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1.
The mean flow within inhomogeneous urban areas is investigated using an urban canopy model. The urban canopy model provides a conceptual and computational tool for representing urban areas in a way suitable for parameterisation within numerical weather prediction and urban air quality models. Average aerodynamic properties of groups of buildings on a neighbourhood scale can be obtained in terms of the geometry and layout of the buildings. These canopy parameters then determine the spatially averaged mean wind speeds within the canopy as a whole. Using morphological data for real cities, computations are performed for representative sections of cities. Simulations are performed to study transitions between different urban neighbourhoods, such as residential areas and city centres. Such transitions are accompanied by changes in mean building density and building height. These are considered first in isolation, then in combination, and the generic effects of each type of change are identified. The simulation of winds through a selection of downtown Los Angeles is considered as an example. An increase in canopy density is usually associated with a decrease in the mean wind speed. The largest difference between mean winds in canopies of different densities occurs near ground level. Winds generally decrease upon encountering a taller canopy of the same density, but this effect may be reversed very near the ground, with possible speed-ups if the canopy is especially tall. In the vicinity of a transition there is an overshoot in the mean wind speed in the bottom part of the canopy. Mechanisms for these effects are discussed.  相似文献   

2.
Accurate simulation of air quality at neighbourhood scales (on order of 1-km horizontal grid spacing) requires detailed meteorological fields inside the roughness sub-layer (RSL). Since the assumptions of the roughness approach, used by most of the mesoscale models, are unsatisfactory at this scale, a detailed urban and rural canopy parameterisation, called DA-SM2-U, is developed inside the Penn State/NCAR Mesoscale Model (MM5) to simulate the meteorological fields within and above the urban and rural canopies. DA-SM2-U uses the drag-force approach to represent the dynamic and turbulent effects of the buildings and vegetation, and a modified version of the soil model SM2-U, called SM2-U(3D), to represent the thermodynamic effects of the canopy elements. The turbulence length scale is also modified inside the canopies. SM2-U(3D) assesses the sensible and latent heat fluxes from rural and urban surfaces in each of the computational layers inside the canopies by considering the shadowing effect, the radiative trapping by the street canyons, and the storage heat flux by the artificial surfaces. DA-SM2-U is tested during one simulated day above the city of Philadelphia, U.S.A. It is shown that DA-SM2-U is capable of simulating the important features observed in the urban and rural RSL, as seen in the vertical profiles of the shear stress, turbulent kinetic energy budget components, eddy diffusivity, potential air temperature, and specific humidity. Within the canopies, DA-SM2-U simulates the decrease of the wind speed inside the dense canopies, the skirting of the flow around the canopy blocks, warmer air inside the vegetation canopy than above open areas during the night and conversely during the day, and constantly warmer air inside the urban canopy. The comparison with measurements shows that the surface air temperature above rural and urban areas is better simulated by DA-SM2-U than by the `standard version' of MM5.  相似文献   

3.
This paper deals with the modelling of the flow in the urban canopy layer. It critically reviews a well-known formula for the spatially-averaged wind profile, originally proposed by Cionco in 1965, and provides a new interpretation for it. This opens up a number of new applications for modelling mean wind flow over the neighbourhood scale. The model is based on a balance equation between the obstacle drag force and the local shear stress as proposed by Cionco for a vegetative canopy. The buildings within the canopy are represented as a canopy element drag formulated in terms of morphological parameters such as λ f and λ p (the ratios of plan area and frontal area of buildings to the lot area). These parameters can be obtained from the analysis of urban digital elevation models. The shear stress is parameterised using a mixing length approach. Spatially-averaged velocity profiles for different values of building packing density corresponding to different flow regimes are obtained and analysed. The computed solutions are compared with published data from wind-tunnel and water-tunnel experiments over arrays of cubes. The model is used to estimate the spatially-averaged velocity profile within and above neighbourhood areas of real cities by using vertical profiles of λ f .  相似文献   

4.
The Atmospheric Boundary Layer Over Baltic Sea Ice   总被引：4，自引：0，他引：4  
A new parametrization for the surface energy balance of urban areas is presented. It is shown that this new method can represent some of the important urban phenomena, such as an urban heat island and the occurrence of a near-neutral nocturnal boundary layer with associated positive turbulent heat fluxes, unlike the traditional method for representing urban areas within operational numerical weather prediction (NWP) models. The basis of the new parametrization is simple and can be applied easily within an operational NWP model. Also, it has no additional computational expense compared to the traditional scheme and is hence applicable for operational forecasting requirements. The results show that the errors for London within the Met Office operational mesoscale model have been significantly reduced since the new scheme was introduced. The bias and root-mean-square (rms) errors have been approximately halved, with the rms error now similar to the model as a whole. The results also show that a seasonal cycle still exists in the model errors, but it is suggested that this may be caused by anthropogenic heat sources that are neglected in the urban scheme.The British Crowns right to retain a non-exclusive royalty-free license in and to any copyright is acknowledged.  相似文献   

5.
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   

6.
The simulation performance over complex building clusters of a wind simulation model(Wind Information Field Fast Analysis model, WIFFA) in a micro-scale air pollutant dispersion model system(Urban Microscale Air Pollution dispersion Simulation model, UMAPS) is evaluated using various wind tunnel experimental data including the CEDVAL(Compilation of Experimental Data for Validation of Micro-Scale Dispersion Models) wind tunnel experiment data and the NJU-FZ experiment data(Nanjing University-Fang Zhuang neighborhood wind tunnel experiment data). The results show that the wind model can reproduce the vortexes triggered by urban buildings well, and the flow patterns in urban street canyons and building clusters can also be represented. Due to the complex shapes of buildings and their distributions, the simulation deviations/discrepancies from the measurements are usually caused by the simplification of the building shapes and the determination of the key zone sizes. The computational efficiencies of different cases are also discussed in this paper. The model has a high computational efficiency compared to traditional numerical models that solve the Navier–Stokes equations, and can produce very high-resolution(1–5 m) wind fields of a complex neighborhood scale urban building canopy(～ 1 km ×1km) in less than 3 min when run on a personal computer.  相似文献   

7.
2014年3—5月T639、ECMWF及日本模式中期预报性能检验   总被引：1，自引：1，他引：0  
刘为一 《气象》2014,40(8):1019-1025
为了更好地应用T639模式中期预报产品,对2014年3—5月T639模式中期预报时效产品进行了天气学检验,并与ECMWF(简称EC)和日本模式(简称JP)进行了对比分析。结果表明:3家模式均对亚洲中高纬大气环流形势的调整和演变具有较好的预报性能,并且可以较好地预报出南支槽东移演变的过程;对于850 hPa温度场,ECMWF模式的预报效果要明显优于T639和日本模式;另外,选取了2014年4月22—24日的强沙尘暴过程进行个例分析,发现ECMWF模式对此次过程的地面高压系统的中期预报指示意义最好。  相似文献   

8.
蔡芗宁 《气象》2010,36(8):106-110
为更好地应用T639模式中期预报产品,对2010年3—5月T639模式96小时预报产品进行了天气学检验,并与ECMWF、日本模式进行了对比分析。结果表明:三种模式均对亚洲中高纬环流形势的调整和演变具有较好的预报性能。综合来看,ECMWF模式对各系统及要素的预报最接近实况;日本模式和T639模式次之。另外,选取了2010年3月19—22日的沙尘天气个例进行分析,发现日本模式对于引发此次沙尘天气的地面强风的中期预报指示意义最好。  相似文献   

9.
2011年3—5月T639、ECMWF及日本模式中期预报性能检验   总被引：3，自引：2，他引：1  
蔡芗宁 《气象》2011,37(8):1026-1030
为了更好地应用T639模式中期预报产品,对2011年3—5月T639模式96小时预报产品进行了天气学检验,并与ECMWF、日本模式进行了对比分析。结果表明：三种模式均对亚洲中高纬环流形势的调整和演变具有较好的预报性能。综合来看,ECMWF模式对各系统及要素的预报最接近实况;日本模式和T639模式次之。另外,选取了2011年4月28-30日的沙尘天气个例进行分析,发现日本模式对于引发此次沙尘天气的地面高压系统的中期预报指示意义最好。  相似文献   

10.
2012年3—5月T639、ECMWF及日本模式中期预报性能检验   总被引：2，自引：2，他引：0  
刘一 《气象》2012,38(8):1017-1022
为了更好地应用T639模式中期预报产品,对2012年3—5月T639模式96、120和144小时预报产品进行了天气学检验,并与ECMWF、日本模式进行了对比分析。结果表明:三种模式均对亚洲中高纬度环流形势的调整和演变具有较好的预报性能。综合来看,ECMWF模式对各系统及要素的预报最接近实况;日本模式和T639模式次之。另外,选取了2012年4月22—23日的沙尘天气个例进行分析,发现日本模式对于此次沙尘天气过程的地面高压系统的中期预报指示意义最好。  相似文献   

11.
The urban impact on the sea breeze is studied by means of a mesoscale model with a detailed urban parameterisation. Four simulations are carried out on an idealised two-dimensional flat domain. In the base case, half of the domain is characterised by seaand the other half by rural land. In the urban case, an urban area 10 km wide is added near the shoreline. Simulations are performed for a moist rural soil (weak sea breeze) and for a dry rural soil (strong sea breeze). Results are analysed in order to evaluate the impact of the city on the wind, temperature and turbulent kinetic energy fields. The dispersion of a passive tracer emitted near the coastline is, also, used in the comparison. Results show that the city accelerates the sea-breeze formation in the morning (combinations of urban circulation and sea breeze), but it slows thesea-breeze front penetration. Moreover, the presence of the city enhances the recirculation processes and strongly modifies the pollutant dispersion. These effects are enhanced for a moist rural soil.  相似文献   

12.
In this study, we analyse the uncertainty of the effect of enhanced greenhouse gas conditions on windiness projected by an ensemble of regional model simulations driven by the same global control and climate change simulations. These global conditions, representative for 1961–1990 and 2071–2100, were prepared by the Hadley Centre based on the IPCC SRES/A2 scenario. The basic data sets consist of simulated daily maximum and daily mean wind speed fields (over land) from the PRUDENCE data archive at the Danish Meteorological Institute. The main focus is on the results from the standard 50 km-resolution runs of eight regional models. The best parameter for determining possible future changes in extreme wind speeds and possible change in the number of storm events is maximum daily wind speed. It turned out during this study that the method for calculating maximum daily wind speed differs among the regional models. A comparison of simulated winds with observations for the control period shows that models without gust parameterisation are not able to realistically capture high wind speeds. The two models with gust parametrization estimate an increase of up to 20% of the number of storm peak (defined as gusts?≥?8 Bft in this paper) events over Central Europe in the future. In order to use a larger ensemble of models than just the two with gust parameterisation, we also look at the 99th percentile of daily mean wind speed. We divide Europe into eight sub-regions (e.g., British Isles, Iberian Peninsula, NE Europe) and investigate the inter-monthly variation of wind over these regions as well as differences between today’s climate and a possible future climate. Results show differences and similarities between the sub-regions in magnitude, spread, and seasonal tendencies. The model ensemble indicates a possible increase in future mean daily wind speed during winter months, and a decrease during autumn in areas influenced by North Atlantic extra-tropical cyclones.  相似文献   

13.
The planetary boundary-layer (PBL) parameterization is a key issue for the definition of initial wind flow fields in diagnostic models. However, PBL theories usually treat separately stable, neutral, and convective stability conditions, so that their implementation in diagnostic wind models is not straightforward. In the present paper, an attempt is made to adopt a comprehensive PBL parameterisation, covering stable/neutral and unstable atmospheric conditions, which appears suitable to diagnostic models. This parameterisation is implemented into our diagnostic mass-consistent code. A validation of the consistency between the implemented PBL parameterisations has been checked through an analysis of the sensitivity of the vertical wind profiles to atmospheric stability.  相似文献   

14.
Two parameterisation schemes for the turbulent surface fluxes and drag coefficients over the Arctic marginal sea-ice zone (MIZ) are (further) developed, and their results are compared with each other. Although the schemes are based on different principles (flux averaging and parameter averaging), the resulting drag coefficients differ only slightly in the case of neutral and stable stratification. For unstable stratification and sea-ice conditions being typical for the north-eastern Fram Strait, the drag coefficients resulting from the parameter-averaging concept are 5–10% larger than those of the flux-averaging concept. At a sea-ice concentration of 45%, the parameter-averaging method overestimates the heat fluxes by a factor of 1.2. An inclusion in the schemes of form drag caused by floe edges and ridges has a much larger effect on the drag coefficient, and on the momentum fluxes, than the choice between the parameter-averaging or flux-averaging methods. Based on sensitivity studies with the flux-averaging scheme, a simple formula for the effective drag coefficient above the Arctic MIZ is derived. It reduces the computational costs of the more complex parameterisations and could also be used in larger scale models. With this simple formula, the effective drag coefficient can be calculated as a function of the sea-ice concentration and skin drag coefficients for water and ice floes. The results obtained with this parameterisation differ only slightly from those using the more complex schemes. Finally, it is shown that in the MIZ, drag coefficients for sea-ice models may differ significantly from the effective drag coefficients used in atmospheric models.  相似文献   

15.
《Atmospheric Research》2010,95(4):743-753
Convection can cause severe precipitation events and is thus of major interest for quantitative precipitation forecasts. Due to its small horizontal scale convection usually has to be parameterized in numerical weather prediction (NWP) models as a subgrid scale phenomenon. Classical mass flux convection schemes assume grid box sizes much larger than the scale of the convective circulation. Thus, the convective mass transport is closed in the local grid column and no net mass transport occurs on the grid scale.In contemporary NWP models with grid sizes of a few kilometers, where convection is already partially resolved, the classical approach leads to a conceptual problem. This can be overcome by a hybrid mass flux convection scheme (HYMACS), in which only the small scale convective updrafts and downdrafts are parameterized, whereas the treatment of the larger scale environmental subsidence is left to the grid scale equations. Different to the classical schemes, HYMACS produces a net convective mass flux exerting pressure gradient forces to the grid scale model.We discuss real cases of different meteorological situations simulated with the operational weather forecast model COSMO of the German Meteorological Service as the hosting model of HYMACS. Precipitation and cloud top pressures simulated with HYMACS are compared with results from classical convection schemes and observational data from the DWD station network and the SEVIRI (MSG) satellite instrument.  相似文献   

16.
As urbanization progresses, more realistic methods are required to analyze the urban microclimate. However, given the complexity and computational cost of numerical models, the effects of realistic representations should be evaluated to identify the level of detail required for an accurate analysis. We consider the realistic representation of surface heating in an idealized three-dimensional urban configuration, and evaluate the spatial variability of flow statistics (mean flow and turbulent fluxes) in urban streets. Large-eddy simulations coupled with an urban energy balance model are employed, and the heating distribution of urban surfaces is parametrized using sets of horizontal and vertical Richardson numbers, characterizing thermal stratification and heating orientation with respect to the wind direction. For all studied conditions, the thermal field is strongly affected by the orientation of heating with respect to the airflow. The modification of airflow by the horizontal heating is also pronounced for strongly unstable conditions. The formation of the canyon vortices is affected by the three-dimensional heating distribution in both spanwise and streamwise street canyons, such that the secondary vortex is seen adjacent to the windward wall. For the dispersion field, however, the overall heating of urban surfaces, and more importantly, the vertical temperature gradient, dominate the distribution of concentration and the removal of pollutants from the building canyon. Accordingly, the spatial variability of concentration is not significantly affected by the detailed heating distribution. The analysis is extended to assess the effects of three-dimensional surface heating on turbulent transfer. Quadrant analysis reveals that the differential heating also affects the dominance of ejection and sweep events and the efficiency of turbulent transfer (exuberance) within the street canyon and at the roof level, while the vertical variation of these parameters is less dependent on the detailed heating of urban facets.  相似文献   

17.
朱丽  苗峻峰  高阳华 《大气科学》2020,44(3):657-678
利用中尺度模式WRF（V3.9）对2016年8月17～18日重庆一次城市热岛环流个例进行了数值模拟，探讨了山地城市热岛环流的三维结构和演变特征，分析了热岛环流期间湍流动能和各项湍流通量的特征。结果表明:15:00（北京时，下同）乡村风开始出现，随着热岛强度增强乡村风增大，18:00热岛环流结构最显著，次日02:00热岛环流结构被破坏，仅低层存在微弱的乡村风。其中，重庆市城市热岛环流最强时，水平尺度约城市尺度的1.5～2倍，垂直厚度约1.3 km，水平风速约2～4 m s?1，最大上升速度约0.5 m s?1。受地形、河流以及背景风的影响，环流呈现非对称的结构，且强度较弱。湍流特征分析结果表明，城市区域的湍流动能明显大于其它区域。此外，城市热岛环流通过湍流运动将郊区的水汽输向城市；高层湍流动量补充边界层中因热岛环流发展而造成的动量耗散。  相似文献   

18.
A major problem in urban climate modelling is determining how the heat fluxes from various canyon surfaces are affected by canyon flow. To address this problem, we developed a water evaporation method involving filter paper to study the distribution of the convective transfer velocity in urban street canyons. In this method, filter paper is pasted onto a building model and the evaporation rate from the paper is measured with an electric balance. The method was tested on 2D (two-dimensional) street canyon models and 3D model arrangements. Moreover, in this technique, it is easy to restrict the flux within an arbitrary surface in question. That is, the evaporation distribution on a surface can be studied by using several small pieces of filter paper. In the 2D case, the wall transfer velocity was strongly dependent on the canyon aspect ratio for perpendicular wind directions and it varied widely with height within both windward and leeward wall surfaces. For 3D cubic arrays, the relation to canyon aspect ratio was largely different from that of the 2D canyon. And, as a case study, the variation of wind direction was investigated for a city-like setting. The area-averaged transfer velocity was insensitive to wind direction but its local deviation was significant. Finally, we measured the transfer velocity for a clustered block array surrounded by relatively wide streets. The effect of spatial heterogeneity on the transfer velocity was significant. Moreover, for a fixed total building volume, the transfer velocity was considerably larger when the building height varied than when it was uniform. Therefore, the water evaporation method with filter paper is expected to be useful for studying the transfer velocity and ventilation rates in urban areas with various canyon shapes.  相似文献   

19.
There are many geometrical factors than can influence the aerodynamic parameters of urban surfaces and hence the vertical wind profiles found above. The knowledge of these parameters has applications in numerous fields, such as dispersion modelling, wind loading calculations, and estimating the wind energy resource at urban locations. Using quasi-empirical modelling, we estimate the dependence of the aerodynamic roughness length and zero-plane displacement for idealized urban surfaces, on the two most significant geometrical characteristics; surface area density and building height variability. A validation of the spatially-averaged, logarithmic wind profiles predicted by the model is carried out, via comparisons with available wind-tunnel and numerical data for arrays of square based blocks of uniform and heterogeneous heights. The model predicts two important properties of the aerodynamic parameters of surfaces of heterogeneous heights that have been suggested by experiments. Firstly, the zero-plane displacement of a heterogeneous array can exceed the surface mean building height significantly. Secondly, the characteristic peak in roughness length with respect to surface area density becomes much softer for heterogeneous arrays compared to uniform arrays, since a variation in building height can prevent a skimming flow regime from occurring. Overall the simple model performs well against available experimental data and may offer more accurate estimates of surface aerodynamic parameters for complex urban surfaces compared to models that do not include height variability.  相似文献   

20.
An urban boundary layer model (UBLM) is improved by incorporating the effect of buildings with a sectional drag coefficient and a height-distributed canopy drag length scale. The improved UBLM is applied to simulate the wind fields over three typical urban blocks over the Beijing area with different height-to-width ratios. For comparisons, the wind fields over the same blocks are simulated by an urban sub-domain scale model resolving the buildings explicitly. The wind fields simulated from the two different methods are in good agreement. Then, two-dimensional building morphological characteristics and urban canopy parameters for Beijing are derived from detailed building height data. Finally, experiements are conducted to investigate the effect of buildings on the wind field in Beijing using the improved UBLM.  相似文献   

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