Computational Fluid Dynamical Modelling of Concentration Fluctuations in an Idealized Urban Area   总被引：2，自引：2，他引：0  

Maya Milliez  Bertrand Carissimo 《Boundary-Layer Meteorology》2008,127(2):241-259

Turbulent mixing induces variability in concentration that is important in many applications, such as reactive plumes, risk assessments or odour impact analyses (when the effects can have time scales on the order of a second). In urban canopies, the variability may be modified by the presence of buildings. Our purpose is to study concentration fluctuation variance in built-up areas using an Eulerian approach. We performed numerical simulations with the computational fluid dynamics model Mercure_Saturne, which is a three-dimensional model adapted to atmospheric flow and pollutant dispersion. We use a k − ϵ turbulence closure and predict the concentration variance with a transport equation model. The model performance is evaluated with the near-full scale experiment MUST (Mock Urban Setting Test), a field experiment conducted in Utah’s West Desert Test Center. The modelled root-mean-square of the concentration fluctuations is compared to measurements for 20 of the MUST trials. The model shows good agreement with the measurements, with the fraction of predictions within a factor of two of observations of 60.1%, with better results for horizontal lines of detectors than for the detectors on vertical masts (with fractions of predictions within a factor of two of observations of respectively 66.4% and 52.6%). The influence of different parameters on the fluctuation variance is also studied and we show the importance of taking into account the stability of the stratification when modelling the turbulent kinetic energy.  相似文献   

Large-eddy Simulations of Flow Over Forested Ridges   总被引：4，自引：4，他引：0  

Andrew N. Ross 《Boundary-Layer Meteorology》2008,128(1):59-76

Large-eddy simulations (LES) of flow over a series of small forested ridges are performed, and compared with numerical simulations using a one-and-a-half order mixing length closure scheme. The qualitative and quantitative similarity between these results provides some confidence in the results of recent analytical and numerical studies of flow over forested hills using first-order mixing length schemes. Time series of model velocities at various locations within the canopy allow the application of various experimental techniques to study the turbulence in the LES. The application of conditional analysis shows that the structure of the turbulence over a forested hill is broadly similar to that over flat ground, with sweeps and ejections dominating. Differences are seen across the hill, particularly associated with regions of mean flow separation and recirculation near the summit and in the lee of the hill. Detailed comparison of derived mixing lengths from the LES with the assumed values used in mixing-length closure schemes show that the mixing length varies with location across the hill and with height in the canopy. This is consistent with previous wind-tunnel measurements, and demonstrates that a constant mixing-length assumption is not strictly valid within the canopy. Despite this, the first-order mixing-length schemes do give similar results both for the mean flow and the turbulence in such situations.  相似文献   

陆面模拟中植被辐射传输参数化方案研究   总被引：2，自引：0，他引：2  

周文艳  郭品文  罗勇  KuoNan Liou  Gu Yu  Xue Yongkang 《气象学报》2008,66(3):359-370

在冠层二流辐射传输模式基础上新发展了一个描述太阳短波辐射在植被中传输的冠层四流辐射传输模式.冠层四流辐射传输模式是在大气辐射传输理论的基础上得到一组描述短波辐射在植被中传输过程的冠层辐射传输基本方程,引进大气中求解辐射传输方程的四流近似解法,并求得冠层四流辐射传输方程的解析解.方程中各项参量能够反映叶子或冠层特殊的几何和光学特征.冠层向上、向下辐射通量取决于冠层散射相函数、叶子在入射光方向投影面积、单个叶子反射率和透射率、叶面积指数以及直射光入射太阳高度角等.四流模式计算叶子水平倾角时对太阳短波辐射的反照率,与二流模式结果比较可以验证模式的理论推导和建模都是正确的:计算结果的比较,表明四流模式在水平叶角分布时计算的冠层反照率与二流模式结果一致,同时直射光从任何太阳高度角入射的冠层反照率结果也一致,从而证明发展的冠层四流辐射传输模式是成功的.模拟试验中将两种模型同时耦合到同一个陆面过程模式中进行比较试验,结果表明,冠层四流辐射传输模式能够得到更精确的植被反照率,从而使得陆面模式计算的地表吸收的净太阳辐射通量更接近于观测值.  相似文献   

应用城市冠层模式研究建筑物形态对城市边界层的影响   总被引：5，自引：1，他引：4  

周荣卫  蒋维楣  何晓凤  刘罡 《气象学报》2008,66(4):489-499

文中将城市冠层模式耦合到南京大学城市尺度边界层模式中,通过模拟对比发现,耦合模式对城市地区气温模拟结果更接近于观测值,尤其是对城市地区夜间气温模拟的改进.运用改进耦合模式通过多个敏感性试验的模拟,从城市面积扩张、建筑物高度增加、建筑物分布密度变化等角度研究城市建筑物三维几何形态变化对城市边界层及城市气象环境的影响,试验结果表明:(1)城市面积扩张使得城市下垫面的热通量增大,热力湍流活动增强,动量通量输送增强,城市湍能增大,湍流扩散系数变大,城市气温升高,且对不同时刻城市区域大气层结稳定度均有不同程度的影响.(2)建筑物高度增加增大了城市下垫面的粗糙度和零平面位移.同时也增大了城市街渠高宽比.城市建筑物越高,白天城市地区地表热通量越小,城市上空大气温度越低,平均风速减小,湍能减小;夜间由于高大建筑物释放储热比低矮建筑物要多,其热力湍流相对活跃,地表热通量增大,使得城市区域气温较高.(3)建筑物密度增大,会减小城市下垫面的粗糙度同时增强街渠对辐射的影响.建筑物密度增大在白天会减小地表热通量和动量通量,使城市气温降低,平均风速增大,城市湍流活动能力减弱;夜间城市释放较多储热使得气温较高.  相似文献   

成都市精细化数值模式研究及试验     

LU Ping  CHEN Zhang 《四川气象》2008,(4)

本文在引入成都市精细化下垫面资料后将G rapes与城市冠层方案相耦合,并在此基础上进行数值试验。初步试验结果表明:该耦合模式能基本表现城市冠层的特征的作用,即能够较好地模拟出城市热岛效应。城市冠层参数化方案及精细化下垫面资料的引入能够影响陆气之间的相互作用,改变大气的能量平衡,致使向上长波辐射增强,感热通量增加,潜热通量减少。城市化效应白天对边界层大气的增温作用还会影响低层大气的层结结构,使之在午后产生弱的上升运动。  相似文献   

Estimation of canopy drying time after rainfall using sap flow measurements in an emergent tree in a lowland mixed‐dipterocarp forest in Sarawak,Malaysia     

Tomonori Kume  Koichiro Kuraji  Natsuko Yoshifuji  Toshiyuki Morooka  Shinji Sawano  Lucy Chong  Masakazu Suzuki 《水文研究》2006,20(3):565-578

This study emphasizes the importance of canopy drying time (CDT) after rainfall in a lowland tropical rain forest. In this study, we estimate CDT using sap flow velocities measured by a heat‐pulse method in an emergent tree in a lowland mixed‐dipterocarp forest. Estimated CDT (ECDT) for each rain event has been defined as the time from rainfall cessation to the specific time derived from the difference between diurnal courses of sap flow velocities on a rainy day versus bright days. ECDT could be derived for 22 rain events that were grouped into two types, depending on whether rainfall ceased before or after noon. The ECDTs were distributed more widely and with greater values when rainfall ceased before noon (Type 1) than after noon (Type 2). The ECDTs of both Type 1 and Type 2 decreased with increases in net radiation (Rn) and vapour pressure deficit (VPD) after rainfall. This result shows that ECDT is determined mainly by post‐rainfall evaporation rates. The sap flow velocity as a detector of canopy wetness worked out well because of the specific rainfall characteristics at this site. The practical limitations of the method using sap flow velocities are discussed in relation to rainfall characteristics and time lags between transpirations and sap flow velocities. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   

A NEW DRY DEPOSITION VELOCITY PATTERN AND ITS PRACTICAL APPLICATION IN HIGH RESOLUTION REGIONAL ACID DEPOSITION MODEL^*     

Lei Xiao''en 《Acta Meteorologica Sinica》1996,10(1):118-128

A new dry deposition velocity pattern (NDDVP) for the study of region-scale dry deposition processes is developed. The mean ratio between NDDVP and 1022 experimental data of dry deposition velocity V_d is 1.06±0.82. The result shows that NDDVP is well consistent with experimental data. Practical cases are forecasted by the high resolution regional acid deposition model (EM3) with both NDDVP and old V_d pattern. The maximum ratio between the central concentrations for SO₄^- can reach 2.4 only due to different V_d patterns. 3-D distributions of species concentrations and dry depositions forecasted by NDDVP are better than those by the old V_d pattern.  相似文献   

A NEW DRY DEPOSITION VELOCITY PATTERN AND ITS PRACTICAL APPLICATION IN HIGH RESOLUTION REGIONAL ACID DEPOSITION MODEL   总被引：4，自引：0，他引：4       下载免费PDF全文

雷孝恩 《Acta Meteorologica Sinica》1996,(1)

A new dry deposition velocity pattern (NDDVP) for the study of region-scale dry depositionprocesses is developed. The mean ratio between NDDVP and 1022 experimental data of dry deposi-tion velocity V_d is 1. 06±0.82. The result shows that NDDVP is well consistent with experimentaldata. Practical cases are forecasted by the high resolution regional acid deposition model (EM3)with both NDDVP and old V_d pattern. The maximum ratio between the central concentrations forSO4 can reach 2.4 only due to different V_d patterns. 3-D distributions of species concentrations anddry depositions forecasted by NDDVP are better than those by the old V_d pattern.  相似文献   

植物冠层内物质交换特征的实验研究     

雷孝恩  田瑞明  韩志伟  邓玉珍  袁素珍  李勤  骆启仁  李真 《大气科学》1996,20(1):28-36

使用湍流和SO2通量梯度测试资料，对植被冠层内物质交换特征进行了详细研究，揭示了一些有意义的结果: 植物的生物过程对物质的吸收作用非常明显； 在冠层内物质通量是随冠层深度加大而明显减小；对高的植被，上层沉积速度(Vgu)大于下层 (Vgd)； 冠层内的沉积速度(Vg) 表现出明显的日变化，冠层内物质的生物吸收作用与太阳总辐射量有直接联系；冠层的Vg与速度尺度V＊和平均风成正变关系；森林的Vg比麦地小；新的Vg理论公式能更好地预测重庆森林和麦地的结果。  相似文献   

Fractal Characteristics of Population Canopy Structure of the Mangrove, Bruguiera gymnorrhiza （L.） Lamk     

LiangShichu WangBosun 《海洋通报(英文版)》2003,5(2):54-61

The fractal characteristics of the canopy structure o! B. gymnorrhiza population are investigated by fractal dimension analysis in the National Shankou Mangrove Nature Reserve. The 3-year-old branches have box dimensions between 1.22 and 1.55, showing thecomplexity degree of branching structure and the ability of occupying and utilizing ecological space. It may be considered that fractal dimension provides a useful index for the study of light utilization efficiencies and growth processes of B. gymnorrhiza. Calculated by using the two-surface method, the fractal dimensions for the crown pattern of individuals with ages of 20 to 50 years range from 2.21 to 2.54, indicating that the filling degree of foliage to a tree crown is relatively low and B. gymnorrhiza has the property of a sun plant.Along with the increase of ages of individuals, the filling degree of foliage to a tree crown changes from high to low, and so does the fractal dimeilsion. The box dimensions obtained from the grayscale curves of population canopy are between 1.47 and 1.61. The greater the box dimension, the more loosely organized the canopy spatial structure, and the more the light spots. The canopy structural information and complexity of a population can be effectively captured by box dimensions obtained from canopy grayscale curves.  相似文献