. O. Krichak P. Alpert T. N. Krishnamurti 《Meteorology and Atmospheric Physics》1997,63(3-4):159-169
Summary A case of development of a meridionally oriented Red Sea Trough (RST) system and its intensification over the Eastern Mediterranean (EM) region during the ALPEX1982 3–5 March period, is investigated. The MM4 mesoscale model of Penn State University/National Center for Atmospheric Research was first applied for a large scale investigation of the processes. The relative roles of the different acting factors, i.e., terrain, latent heat release and the surface fluxes were calculated employing the factor separation method. Topography and sensible heat flux were found to be the dominant ones.The high resolution non-hydrostatic RAMS 3a model of Colorado State University with nested grids of 100 and 20 km illustrated the finer details of the cyclogenetic processes in the mountainous area of the Abyssinean Highlands, Ethiopia, and the Arabian peninsula, where initiation of the trough took place.Results of the factor separation showed that the topography blocking acted as a cyclolytic factor, preventing the process of the northward trough propagation. The situation changed only after about 30 h of the simulation, when the trough already propagated into the EM area after intensification of the mid-tropospheric westerlies over the central part of the Red Sea area. Starting from this time, terrain was acting as one of two major cyclogenetic factors. The second local effect also working as a cyclogenetic one was the sensible heat flux. Its role was especially important after 36 h of the simulations when strong winds over the sea area caused more active heat transfer from the sea surface to the atmosphere.With 7 Figures 相似文献
13.
A three-level model system for the prediction of local flows in mountainous terrain is described. The system is based upon
an operational weather prediction model with a horizontal grid spacing of about 10 km. The large-scale flow is transformed
to a more detailed terrain, first by a mesoscale model with grid spacing of about 1 km, and then by a local-scale model with
a grid spacing of about 0.2 km. The weather prediction model is hydrostatic, while the two other models are non-hydrostatic.
As a case study the model system has been applied to estimate wind and turbulence over Várnes airport, Norway, where data
on turbulent flight conditions were provided near the runway. The actual case was chosen due to previous experiences, which
indicate that south-easterly winds may cause severe turbulence in a region close to the airport. Local terrain induced turbulence
seems to be the main reason for these effects. The predicted local flow in the actual region is characterized by narrow secondary
vortices along the flow, and large turbulent intensity associated with these vortices. A similar pattern is indicated by the
sparse observations, although there seems to be a difference in mean wind direction between data and predictions. Due to fairly
coarse data for sea surface temperature, errors could be induced in the turbulence damping via the Richardson number. An adjustment
for this data problem improved the predictions. 相似文献
14.
西北干旱区对流边界层发展的热力机制模拟研究 总被引：4，自引：0，他引：4
观测发现,西北干旱区对流边界层高度在夏、冬两季极大值分别达到4 km和800 m.利用热力学方法对西北干旱地区深厚对流边界层的原因进行了定量分析.用建立的热力数值模型模拟表明,平均而言,夏季和冬季对流边界层高度的51.5％和61.4％分别由感热产生,对于夏、冬季对流边界层高度可分别达到4 km和800 m的极大值,感热贡献更大,为63.4％和73.1％,感热对对流边界层的贡献率冬季明显大于夏季;夏、冬季浮力夹卷对对流边界层的贡献是18.1％和9.4％;包括感热和浮力夹卷的热力因素可以解释夏、冬季对流边界层高度的69.6％和70.8％,而对于对流边界层极大值,则其解释可高达81％和84.6％.热力因素是西北干旱区对流边界层产生的主要原因,其中,感热又是最根本原因. 相似文献
15.
The Influence of Thermally-Induced Mesoscale Circulations on Turbulence Statistics Over an Idealized Urban Area Under a Zero Background Wind 总被引：2，自引：2，他引：0
Weiguo Wang 《Boundary-Layer Meteorology》2009,131(3):403-423
The influence of mesoscale circulations induced by urban-rural differential surface sensible heat flux and roughness on convective
boundary-layer (CBL) flow statistics over an isolated urban area has been examined using large-eddy simulation (LES). Results
are analyzed when the circulations influence the entire urban area under a zero background wind. For comparison, the CBL flow
over an infinite urban area with identical urban surface characteristics under the same background meteorological conditions
is generated as a control case (without circulations). The turbulent flow over the isolated urban area exhibits a mix of streaky
structure and cellular pattern, while the cellular pattern dominates in the control case. The mixed-layer height varies significantly
over the isolated urban area, and can be lower near the edge of the urban area than over the rural area. The vertical profiles
of turbulence statistics over the isolated urban area vary horizontally and are dramatically different from the control case.
The turbulent kinetic energy (TKE) sources include wind shear, convergence, and buoyancy productions, compared to only buoyancy
production in the control case. The normalized vertical velocity variance is reduced compared to the control case except in
the central urban area where it is little affected. The low-level flow convergence is mainly responsible for the enhanced
horizontal velocity variance in the central urban area, while wind shear is responsible for the additional local maximum of
the horizontal velocity variance near the middle of the CBL outside the central area. Parameterizations in the prognostic
equation for TKE used in mesoscale models are evaluated against the LES results over the isolated urban area. We also discuss
conditions under which the urban-induced circulations occur and when they may affect the entire urban area. Given that urban-induced
circulations can influence the entire urban area within hours for an urban area of a realistic size, it is inappropriate to
directly apply empirical relations of turbulence statistics derived under horizontally-homogenous flow conditions to an urban
area. 相似文献
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Terrain characteristics can be accurately represented in spectrum space. Terrain spectra can quantitatively reflect the effect of topographic dynamic forcing on the atmosphere. In wavelength space, topographic spectral energy decreases with decreasing wavelength, in spite of several departures. This relationship is approximated by an exponential function. A power law relationship between the terrain height spectra and wavelength is fitted by the least-squares method, and the fitting slope is associated with grid-size selection for mesoscale models. The monotonicity of grid size is investigated, and it is strictly proved that grid size increases with increasing fitting exponent, indicating that the universal grid size is determined by the minimum fitting exponent. An example of landslide-prone areas in western Sichuan is given, and the universal grid spacing of 4.1 km is shown to be a requirement to resolve 90% of terrain height variance for mesoscale models, without resorting to the parameterization of subgrid-scale terrain variance. Comparison among results of different simulations shows that the simulations estimate the observed precipitation well when using a resolution of 4.1 km or finer. Although the main flow patterns are similar, finer grids produce more complex patterns that show divergence zones, convergence zones and vortices.Horizontal grid size significantly affects the vertical structure of the convective boundary layer. Stronger vertical wind components are simulated for finer grid resolutions. In particular, noticeable sinking airflows over mountains are captured for those model configurations. 相似文献
20.
An Urban Surface Exchange Parameterisation for Mesoscale Models 总被引：9，自引：11，他引：9
A scheme to represent the impact of urban buildings on airflow in mesoscale atmospheric models is presented. In the scheme, the buildings are not explicitly resolved, but their effects on the grid-averaged variables are parameterised. An urban quarter is characterised by a horizontal building size, a street canyon width and a building density as a function of height. The module computes the impact of the horizontal (roof and canyon floor) and vertical (walls) surfaces on the wind speed, temperature and turbulent kinetic energy. The computation of the shortwave and longwave radiation, needed to compute the temperature of the urban surfaces, takes into account the shadowing and radiation trapping effects induced by the urban canyons. The computation of the turbulent length scales in the TKE equation is also modified to take into account the presence of the buildings.The parameterisation is introduced into a mesoscale model and tested in a bidimensional case of a city over flat terrain. The new parameterisation is shown to be able to reproduce the most important features observed in urban areas better than the traditional approach which is based only on the modification of the roughness length, thereby retaining the Monin–Obukhov similarity theory. The new surface exchange parameterisation is furthermore shown to have a strong impact on the dispersion characteristics of air pollutants in urban areas. 相似文献