A Physically-Based Scheme For The Urban Energy Budget In Atmospheric Models

An urban surface scheme for atmospheric mesoscale models ispresented. A generalization of local canyon geometry isdefined instead of the usual bare soil formulation currently usedto represent cities in atmospheric models. This allows refinement ofthe radiative budgets as well as momentum, turbulent heat and ground fluxes.The scheme is aimed to be as general as possible, in order to representany city in the world, for any time or weather condition(heat island cooling by night, urban wake, water evaporation after rainfalland snow effects).Two main parts of the scheme are validated against published data.Firstly, it is shown that the evolution of the model-predictedfluxes during a night with calm winds is satisfactory, considering both the longwave budget and the surface temperatures. Secondly, the original shortwave scheme is tested off-line and compared to the effective albedoof a canyon scale model. These two validations show that the radiative energy input to the urban surface model is realistic.Sensitivity tests of the model are performed for one-yearsimulation periods, for both oceanic and continental climates. The scheme has the ability to retrieve, without ad hoc assumptions, the diurnal hysteresis between the turbulent heat flux and ground heat flux. It reproduces the damping of the daytime turbulent heat flux by the heat storage flux observed in city centres. The latent heat flux is negligible on average,but can be large when short time scales are considered (especially afterrainfall). It also suggests that in densely built areas, domesticheating can overwhelm the net radiation, and supply a continuous turbulentheat flux towards the atmosphere. This becomes very important inwinter for continental climates. Finally, a comparison with a vegetation scheme shows that the suburban environment can be represented with a bare soil formulation for large temporal or spatial averages (typical of globalclimatic studies), but that a surface scheme dedicated to the urban surface is necessary when smaller scales are considered: town meteorological forecasts, mesoscale or local studies.     

Development of a Multi-Layer Urban Canopy Model for the Analysis of Energy Consumption in a Big City: Structure of the Urban Canopy Model and its Basic Performance

A multilayer one-dimensional canopy model was developed to analyze the relationship between urban warming and the increase in energy consumption in a big city. The canopy model, which consists of one-dimensional diffusion equations with a drag force, has three major parameters: building width, distance between buildings, and vertical floor density distribution, which is the distribution of a ratio of the number of the buildings that are taller than some level to all the buildings in the area under consideration. In addition, a simplified radiative process in the canopy is introduced. Both the drag force of the buildings and the radiative process depend on the floor density distribution. The thermal characteristics of an urban canopy including the effects of anthropogenic heat are very complicated. Therefore, the focus of this research is mainly on the basic performance of an urban canopy without anthropogenic heat. First, the basic thermal characteristics of the urban canopy alone were investigated. The canopy model was then connected with a three-dimensional mesoscale meteorological model, and on-line calculations were performed for 10 and 11 August, 2002 in Tokyo, Japan. The temperature near the ground surface at the bottom of the canopy was considerably improved by the calculation with the canopy model. However, a small difference remained between the calculation and the observation for minimum temperature. Deceleration of the wind was well reproduced for the velocity at the top of the building by the calculation with the canopy model, in which the floor density distribution was considered.     

Study on Effects of Building Morphology on Urban Boundary Layer Using an Urban Canopy Model

An urban canopy model is incorporated into the Nanjing University Regional Boundary Layer Model. Temperature simulated by the urban canopy model is in better agreement with the observation, especially in the night time, than that simulated by the traditional slab model. The coupled model is used to study the effects of building morphology on urban boundary layer and meteorological environment by changing urban area, building height, and building density. It is found that when the urban area is expanded, the urban boundary layer heat flux, thermal turbu- lence, and the turbulent momentum flux and kinetic energy all increase or enhance, causing the surface air temperature to rise up. The stability of urban atmospheric stratiˉcation is a?ected to diffierent extent at diffierent times of the day. When the building height goes up, the aerodynamic roughness height, zero plane displacement height of urban area, and ratio of building height to street width all increase. Therefore, the increase in building height results in the decrease of the surface heat flux, urban surface temperature, mean wind speed, and turbulent kinetic energy in daytime. While at night, as more heat storage is released by higher buildings,thermal turbulence is more active and surface heat flux increases, leading to a higher urban temperature. As the building density increases, the aerodynamic roughness height of urban area decreases, and the effect of urban canopy on radiation strengthens. The increase of building density results in the decrease in urban surface heat flux, momentum flux, and air temperature, the increase in mean wind speed, and the weakening of turbulence in the daytime. While at night, the urban temperature increases due to the release of more heat storage.     

Study on Effects of Building Morphology on Urban Boundary Layer Using an Urban Canopy Model

An urban canopy model is incorporated into the Nanjing University Regional Boundary Layer Model. Temperature simulated by the urban canopy model is in better agreement with the observation, especially in the night time, than that simulated by the traditional slab model. The coupled model is used to study the effects of building morphology on urban boundary layer and meteorological environment by changing urban area, building height, and building density.It is found that when the urban area is expanded, the urban boundary layer heat flux, thermal turbulence, and the turbulent momentum flux and kinetic energy all increase or enhance, causing the surface air temperature to rise up. The stability of urban atmospheric stratification is affected to different extent at different times of the day.When the building height goes up, the aerodynamic roughness height, zero plane displacement height of urban area, and ratio of building height to street width all increase. Therefore, the increase in building height results in the decrease of the surface heat flux, urban surface temperature, mean wind speed, and turbulent kinetic energy in daytime. While at night, as more heat storage is released by higher buildings, thermal turbulence is more active and surface heat flux increases, leading to a higher urban temperature.As the building density increases, the aerodynamic roughness height of urban area decreases, and the effect of urban canopy on radiation strengthens. The increase of building density results in the decrease in urban surface heat flux, momentum flux, and air temperature, the increase in mean wind speed, and the weakening of turbulence in the daytime. While at night, the urban temperature increases due to the release of more heat storage.     

Thermal Energy Balance Analysis of the Tokyo Metropolitan Area Using a Mesoscale Meteorological Model Incorporating an Urban Canopy Model

The summer climate around the Tokyo metropolitan area has been analysed on an urban scale, and the regional characteristics of the thermal energy balance of a bayside business district in the centre of Tokyo (Otemachi) have been compared with an inland residential district (Nerima), using a mesoscale meteorological model incorporating an urban canopy model. From the results of the analysis, the mechanism of diurnal change in air temperature and absolute humidity in these areas is quantitatively demonstrated, with a focus on the thermal energy balance. Moreover, effective countermeasures against urban heat-islands are considered from the viewpoint of each region’s thermal energy balance characteristics. In addition to thermal energy outflux by turbulent diffusion, advection by sea-breezes from Tokyo Bay discharges sensible heat in Otemachi. This mitigates temperature increases during the day. On the other hand, because all sea-breezes must first cross the centre of Tokyo, it has less of a cooling effect in Nerima. As a result, the air temperature during the day in Nerima is higher than that in Otemachi.     

应用城市冠层模式对建筑物表面太阳辐射的分析

利用城市冠层模式对建筑物表面的太阳辐射进行模拟,并将模拟结果与部分观测作了对比,一致性较好。在此基础上重点研究由于建筑物之间的遮蔽效应和墙面间的反射作用对建筑物各墙面所接收到的太阳辐射的影响。结果表明:(1)冬季建筑物墙面之间的遮蔽效应会明显减少建筑物的太阳辐照度,而且在早晨和傍晚尤为明显,在实际对太阳能利用的过程中,应该考虑这种遮蔽效应,否则会高估壁面所获得的太阳能。墙面之间的反射会小幅增加壁面太阳辐照度10～20 W.m-2。(2)建筑物墙面间的遮蔽效应和反射辐射随建筑群的高度和密度的增加而增大,特别在冬季,高而密的城市建筑区域,由于墙面的遮蔽,会使得壁面的日照时数大大减少。(3)纬度能够影响墙面遮蔽时间和太阳辐射相对于墙面的倾斜角,冬季,纬度越高,南墙日照时数越小;夏季则反之。     

一种单层城市冠层模式的建立及数值试验研究

本文在引进先进的城市地表能量平衡方案 (Town Energy Balance, 简称TEB) 的基础上建立了一个单层城市冠层模式, 并对南京市典型居民区1 km2范围内的局地尺度地表能量平衡各分量进行离线模拟, 将模拟结果与同期观测值作了比对, 发现: TEB方案对城市地表能量平衡各分量的模拟效果良好, 而该方案的模拟性能受建筑物表面材料反照率取值的影响较大。在离线研究的基础上, 本文又将TEB方案成功耦合到南京大学区域边界层模式 (NJU－RBLM) 中, 作为该模式的地表能量平衡参数化方案之一, 分别选取该边界层模式中原有的地表能量平衡参数化方案SVAT (Soil－Vegetation－Atmosphere－Transfer model) 和新引入的TEB方案对冬夏两季不同个例进行模拟, 以常规近地面气温观测资料和Landsat卫星观测的地表反照率资料对模拟结果进行比较, 结果表明: TEB方案对原大气边界层模式的模拟效果有明显改善, 对近地面热力场的改善效果尤为明显, 可以很好地模拟出城市冠层中的“陷阱效应”。     

Including the Urban Canopy Layer in a Lagrangian Particle Dispersion Model

Boundary-Layer Meteorology - When wind blows over the ocean, short wind-waves (of wavelength smaller than 10 m) are generated, rapidly reaching an equilibrium with the overlying turbulence (at...     

Momentum Transfer and Turbulent Kinetic Energy Budgets within a Dense Model Canopy

Second-order closure models for the canopy sublayer (CSL) employ aset of closure schemes developed for `free-air' flow equations andthen add extra terms to account for canopy related processes. Muchof the current research thrust in CSL closure has focused on thesecanopy modifications. Instead of offering new closure formulationshere, we propose a new mixing length model that accounts for basicenergetic modes within the CSL. Detailed flume experiments withcylindrical rods in dense arrays to represent a rigid canopy areconducted to test the closure model. We show that when this lengthscale model is combined with standard second-order closureschemes, first and second moments, triple velocity correlations,the mean turbulent kinetic energy dissipation rate, and the wakeproduction are all well reproduced within the CSL provided thedrag coefficient (C_D) is well parameterized. The maintheoretical novelty here is the analytical linkage betweengradient-diffusion closure schemes for the triple velocitycorrelation and non-local momentum transfer via cumulant expansionmethods. We showed that second-order closure models reproducereasonably well the relative importance of ejections and sweeps onmomentum transfer despite their local closure approximations.Hence, it is demonstrated that for simple canopy morphology (e.g.,cylindrical rods) with well-defined length scales, standard closureschemes can reproduce key flow statistics without much revision.When all these results are taken together, it appears that thepredictive skills of second-order closure models are not limitedby closure formulations; rather, they are limited by our abilityto independently connect the drag coefficient and the effectivemixing length to the canopy roughness density. With rapidadvancements in laser altimetry, the canopy roughness densitydistribution will become available for many terrestrialecosystems. Quantifying the sheltering effect, the homogeneity andisotropy of the drag coefficient, and more importantly, thecanonical mixing length, for such variable roughness density isstill lacking.     

A Vegetated Urban Canopy Model for Meteorological and Environmental Modelling

An urban canopy model is developed for use in mesoscale meteorological and environmental modelling. The urban geometry is composed of simple homogeneous buildings characterized by the canyon aspect ratio (h/w) as well as the canyon vegetation characterized by the leaf aspect ratio (σ _l ) and leaf area density profile. Five energy exchanging surfaces (roof, wall, road, leaf, soil) are considered in the model, and energy conservation relations are applied to each component. In addition, the temperature and specific humidity of canopy air are predicted without the assumption of thermal equilibrium. For radiative transfer within the canyon, multiple reflections for shortwave radiation and one reflection for longwave radiation are considered, while the shadowing and absorption of radiation due to the canyon vegetation are computed by using the transmissivity and the leaf area density profile function. The model is evaluated using field measurements in Vancouver, British Columbia and Marseille, France. Results show that the model quite well simulates the observations of surface temperatures, canopy air temperature and specific humidity, momentum flux, net radiation, and energy partitioning into turbulent fluxes and storage heat flux. Sensitivity tests show that the canyon vegetation has a large influence not only on surface temperatures but also on the partitioning of sensible and latent heat fluxes. In addition, the surface energy balance can be affected by soil moisture content and leaf area index as well as the fraction of vegetation. These results suggest that a proper parameterization of the canyon vegetation is prerequisite for urban modelling.     

A Simple Physically-Based Model for Wind-Turbine Wake Growth in a Turbulent Boundary Layer

A systematic evaluation is conducted of the smoothed spectrum, which is a spectral estimate obtained by averaging over a window of contiguous frequencies. The technique is extended to the ogive, as well as to the cross-spectrum. It is shown that, combined with existing variance estimates for the periodogram, the variance—and therefore the random error—associated with these estimates can be calculated in a straightforward way. The smoothed spectra and ogives are biased estimates; with simple power-law analytical models, correction procedures are devised, as well as a global constraint that enforces Parseval’s identity. Several new results are thus obtained: (1) The analytical variance estimates compare well with the sample variance calculated for the Bartlett spectrum and the variance of the inertial subrange of the cospectrum is shown to be relatively much larger than that of the spectrum. (2) Ogives and spectra estimates with reduced bias are calculated. (3) The bias of the smoothed spectrum and ogive is shown to be negligible at the higher frequencies. (4) The ogives and spectra thus calculated have better frequency resolution than the Bartlett spectrum, with (5) gradually increasing variance and relative error towards the low frequencies. (6) Power-law identification and extraction of the rate of dissipation of turbulence kinetic energy are possible directly from the ogive. (7) The smoothed cross-spectrum is a valid inner product and therefore an acceptable candidate for coherence and spectral correlation coefficient estimation by means of the Cauchy–Schwarz inequality. The quadrature, phase function, coherence function and spectral correlation function obtained from the smoothed spectral estimates compare well with the classical ones derived from the Bartlett spectrum.     

一种城市地表能量平衡模式在上海的模拟评估

利用一整年的上海城区常规气象和地表能量平衡观测资料,驱动和检验了局地城市地表能量(水分)平衡模式(SUEWS/LUMPS)在上海地区的模拟能力,并对模式输入参数进行了部分本地化。模拟结果表明,SUEWS模式较好地再现了各辐射通量的日变化形态,对净辐射通量(Q~*)中午日峰值低估约为25 W·m~(-2);模式对四个季节向下长波辐射通量(L_↓)的日变化幅度均被低估,对向上长波辐射通量(L_↑)的模拟明显优于L_↓。SUEWS/LUMPS模式对感热通量(Q_H)各季节(春季除外)日峰值出现时次均有准确模拟,而对Q_H量值各季节均为低估;SUEWS模式在夏、秋季对白天潜热通量(Q_E)的模拟均优于LUMPS模式,而在冬、春季的模拟情况两者接近;SUEWS模式成功再现了储热通量(△Q_S)冬、春、秋季早、晚正负值转换时间,而在夏季滞后了2 h,模拟的△Q_S量值季节差异性较大。对模式误差随气温、风速及风向变化进行分析表明,在较高气温和较大风速下,Q_H、Q_E均表现为低估误差增大,而△Q_S则相反,表现为更显著地高估;风向的影响主要表现为模式未考虑东面密集建筑群而使得Q_H较明显低估约为-50 W·m~(-2),而西侧公园绿地的存在使得Q_H高估约15 W·m~(-2)。     

Further Development of the Vegetated Urban Canopy Model Including a Grass-Covered Surface Parametrization and Photosynthesis Effects

The vegetated urban canopy model (VUCM), which includes parametrizations of urban physical processes for artificial surfaces and vegetated areas in an integrated system, has been further developed by including physical processes associated with grass-covered surfaces in urban pervious surfaces and the photosynthesis effects of urban vegetation. Using measurements made from three urban/suburban sites during the BUBBLE field campaign in 2002, the model’s performance in modelling surface fluxes (momentum flux, net radiation, sensible and latent heat fluxes and storage heat flux) and canopy air conditions (canopy air temperature and specific humidity) was critically evaluated for the non-precipitation and the precipitation days. The observed surface fluxes at the urban/suburban sites were significantly altered by precipitation as well as urban vegetation. Especially, the storage heat at urban surfaces and underlying substrates varied drastically depending on weather conditions while having an important role in the formation of a nocturnal urban surface layer. Unlike the nighttime canopy air temperature that was largely affected by the storage-heat release, the daytime canopy air conditions were highly influenced by the vertical turbulent exchange with the overlying atmosphere. The VUCM well reproduced these observed features in surface fluxes and canopy air conditions at all sites while performing well for both the non-precipitation and the precipitation days. The newly implemented parametrizations clearly improved the model’s performance in the simulation of sensible and latent heat fluxes at the sites, more noticeably at the suburban site where the vegetated area fraction is the largest among the sites. Sensitivity analyses for model input parameters in VUCM elucidated the relative importance of the morphological, aerodynamic, hydrological and radiative/thermal properties in modelling urban surface fluxes and canopy air conditions for daytime and nighttime periods. These results suggest that the VUCM has great potential for urban atmospheric numerical modelling for a range of cities and weather conditions in addition to having a better physical basis in the representation of urban vegetated areas and associated physical processes.     

A Simple Model for Spatially-averaged Wind Profiles Within and Above an Urban Canopy

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 .     

A Stochastic Single-Particle Lagrangian Model for the Concentration Fluctuations in a Plume Dispersing Inside an Urban Canopy

A new approach for estimating concentration fluctuations intensity in dense built-up environments using a Lagrangian stochastic (LS) particle model is described. Following past success in modelling the dynamics of concentration variance as a diffusion-advection process, the ensemble-averaged concentration variance is represented by particles that advect and diffuse throughout the computational domain. The calculation of the concentration variance is addressed by assuming an appropriate distribution of effective variance sources for a given mean concentration field. Dissipation is treated by allowing the variance carried by every particle to decay exponentially with a locally-estimated decay time. The approach has the benefit of easily handling complex boundary conditions. It can also be easily and naturally implemented as an extension to an existing LS model, which is used for mean concentration estimations. The method differs from existing two-particle methods that demand knowledge of the structure function of the flow. It is also more computationally efficient than micro-mixing approaches that involve maintaining high population levels of particles in every grid volume. The model is compared with high frequency concentration measurements, taken as part of the JU2003 (Joint Urban 2003) experiment that was carried out in Oklahoma City. Good agreement is observed.     

Thermal Stratification Effects on Flow Over a Generic Urban Canopy

The influence of local surface heating and cooling on flow over urban-like roughness is investigated using large-eddy simulations. By adjusting the incoming or outgoing heat flux from the ground surface, various degrees of local thermal stratification, represented by a Richardson number \((Ri_\tau )\) , were attained. Drag and heat transfer coefficients, turbulence structure, integral length scales, and the strength of quadrant events that contribute to momentum and heat fluxes were obtained and are compared with locally stable, neutral and unstable flows. With increasing \(Ri_\tau \) , or equivalently as the flow characteristics change from local thermal instability to stability, a gradual decline in the drag and heat transfer coefficients is observed. These values are found to be fairly independent of the type of thermal boundary condition (constant heat flux or constant temperature) and domain size. The maps of anisotropy invariants showed that for the values of \(Ri_\tau \) considered, turbulence structures are almost the same in shape for neutral and unstable cases but differ slightly from those in the stable case. The degree of anisotropy is found to decrease as \(Ri_\tau \) increases from \(-2\) to 2.5. Compared to the neutral case, the integral length scales are shortened in the streamwise and vertical direction by ground cooling, but enhanced in the vertical direction with ground heating. Quadrant analysis showed that an increase in floor heating increases the strength of ejections above the canopy. However, the contributions of updrafts or downdrafts to the heat flux are found not to be significantly influenced by the type of local thermal stratification for the values of \(Ri_\tau \) considered. From the octant analysis, the transport mechanisms of momentum and heat above the canopy are found to be very similar in both locally unstable and stable flows.     

A Simple Energy Balance Model for Regular Building Arrays

A simple urban energy balance model for mesoscale simulations (SUMM) was tested using results from an outdoor scale-model experiment. The model geometry is assumed to be an infinitely extended regular array of uniform buildings, each of which is composed of six faces (roof, floor, and four vertical walls). The SUMM explicitly considers the three dimensionality of surface geometry and theoretically predicts the energy balance at each face without time-consuming iterations. The SUMM was compared with outdoor scale-model experiments. The simulated energy balance and surface temperatures agree well with the values measured on a reduced-scale hardware model corresponding to the numerical model geometry.     

基于Ｌｏｔｋａ Ｖｏｌｔｅｒｒａ方程的基因调控网络的稳定性分析

在2007年7—8月棉花生长旺季,将冠层按高度分多层,通过简单的试验方法确定了棉冠内叶片丛聚指数（clumping index,Ω）,以此区分计算出田间各层逐时阳叶（受光叶）和阴叶（被遮荫叶）叶面积指数的动态值。在8月23日用LI-6400R便携式光合测量系统分多层分别测定阴、阳叶单张叶片净光合作用的日变化,结合阴、阳叶面积权重,探讨利用Ω区分阴阳叶之后对整个冠层日总光合作用的影响。试验结果表明：（1）花铃期棉花冠层内Ω约为0.68;（2）利用Ω计算得到8月23日冠层日间（09：00—18：00）总净光合作用日平均值大约为20.3μmol·m-2·s-1,其中阳叶贡献约占总量的72%,阴叶约占总量的28%;（3）上层叶片贡献约占总量的75%,中层叶片约占总量的22%,下层叶片约占总量的3%。     

Development of a Three-Dimensional Urban Energy Model for Predicting and Understanding Surface Temperature Distribution

The Model for Urban Surface Temperature, a three-dimensional approach, is developed for a realistically complex city with considerations of the energy exchange processes at the urban surface. The discrete transfer method and Gebhart absorption factor method are used for the shape factor estimation and multiple reflection calculation, respectively. The surface energy balance model is evaluated against existing field measurements that pertain to idealized urban geometry. It performs well in terms of predicting surface temperature and heat fluxes by allowing for detailed urban surface properties and meteorological conditions. The compressed row storage scheme is applied to calculate the transfer of surface thermal radiation, which dramatically reduces the computational requirements. This strategy permits the rigorous consideration of multiple reflections in a realistic urban area with hundreds of buildings. The result illustrates that considering only the first reflection is a good approach when the urban area is comprised of typical urban materials, e.g. materials with high emissivity and low albedo, because relatively accurate computational results can be obtained rapidly by avoiding the multiple reflection calculation.