An Urban Surface Exchange Parameterisation for Mesoscale Models

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.     

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。     

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.     

Simulating Australian Urban Climate in a Mesoscale Atmospheric Numerical Model

We develop an urban canopy scheme coupled to a mesoscale atmospheric numerical model and evaluate the simulated climate of an Australian city. The urban canopy scheme is based on the Town Energy Budget approach, but is modified to efficiently represent the predominately suburban component of Australian cities in regional climate simulations. Energy conservation is improved by adding a simple model of air-conditioning to prevent the urban parametrization acting as an energy sink during the Australian summer. In-canyon vegetation for suburban areas is represented by a big-leaf model, but with a largely reduced set of prognostic variables compared to previous approaches. Although we have used a recirculation/venting based parametrization of in-canyon turbulent heat fluxes that employs two canyon wall energy budgets, we avoid using a fixed canyon orientation by averaging the canyon fluxes after integrating over 180° of possible canyon orientations. The urban canopy scheme is evaluated by simulating the climate for Melbourne, Australia after coupling it to The Air Pollution Model. The combined system was found to predict a realistic climatology of air temperatures and winds when compared with observations from Environmental Protection Authority monitoring stations. The model also produced a plausible partitioning of the urban energy budget when compared to urban flux-tower studies. Overall, the urban canyon parametrization appears to have reasonable potential for studying present and predicting changes in future Australian urban climates in regional climate simulations.     

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.     

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.     

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.     

Surface heating in relation to air temperature,wind and turbulence in an urban street canyon

Wind and temperature measurements from within and above a deep urban canyon (height/width = 2.1) were used to examine the thermal structure of air within the canyon, exchange of heat with the overlying atmosphere, and the possible impacts of surface heating on within-canyon air flow. Measurements were made over a range of seasons and primarily analysed for sunny days. This allowed the study of temperature differences between opposing canyon walls and between wall and air of more than 15°C in summer. The wall temperature patterns follow those of incoming solar radiation loading with a secondary daytime effect from the longwave exchange between the walls. In winter, the canyon walls receive little direct solar radiation, and temperature differences are largely due to anthropogenic heating of the building interiors. Cool air from aloft and heated air from canyon walls is shown to circulate within the canyon under cross-canyon flow. Roofs and some portions of walls heat up rapidly on clear days and have a large influence on heat fluxes and the temperature field. The magnitude and direction of the measured turbulent heat flux also depend strongly on the direction of flow relative to surface heating. However, these spatial differences are smoothed by the shear layer at the canyon top. Buoyancy effects from the heated walls were not seen to have as large an impact on the measured flow field as has been shown in numerical experiments. At night canyon walls are shown to be the source of positive sensible heat fluxes. The measurements show that materials and their location, as well as geometry, play a role in regulating the heat exchange between the urban surface and atmosphere.     

Scalar Fluxes from Urban Street Canyons Part II: Model

A practical model is developed for the vertical flux of a scalar, such as heat, from an urban street canyon that accounts for variations of the flow and turbulence with canyon geometry. The model gives the magnitude and geometric dependence of the flux from each facet of the urban street canyon, and is shown to agree well with wind-tunnel measurements described in Part I. The geometric dependence of the flux from an urban street canyon is shown to be determined by two physical processes. Firstly, as the height-to-width ratio of the street canyon increases, so does the roughness length and displacement height of the surface. This increase leads to a reduction in the wind speed in the inertial sublayer above the street canyons. Since the speed of the circulations in the street are proportional to this inertial sublayer wind speed, the flux then reduces with the inertial sublayer wind speed. This process is dominant at low height-to-width ratios. Secondly, the character of the circulations within the street canyon also varies as the height-to-width ratio increases. The flow in the street is partitioned into a recirculation region and a ventilated region. When the street canyon has high height-to-width ratios the recirculation region occupies the whole street canyon and the wind speeds within the street are low. This tendency decreases the flux at high height-to-width ratios. These processes tend to reduce the flux density from the individual facets of the street canyon, when compared to the flux density from a horizontal surface of the same material. But the street canyon has an increased total surface area, which means that the total flux from the street canyon is larger than from a horizontal surface. The variations in scalar flux from an urban street canyon with geometry is over a factor of two, which means that the physical mechanisms responsible should be incorporated into energy balance models for urban areas.     

Radiative Exchange in an Urban Street Canyon

The influence of building geometry on the radiation terms ofthe surface energy balance is a principal reason for surfacetemperature differences between rural and urban areas.Methods exist to calculate the radiation balance in an urban area,but their validity across the range of urban geometries andmaterials has not been carefully considered.Here the exchange of diffuse radiation in an urban street canyon isinvestigated using a method incorporating all reflections of radiation.This exact solution is compared to two commonly used approximationsthat retain either no reflections, or just one reflection of radiation.The area-averaged net radiative flux density from the facets of the canyondecreases in magnitude monotonically as the canyon aspect ratio increases.The two approximate solutions possess unphysical differences from thismonotonic decrease for high canyon aspect ratios or low materialemissivities/high material albedos.The errors of the two approximate solutions are small for near blackbodymaterials and small canyon aspect ratios but can be an order ofmagnitude for intermediate material properties and deep street canyons.Urban street canyon models need to consider at least one reflectionof radiation and multiple reflections are desirable for full applicability.     

不均匀地表产生的中尺度通量的数值试验

采用北京大学三维的复杂地形中尺度模式,结合陆面过程模式(SiB),模拟了草原和沙漠并存的下垫面的边界层大气运动.利用SiB模式计算了地表辐射、感热、潜热通量,并且预报地表温度.中尺度模式则模拟了沙漠地区受热抬升,形成的辐合运动,垂直速度的分布,不同高度上水平流场的变化以及中尺度动量和热量通量,把中尺度通量跟湍流通量进行了比较,以确定这种中尺度运动在GCM模式的参数化过程中的重要性.试验表明中尺度通量尤其是热量通量要比湍流通量大很多.     

Long-wave radiative flux divergence and nocturnal cooling of the urban atmosphere

Results from measurements of long-wave radiative flux divergence on calm, cloudless nights from within an urban canyon are presented. Results show the existence of three-dimensional (volume) radiative divergence in the canyon-air system. The results are compared with air temperature cooling rates (energy storage change) in the canyon. With calm, cloudless conditions, radiative divergence is the dominant mechanism controlling air temperature changes. In the early part of the night this results in air cooling, but radiative warming was commonly observed after midnight. Canyon-air volume radiative divergence is found to be considerably less than that observed previously above roof-level, and in rural areas. Measured cooling rates in the canyon are poorly predicted by a surface-oriented cooling approach. This again points to the importance of the atmospheric radiation balance.     

Simulation of surface urban heat islands under ‘IDEAL’ conditions at night part 1: Theory and tests against field data

Observations show that the urban heat island in the atmospheric layer below roof level is most strongly developed during calm, cloudless conditions at night. This paper outlines two versions of a numerical model to describe the cooling of rural and street canyon surfaces under these conditions using surface thermal and radiative properties and the radiative geometry of the canyons. One version uses a full system of differential equations and the other the simpler force-restore approach. The two approaches are shown to be in general agreement and the output of the simpler model is shown to give a faithful representation of cooling of rural and urban surfaces, and therefore heat islands, when compared with field observations.     

Simulation of a Summer Urban Breeze Over Paris

Numerical simulations for an anticyclonic summer episode in the Paris area have been performed at the meso- scale for a 48-hour period, and compared to observations from a dense operational observational network. The meteorological stations have been classified, according to the extent of urbanization of their surroundings, into four classes (central Paris, urban, suburban, and rural). The atmospheric model, coupled with an urban surface scheme, correctly reproduces the temperature (within 1 K from the observations) and humidity. The intense urban heat island during the night is also well represented.Following the validation, the model is used to quantify atmospheric effects of Paris on the boundary layer, through a comparison with a purely rural simulation. At night, the model simulates a neutral or even slightly unstable boundary layer to a depth of 200 m over the city. In contrast, a very stable layer formed in the countryside. During the day, the boundary layer was more turbulent and 500 m deeper over Paris; vertical velocities of up to 1 m s^-1 were created over the city. This leads to an urban breeze with convergence at low levels (with winds around 5 to 7 m s^-1), and divergence at the boundary-layer top (with similar wind speeds). The horizontal extent of the breeze reaches for more than 50 km from the city centre, and could have an important impact on pollutant diffusion in the area for calm days.Finally, three other spring cases are presented briefly. These show that an urban breeze develops if the synoptic wind is weak enough or disorganized; an urban plume develops otherwise.     

A Double-Canyon Radiation Scheme for Multi-Layer Urban Canopy Models

We develop a double-canyon radiation scheme (DCEP) for urban canopy models embedded in mesoscale numerical models based on the Building Effect Parametrization (BEP). The new scheme calculates the incoming and outgoing longwave and shortwave radiation for roof, wall and ground surfaces for an urban street canyon characterized by its street and building width, canyon length, and the building height distribution. The scheme introduces the radiative interaction of two neighbouring urban canyons allowing the full inclusion of roofs into the radiation exchange both inside the canyon and with the sky. In contrast to BEP, we also treat direct and diffuse shortwave radiation from the sky independently, thus allowing calculation of the effective parameters representing the urban diffuse and direct shortwave radiation budget inside the mesoscale model. Furthermore, we close the energy balance of incoming longwave and diffuse shortwave radiation from the sky, so that the new scheme is physically more consistent than the BEP scheme. Sensitivity tests show that these modifications are important for urban regions with a large variety of building heights. The evaluation against data from the Basel Urban Boundary Layer Experiment indicates a good performance of the DCEP when coupled with the regional weather and climate model COSMO-CLM.     

An Improved Three-Dimensional Simulation of the Diurnally Varying Street-Canyon Flow

The impact of diurnal variations of the heat fluxes from building and ground surfaces on the fluid flow and air temperature distribution in street canyons is numerically investigated using the PArallelized Large-eddy Simulation Model (PALM). Simulations are performed for a 3 by 5 array of buildings with canyon aspect ratio of one for two clear summer days that differ in atmospheric instability. A detailed building energy model with a three-dimensional raster-type geometry—Temperature of Urban Facets Indoor-Outdoor Building Energy Simulator (TUF-IOBES)—provides urban surface heat fluxes as thermal boundary conditions for PALM. In vertical cross-sections at the centre of the spanwise canyon the mechanical forcing and the horizontal streamwise thermal forcing at roof level outweigh the thermal forces from the heated surfaces inside the canyon in defining the general flow pattern throughout the day. This results in a dominant canyon vortex with a persistent speed, centered at a constant height. Compared to neutral simulations, non-uniform heating of the urban canyon surfaces significantly modifies the pressure field and turbulence statistics in street canyons. Strong horizontal pressure gradients were detected in streamwise and spanwise canyons throughout the day, and which motivate larger turbulent velocity fluctuations in the horizontal directions rather than in the vertical direction. Canyon-averaged turbulent kinetic energy in all non-neutral simulations exhibits a diurnal cycle following the insolation on the ground in both spanwise and streamwise canyons, and it is larger when the canopy bottom surface is paved with darker materials and the ground surface temperature is higher as a result. Compared to uniformly distributed thermal forcing on urban surfaces, the present analysis shows that realistic non-uniform thermal forcing can result in complex local airflow patterns, as evident, for example, from the location of the vortices in horizontal planes in the spanwise canyon. This study shows the importance of three-dimensional simulations with detailed thermal boundary conditions to explore the heat and mass transport in an urban area.     

Sensitivity study and validation of a land surface parameterization using the HAPEX-MOBILHY data set

A simple parameterization of land surface processes, amenable to the structure of a two-layer soil model, including a representation of the vegetation, has been designed for use in meteorological models. Prior to implementation in a mesoscale model, it is necessary to check the components and to verify the good working order of the parameterization as a whole. The aims of this paper then are: (i) evaluation and a sensitivity study of the various components of the model, specifying the needed accuracy for the parameters; (ii) micrometeorological validation of the model against the HAPEX-MOBILHY data set.First, we present the basic scheme. The focus is on the parameterization of surface resistance, and especially on its relationship with soil moisture.A sensitivity study is then performed through a set of one-dimensional simulations which allow a full interaction between the ground and the atmosphere. Above bare ground, it is shown that both soil texture and initial moisture greatly influence the outcome of the simulation. Latent heat flux ranges from that associated with potential evaporation through a switch-like behavior to that of dry soil. Next, the effects of transpiring vegetation canopies on the physical processes involved and the surface energy balance are examined. The sensitivity of the latent heat flux to changes in the soil and canopy parameters is emphazised; the major influence of the initial mean soil moisture and of the vegetation cover is pointed out. Finally, the evolution of the boundary layer in response to various surface conditions is studied.A validation of the land surface scheme is conducted through daily cycles during cloudless days. Simulated turbulent fluxes are successfully compared to micrometeorological measurements over a maize field at different growth stages. Over a pine forest, the correct simulation of the turbulent fluxes is obtained with an adequate parameterization of the surface resistance accounting for the atmospheric moisture deficit.     

Intelligent urban forms (IUF) a new climate-concerned,urban planning strategy

Summary Adaptive geometrical configurations are presented, which aim to create intelligent urban forms, and which include screening methods applicable to the linear- and grid-type building layouts. They are especially suitable for mid-latitude cities characterized by seasonally swinging climates which necessitate heating in winter and cooling in summer. The screens are envisaged as shading devices in the summer blocking the incoming solar radiation during day-time (that is, on-position), while being removed at night to enhance nocturnal radiative cooling (that is, off-position). In winter they are assumed to be in off-position during sunshine hours to promote the access of solar radiation and in on-position at night to obstruct the sky energy sink and reduce radiant heat losses. Their implications for the urban street canyon climate and the thermal performance of the built environment are simulated using the cluster thermal time constant (CTTC) model. The diurnal variation of both the ambient air temperature and net radiant flux within the urban canopy layer serve as criteria by which the climatetempering effectiveness of the screens is assessed.With 9 Figures     

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.