应用城市冠层模式研究建筑物形态对城市边界层的影响

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.     

成都精细下垫面信息对城市气象影响的模拟试验

为了提高成都市精细化天气预报水平,使用成都地区精细下垫面土地利用资料,在WRF中耦合了单层城市冠层模式,对2008年7月6 日晴空背景下的成都城市气象特征进行了模拟,并和使用旧土地利用资料、slab模式的模拟结果进行了对比分析.模拟结果表明城区因为不透水下垫面的增加,使得地表蒸发和地表水汽通量显著减小,潜热通量减小,感...     

太原城市下垫面扩张对边界层特征影响的个例研究

通过高分辨率卫星夜间灯光数据获取最新的城市地表分布,并利用高分辨率数值模式对2013年8月14~16日太原区域的一次高温过程进行研究,探讨城市下垫面扩张对大气边界层的影响。结果表明：基于DMSP/OLS夜间灯光数据对模式中地表参数修正后,能够更准确地反映太原主城区和高速公路沿线小规模建筑群的扩张,有效改善了模式的预报性能,显著提高对近地面气温、地表温度的预报能力。城市下垫面的扩张,使城区夜间升温明显,热岛强度增强。与1992年的城市化状况相比,晴空天气条件下,2012年太原城区夜间气温上升5℃,热岛强度升高2~3℃。城市下垫面扩张,改变了地表能量分配关系,使得地表感热传输明显加强,潜热通量明显减弱,城市冠层作用下的储热能力增强。边界层内部湍流交换、水汽输送等的进一步研究表明：城市地表水汽输送减弱,边界层水汽含量减少,2~4 km高度的水汽含量增加,湍流动能的影响高度增高,湍流混合加剧;14：00,城区边界层高度抬高了800 m,城市上空混合层加深,持续时间更长。     

人为热对城市边界层结构影响研究

为研究不同人为热源引入方案对城市边界层结构模拟性能的影响，以杭州地区为例，在区域边界层模式（RBLM）中引入一种新人为热源处理方案，即对城市中的人为热排放分层考虑，将低层的人为热源加入地表能量平衡方程，将高层人为热源分布与建筑物高度和密度联系起来，加入热量方程中，同时考虑了人为热源强度的日变化。数值试验结果表明，这是一种比较合理的处理方案。人为热源引入方案对城市边界层结构的影响表现在:气温、湍流动能增加，并通过湍流交换输送到较高层大气；大气不稳定度增加，混合层高度最高抬升了400 m；城市地区上升速度增加，热岛环流加强；白天人为热源一般为太阳辐射的10%～20%，对地气交换的影响较小。夜间没有了太阳辐射能量，对地气交换的影响比日间更明显；冬季低层湍流活动加强，湍能约增加40%，大气层结稳定度降低。     

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.     

Evaluation of the Urban Tile in MOSES using Surface Energy Balance Observations

The UK Met Office has introduced a new scheme for its urban tile in MOSES 2.2 (Met Office Surface Exchange Scheme version 2.2), which is currently implemented within the operational Met Office weather forecasting model. Here, the performance of the urban tile is evaluated in two urban areas: the historic core of downtown Mexico City and a light industrial site in Vancouver, Canada. The sites differ in terms of building structures and mean building heights. In both cases vegetation cover is less than 5%. The evaluation is based on surface energy balance flux measurements conducted at approximately the blending height, which is the location where the surface scheme passes flux data into the atmospheric model. At both sites, MOSES 2.2 correctly simulates the net radiation, but there are discrepancies in the partitioning of turbulent and storage heat fluxes between predicted and observed values. Of the turbulent fluxes, latent heat fluxes were underpredicted by about one order of magnitude. Multiple model runs revealed MOSES 2.2 to be sensitive to changes in the canopy heat storage and in the ratio between the aerodynamic roughness length and that for heat transfer (temperature). Model performance was optimum with heat capacity values smaller than those generally considered for these sites. The results suggest that the current scheme is probably too simple, and that improvements may be obtained by increasing the complexity of the model.     

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.     

Modeling the Warming Impact of Urban Land Expansion on Hot Weather Using the Weather Research and Forecasting Model: A Case Study of Beijing,China

The impacts of three periods of urban land expansion during 1990–2010 on near-surface air temperature in summer in Beijing were simulated in this study, and then the interrelation between heat waves and urban warming was assessed. We ran the sensitivity tests using the mesoscale Weather Research and Forecasting model coupled with a single urban canopy model,as well as high-resolution land cover data. The warming area expanded approximately at the same scale as the urban land expansion. The average regional warming induced by urban expansion increased but the warming speed declined slightly during 2000–2010. The smallest warming occurred at noon and then increased gradually in the afternoon before peaking at around 2000 LST—the time of sunset. In the daytime, urban warming was primarily caused by the decrease in latent heat flux at the urban surface. Urbanization led to more ground heat flux during the day and then more release at night, which resulted in nocturnal warming. Urban warming at night was higher than that in the day, although the nighttime increment in sensible heat flux was smaller. This was because the shallower planetary boundary layer at night reduced the release efficiency of near-surface heat. The simulated results also suggested that heat waves or high temperature weather enhanced urban warming intensity at night. Heat waves caused more heat to be stored in the surface during the day, greater heat released at night, and thus higher nighttime warming. Our results demonstrate a positive feedback effect between urban warming and heat waves in urban areas.     

Numerical Study of Winter Urban Boundary Layer Structure over Beijing Area

1. Introduction The urban canopy layer (UCL) (Roth, 2000) is defined as a layer in the vertical structure of urban boundary layer (UBL) ranging from the surface to the top of buildings. The urban infrastructures within UCL, such as buildings with different heights, the het- erogeneity of urban land cover and the anthropogenic activity, all directly influence the thermal and dynam- ical structures of atmosphere and net radiation budget at the surface, meanwhile, affect the structure of ur- …     

成都地区城市化对气候影响的模拟研究

为了研究成都地区城市化对当地气候的影响,利用不同时期的下垫面土地利用类型数据和耦合单层城市冠层模型(UCM)的WRF（Weather Research and Forecasting）模式对成都夏季和冬季城市化效应进行了模拟研究,得到以下主要结论:1）成都地区城市化使夏季城区上空出现增温区域。城区地表气温升高约2.8°C,边界层高度升高约150 m,冬季地表气温平均升高约0.6°C,边界层高度升高约25 m。夏冬两季气温日较差均减小。2）受城市化影响,成都地区夏季和冬季2 m相对湿度减小,感热通量增加,潜热通量减小,且夏季变化程度强于冬季。3）城市化使地表的粗糙度增加,进而使夏季和冬季风速在城区减小,减小约0.1~0.6 m s?1,但夏季风速减小区域较冬季更大。城市化还使城市上空低层散度减小,辐合作用增强,垂直速度增大,夏季水汽往高层输送明显。4）夏季,城市化作用使日平均和白天时段降水量在城区的迎风区和下风区均增加,夜间降水量在下风区域增加,对迎风区域影响不明显。     

Simulation of Meteorological Fields Within and Above Urban and Rural Canopies with a Mesoscale Model

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.     

Mean Winds Through an Inhomogeneous Urban Canopy

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.     

城市街区形态及冷却屋顶对冠层内辐射热通量的影响

在当前中国城市化进程愈演愈烈的情形下,城市热岛冷却效应的研究对于确立城市生态环境可持续化发展的正确途径等有重要意义。采用离线城市冠层模型分析了城市冠层中街区形态和屋顶材料的变化对辐射热量、表面温度及冠层内气温的影响。研究发现:建筑物高度、宽度以及街道宽度等参数的改变对冠层各表面温度的影响较大,当街道宽度增加3 m时,地面温度升高约3.5 K。但是街道宽度增加,多重反射导致的辐射截陷效应减弱,墙面上更多的热量释放出去,各墙面温度降低约1.5 K;冠层气温先增加,日出后降低约0.4 K。屋顶材料的改变对辐射及热通量和表面温度也有较大影响,与灰色水泥屋顶相比,采用高反照率白色涂料冷却屋顶后,屋顶净辐射热量损失约380 W m-2,屋顶表面温度降低约10 K。冠层内街区形态和屋顶材料对城市辐射热环境产生直接的影响。     

人为热源和城市绿化对冬季山谷城市边界层结构影响的数值模拟

为了探讨人为热源和城市绿化对城市边界层的影响,本文在RAMS模式中引入周期性日变化的人为热源和人工改变下垫面状况,初步模拟分析了人为热源和城市绿化对城市边界层结构的影响.结果表明:人为热源具有明显的增温效应,增强了城市的湍流交换,白天增加了大气不稳定度,促进了混合层的发展,夜间降低了大气稳定度,减弱了城市夜间逆温.城市绿化减小了地表反照率,增加了到达地面的净辐射,模拟期间土壤冻结,增加的净辐射其中一部分以感热的形式来加热大气;绿化后地气之间的湍流交换增强,增加了大气不稳定度,减弱了白天高空逆温;本文还讨论了不同绿化布局对白天高空逆温的影响以及人为热源和城市绿化之间的非线性相互作用.     

Impact of Eddy Characteristics on Turbulent Heat and Momentum Fluxes in the Urban Roughness Sublayer

Eddy-covariance observations above the densely built-up Centre of Nanjing were made from December 2011 to August 2012. Separate eddy-covariance systems installed at two levels on a 36-m tower located on a rooftop were operated simultaneously, and observations grouped into two sectors (A, B) according to the prevalent wind directions. For sector A, where the nearby buildings are all below the lower measurement level, the sensible heat and momentum fluxes are generally greater at the upper level. For sector B, where several high-rise buildings are located upwind, the sensible heat and momentum fluxes at the upper level are close to those at the lower level. The analysis shows that the turbulent eddy characteristics differ between the two wind sectors, leading to a different behaviour of turbulent exchange between the two levels. A hypothesis is proposed that addresses the vertical variation of turbulent fluxes in the urban roughness sublayer (RSL). For sector A, the buildings block the flow, change the trajectory of scalars, and distort the footprint of scalar fluxes; this ‘blocking effect’ is believed to lead to a smaller sensible heat flux above the canopy layer. Such an effect should decrease with height in the RSL, explaining the increase of the observed turbulent heat flux with height. In addition, the presence of non-uniform building heights adversely affects turbulence organization around the canopy top, and likely elevates the inflection point of the mean flow to a higher elevation close to the upper measurement level, where larger shear results in a larger momentum flux. For sector B, wake effects from the nearby high-rise buildings strongly reduce turbulence organization at higher elevations, leading to similar sensible heat and momentum fluxes at both measurement levels.     

MM5模式中城市冠层参数化方案的设计及其数值试验

文中在综合国外一些较先进的中尺度模式城市作用参数化方案的基础上 ,从城市下垫面结构对城市边界层大气作用的物理机制及实际应用两方面出发 ,对城市下垫面结构和人为活动等因素对边界层结构的影响及中尺度模式中城市化作用的合理体现等问题进行了较全面的考虑 ,改进和设计出能够较全面、细致地描述城市结构对大气边界层动力、热力结构的影响 ,且适合中尺度模式结构特点的城市冠层参数化方案 (UCP) ,并实现了其与MM5模式的耦合。进行了耦合后的UCP方案及采用原城市作用方案的MM5模式对BECAPEX试验期间北京地区气象条件多重嵌套细尺度进行了模拟试验 ,并与观测结果对比 ,结果表明 :相比于MM 5模式中原有表示城市作用的参数化方案来讲 ,设计的UCP方案在很大程度上提高了MM 5模式对城市边界层热力和动力结构的模拟能力。     

北京城市下垫面边界层湍流统计特征

分析了1998年10月在北京325 m气象塔47 m、120m及280m 3层取得的湍流资料,讨论了湍流宏观统计特征量等随稳定度变化的规律.结果表明,47 m及120 m观测到的感热通量的日变化基本一致,城市边界层中近地层高度可达100m左右,这主要是由于下垫面粗糙度很大所致.同时发现当冷空气来临时,边界层中的感热通量会有明显增大.     

复杂地形条件下城市热岛及局地环流特征的数值模拟

应用基于多层城市冠层方案BEP(Building Environment Parameterization)增加室内空调系统影响的建筑物能量模式BEM(Building Energy Model)方案的WRF模式,模拟研究重庆热岛的特征、成因以及局地环流对热岛形成的影响。文中共有两个算例,一为重庆真实下垫面算例,称之为URBAN算例,二为将城市下垫面替换为耕地下垫面的对比算例,称之为NOURBAN算例。结果表明:1)WRF方案模拟结果与观测2 m气温的对比吻合较好,误差主要出现在正午温度峰值和凌晨温度谷值处,由城市下垫面特性及城市内建筑分布误差引起。2)BEP+BEM方案较好地模拟出了重庆地区的热岛分布的空间和时间特征。重庆市温度的分布受地形和城市下垫面的双重影响,越靠近城区,温度的分布受城市化影响就越大,在海拔低处,温度就越高。3)城区立体三维表面对辐射的陷阱作用导致城市表面总体反射率小,向上短波辐射小于郊区约20 W/m~2。城市表面以感热排放为主,而郊区则表现为潜热的作用占主导。夜间城市地表储热以及空调废热向大气释放,是城市热岛形成的重要原因。4)模拟区域背景风场主要为东南风,局地环流呈现出越靠近山区风速越大、城市区域风速较小的特性,体现了城市密集的建筑群对低层大气流场的空气动力学效应,以及复杂山谷地形的山谷风环流特性。在市区的西侧和东南侧均有高大山脉阻挡,山脉对城市出流的阻碍作用、气流越山与绕流运动对城市热岛的形成有一定影响。     

The Influence Of Urban Canopy Configuration On Urban Albedo

We propose a calculation method for shortwave radiation flux and longwave radiation flux within the urban canopy and investigate the influence of urban canopy configuration on net radiation flux. In the assumed urban configuration, buildings of equal size are arranged in a regular lattice within the urban canopy. The net shortwave radiation flux and longwave radiation flux within the urban canopy were calculated by the photon tracking method based on the Monte Carlo method. The albedo value obtained by this method shows close agreement with experimental data, and the average sky view factor shows almost perfect agreement with the theoretical value. Moreover, we calculated the urban albedo for the urban canopy configuration including roads and building height distribution.%Moreover, we calculated net radiation within the urban canopy in %consideration of roads and building height distribution.We found that the sky view factor of the ground surface is high when building coverage is low, building height is low, open space by roads exists, and building height is non-uniform. Moreover, we found that the albedo value is high when building height is small, open space by roads is wide, and building height is uniform. The albedo value was found to vary in a complicated manner with change in building coverage.