An analytical study on the urban boundary layer

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The New Canadian Urban Modelling System: Evaluation for Two Cases from the Joint Urban 2003 Oklahoma City Experiment

A new Canadian numerical urban modelling system has been developed at the Meteorological Service of Canada to represent surface and boundary-layer processes in the urban environment. In this system, urban covers are taken into account by including the Town Energy Balance urban-canopy parameterization scheme in the Global Environmental Multiscale meteorological model. The new modelling system is run at 250-m grid size for two intensive observational periods of the Joint Urban 2003 experiment that was held in Oklahoma City, U.S.A. An extensive evaluation against near-surface and upper-air observations has been performed. The Town Energy Balance scheme correctly simulates the urban micro-climate, more particularly the positive nighttime urban heat island, and also reproduces the “cool” island during the morning but does not succeed in maintaining it during all of the daytime period. The vertical structure of the boundary layer above the city is reasonably well simulated, but the simulation of the nocturnal boundary layer is difficult, due to the complex interaction with the nighttime southerly low-level jet that crosses the domain. Sensitivity tests reveal that the daytime convective boundary layer is mainly driven by dry soil conditions in and around Oklahoma City and that the nighttime low-level jet reinforces the urban heat island in the first 300m through large-scale advection, leading to the development of a less stable layer above the city.     

不稳定层结下的热岛环流

采用对数压力坐标系的大气热力、动力方程组，分析了由于城市的加热和摩擦作用，在大气层结不稳定情况下的热岛环流，给出了表征热岛基本特征的垂直运动，水平运动和温度场的空间结构，从理论上证实了热岛环流在不稳定城市边界层中存在的可能性，并得出了如下主要结论:(1) 垂直运动在市区是上升运动，在郊区是下沉运动，在低层z=150m处有一闭合中心；(2) 流场在市区上空呈一层波动，波谷在上风区，波峰在下风区，波长为城市半宽的4倍；(3) 地面的高温区出现在城市的下风区，且无逆温层出现。     

夜间城市大气边界层和气溶胶的相互作用

本文利用能量闭合的二维非线性、非定常模式，结合地面热量平衡方程，研究了夜间城市边界层和气溶胶的相互作用问题。结果表明:气溶胶在夜间对大气低层起保温作用，对大气上层起冷却作用；使大气低层稳定度减小，上层稳定度增加；此外，气溶胶还能够削弱贴地逆温强度。在正常城市气溶胶污染情况下，气溶胶对城市热岛强度影响不大，但可使城市热岛环流稍有增加。夜间城市边界层对气溶胶的反馈作用使大气下层气溶胶浓度减小，上层气溶胶浓度增加。上述部分结论得到了在天津取得的城市热岛观测资料的直接验证。     

Measurements Of Thermal Updraft Intensity Over Complex Terrain Using American White Pelicans And A Simple Boundary-Layer Forecast Model

An examination of boundary-layer meteorological and avian aerodynamic theories suggests that soaring birds can be used to measure the magnitude of vertical air motions within the boundary layer. These theories are applied to obtain mixed-layer normalized thermal updraft intensity over both flat and complex terrain from the climb rates of soaring American white pelicans and from diagnostic boundary-layer model-produced estimates of the boundary-layer depth z_i and the convective velocity scale w_*. Comparison of the flatland data with the profiles of normalized updraft velocity obtained from previous studies reveals that the pelican-derived measurements of thermal updraft intensity are in close agreement with those obtained using traditional research aircraft and large eddy simulation (LES) in the height range of 0.2 to 0.8 z_i. Given the success of this method, the profiles of thermal vertical velocity over the flatland and the nearby mountains are compared. This comparison shows that these profiles are statistically indistinguishable over this height range, indicating that the profile for thermal updraft intensity varies little over this sample of complex terrain. These observations support the findings of a recent LES study that explored the turbulent structure of the boundary layer using a range of terrain specifications. For terrain similar in scale to that encountered in this study, results of the LES suggest that the terrain caused less than an 11% variation in the standard deviation of vertical velocity.     

Effects of Thermal Stability and Incoming Boundary-Layer Flow Characteristics on Wind-Turbine Wakes: A Wind-Tunnel Study

Wind-tunnel experiments were carried out to study turbulence statistics in the wake of a model wind turbine placed in a boundary-layer flow under both neutral and stably stratified conditions. High-resolution velocity and temperature measurements, obtained using a customized triple wire (cross-wire and cold wire) anemometer, were used to characterize the mean velocity, turbulence intensity, turbulent fluxes, and spectra at different locations in the wake. The effect of the wake on the turbulence statistics is found to extend as far as 20 rotor diameters downwind of the turbine. The velocity deficit has a nearly axisymmetric shape, which can be approximated by a Gaussian distribution and a power-law decay with distance. This decay in the near-wake region is found to be faster in the stable case. Turbulence intensity distribution is clearly non-axisymmetric due to the non-uniform distribution of the incoming velocity in the boundary layer. In the neutral case, the maximum turbulence intensity is located above the hub height, around the rotor tip location and at a distance of about 4–5.5 rotor diameters, which are common separations between wind turbines in wind farms. The enhancement of turbulence intensity is associated with strong shear and turbulent kinetic energy production in that region. In the stable case, the stronger shear in the incoming flow leads to a slightly stronger and larger region of enhanced turbulence intensity, which extends between 3 and 6 rotor diameters downwind of the turbine location. Power spectra of the streamwise and vertical velocities show a strong signature of the turbine blade tip vortices at the top tip height up to a distance of about 1–2 rotor diameters. This spectral signature is stronger in the vertical velocity component. At longer downwind distances, tip vortices are not evident and the von Kármán formulation agrees well with the measured velocity spectra.     

Spatial variation of the evolution and structure of the urban boundary layer

The spatial variation of the nocturnal urban boundary-layer structure is described and the time variation of the mixing height, and the nocturnal inversion top and strength after sunrise is presented for urban sites located upwind, downwind, and near the center of the heat island, and at an upwind rural site. Observations were derived from high resolution temperature profiles obtained by a helicopter during 35 intensive morning experiments in St. Louis, Missouri.The nocturnal urban boundary layer increased in depth from the upwind edge of the urban area. Far downwind, in suburban and rural areas, a remnant of the urban boundary layer existed between a stable surface-based layer and an upper inversion that resembled the upwind rural inversion.The mixing height (base of the inversion) evolved in a parabolic manner after sunrise at the urban locations. A rise in the inversion top after sunrise at the urban locations is believed to be due to low-level convergence which caused the entire inversion layer to be lifted. Due to large horizontal temperature gradients associated with the urban heat island, cold air advection tended to counteract the urban-induced lifting effect by inhibiting mixing-height growth at urban locations upwind of the heat-island center. Advection also caused the maximum height and fastest growth rate of the urban mixed layer to be shifted downwind of the urban area with time. However, mean mixing-height growth rates at various urban locations did not differ significantly. The rural mixing-height growth rate was about twice as large as urban values for up to 3 hr after sunrise. Spatial differences in the mixing height became small near the time of inversion dissipation, which appeared to occur at about the same time at all locations.On assignment from the National Oceanic and Atmospheric Administration, U.S. Department of Commerce.     

Turbulent transport from an arctic lead: A large-eddy simulation

The upward transfer of heat from ocean to atmosphere is examined for an Arctic lead, a break in the Arctic ice which allows contact between the cold atmosphere and the relatively warm ocean. We employ a large-eddy model to compute explicitly the three-dimensional turbulent response of the atmosphere to a lead of 200 m width. The surface heat flux creates a turbulent plume of individual quasi-random eddies, not a continuous updraft, which penetrate into the stable atmosphere and transport heat upward.Maximum updraft velocities and turbulence occur downwind of the lead rather than over the lead itself, because the development time of an individual thermal eddy is longer than its transit time across the lead. The affected vertical region, while shallow over the lead itself, grows to a height of 65m at 600 m downwind of the lead; beyond that, the depth of the turbulent region decreases as the eddies weaken. The maximum vertical turbulent heat flux occurs at the downwind edge of the lead, beyond which a relative maximum extends upward into the plume. Negative surface heat flux immediately downwind of the lead creates a growing stable layer, but above that internal boundary layer the turbulent heat flux is still positive. Updraft maxima are typically 28 cm/s, but compensating downdrafts result in time-averaged vertical velocities of less than 1 cm/s in the plume. Conditional sampling separates the updraft and downdraft contributions. Formulas for the horizontal eddy development distance and for the vertical plume penetration height are presented. The relative importance of mean and turbulent transport is compared for both vertical and horizontal heat transfer: turbulence dominates the vertical heat transport whereas mean advection dominates the horizontal transport, these offsetting transports producing a quasi-stationary state.     

北京海淀地区大气边界层的数值模拟研究

采用北京大学的三维复杂地形中尺度数值模式,利用Landsat-TM卫星影像图得到的地表状况计算陆气之间的能量平衡过程,模拟了北京海淀地区大气边界层的风、温场结构以及污染物浓度分布,进而模拟了由于汽车尾气的排放而转化成的气溶胶的浓度分布.模拟表明城郊之间存在热岛效应,边界层风场受到热岛的热力作用以及地形的动力作用影响.污染物及气溶胶浓度也存在城乡差别,最大浓度出现在城区的下风方向.     

Boundary-Layer Adjustment Over Small-Scale Changes of Surface Heat Flux

Four months of eddy correlation data collected over a grass field and a nearby sage brush community are analyzed to examine the adjustment of the boundary-layer structure as it flows from the heated brush to the snow-covered grass. The grass site includes a 34-m tower with seven levels of eddy correlation data. The midday heat flux over the snow-covered grass and bare ground surfaces is often downward particularly with melting conditions, while the corresponding heat flux over the brush is almost always upward. For most of these cases, a stable internal boundary layer over the snow is well defined in terms of vertical profiles of the buoyancy flux over the snow-covered grass. The stable internal boundary layer is generally embedded within a deeper layer of flux divergence corresponding to increasing upward heat flux with height above the internal boundary layer. With thin snow cover, the surface heat flux over the grass is weak upward due to heating of grass protruding above the snow so that the flow adjusts to a decrease of the upward surface heat flux in the downwind direction. This common case of an adjusting boundary layer contrasts with the formation of an internal boundary layer due to a change of sign of the surface heat in flux the downwind direction. The adjustment of the boundary layer to the decrease of the surface heat flux leads to vertical divergence of the upward heat flux in contrast to the usual heated boundary layer over homogeneous surfaces. The consequences of the cooling due to the vertical divergence of the heat flux are discussed in terms of the heat budget of the adjusting and internal boundary layers.     

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...     

中国地区城市热岛环流研究进展

城市热岛环流是由城市热岛激发的、存在于边界层的局地环流。这一环流常影响城市区域污染物的扩散及城市降水,是城市气象学的重点研究对象。在过去30多年间,中国城市热岛环流的研究不断取得突破,研究成果丰富。为了今后更有效地开展相关研究,本文从观测分析、理论与数值模拟研究两方面,回顾并总结了我国在热岛环流研究方面取得的成果,包括热岛环流的时空特征、产生机制,以及热岛环流与其他局地环流的相互作用。此外,本文总结了热岛环流对城市及其周边地区污染物水平和垂直分布的影响。     

Formation of horizontal convective rolls in urban areas

The formation of horizontal convective rolls (HCRs) in urban areas is investigated in this paper using observations and fine-scale numerical simulations. Cloud streets organized parallel to the mean boundary-layer wind (a manifestation of HCRs) are seen in the Fengyun-2C satellite imagery around local noon in Beijing. Observed vertical velocity and horizontal wind fields from an urban wind profiler suggest that the time scale for alternating updraft and downdraft in the boundary layer is about 30 min, and the length of the updraft/downdraft is about 9 km. Numerical simulations show that most HCRs occur in the urban areas with − z_i / L < 25 (z_i: the boundary-layer depth, L: the Monin–Obukhov length). Sensitivity tests reveal that HCRs are common in urban boundary layers, while rural areas are more conducive to forming cellular convection; the aspect ratio of HCRs in urban areas is smaller than the typical value over natural landscapes.     

城市大气边界层的数值研究

本文利用能量闭合的二维非线性、非定常模式,结合地面热量平衡方程,研究了城市热岛和城市热岛环流以及其它相关的城市边界层参量的发展演变规律;同时探讨了在城市污染面源的情况下,污染物的扩散、输送问题.模拟结果得到了天津市城市热岛观测资料的证实.     

Urban impacts on precipitation

Weather and climate changes caused by human activities (e.g., greenhouse gas emissions, deforestation, and urbanization) have received much attention because of their impacts on human lives as well as scientific interests. The detection, understanding, and future projection of weather and climate changes due to urbanization are important subjects in the discipline of urban meteorology and climatology. This article reviews urban impacts on precipitation. Observational studies of changes in convective phenomena over and around cities are reviewed, with focus on precipitation enhancement downwind of cities. The proposed causative factors (urban heat island, large surface roughness, and higher aerosol concentration) and mechanisms of urban-induced and/or urban-modified precipitation are then reviewed and discussed, with focus on downwind precipitation enhancement. A universal mechanism of urban-induced precipitation is made through a thorough literature review and is as follows. The urban heat island produces updrafts on the leeward or downwind side of cities, and the urban heat island-induced updrafts initiate moist convection under favorable thermodynamic conditions, thus leading to surface precipitation. Surface precipitation is likely to further increase under higher aerosol concentrations if the air humidity is high and deep and strong convection occurs. It is not likely that larger urban surface roughness plays a major role in urbaninduced precipitation. Larger urban surface roughness can, however, disrupt or bifurcate precipitating convective systems formed outside cities while passing over the cities. Such urban-modified precipitating systems can either increase or decrease precipitation over and/or downwind of cities. Much effort is needed for in-depth or new understanding of urban precipitation anomalies, which includes local and regional modeling studies using advanced numerical models and analysis studies of long-term radar data.     

Numerical simulation of the anthropogenic heat effect on urban boundary layer structure

In this paper, several methods of incorporating anthropogenic heat release into the boundary layer are compared. The best scheme was one that included anthropogenic heat release in both the surface energy balance equation and the thermodynamic equations. In addition, it included diurnal variations and a distribution of heat based on building concentrations. We further investigated the influence of anthropogenic heat release on urban boundary layer structure and the urban heat island, and found that the contribution of anthropogenic heat release to the urban heat island is greatest in the evening and at night, and least at noon. The daily average contribution ratio of anthropogenic heat to urban heat island intensity in the winter is 54.5%, compared with just 43.6% in the summer. Anthropogenic heat strengthens the vertical movement of urban surface air flow, changing the urban heat island circulation. It also makes the urban boundary layer more turbulent and unstable, especially in the morning and evening. The degree of influence of anthropogenic heat release on local boundary layer structure depends on its importance to the surface energy budget.     

The Ekman boundary layer over orography: An analysis of vertical motion

A model of the planetary boundary layer is used to determine the field of vertical motion over large-scale orography. This model represents Ekman boundary-layer dynamics modified by the inclusion of accelerations of the geostrophic wind under the geostrophic momentum approximation. The orography is represented by a circular mountain. The inviscid solution is provided by the sum of a constant translation and a steady, uniform potential vorticity, anticyclonic vortex. The boundary-layer solution vanishes on the mountain, but is matched to the inviscid solution as the top of the boundary layer is approached. The vertical velocity field at the top of the boundary layer is determined by integration of the continuity equation. The field of motion is largely determined by descent from above into the anticyclonic circulation, as in the classical Ekman model. Contributions that arise from the inclusion of accelerations are associated with boundary-layer advection and ageostrophic divergence that produce vorticity tendencies. Finally, the boundary-layer vertical motion is shown to be comparable in magnitude to the vertical motion forced by inviscid flow over the orography, although the distributions of each are significantly different. Effects of mountain asymmetry and a changing pressure field, that can be treated more fully by numerical model simulations, are not considered in the present study.On leave at the University of Colorado, 1990.     

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。     

北京夏季城市热岛现状及楔形绿地规划对缓解城市热岛的作用

通过遥感技术与地面测定相结合的方法，对北京城市热岛现状作观测研究，得到北京城市地面的温度分布特点。使用北京大学城市边界层模式从气象观点就“楔形绿地”规划对北京城市气候的影响进行研究和评价，模式通过对城市地表复杂性和多样性的特征进行细致描述，建立了一个细致模拟城市特点的城市边界层能量平衡模式，并用此能量平衡模式得到的地面温度作为下边界条件，中尺度气象模式MM5做初始场和侧边界条件，建立一个最小分辨率为500 m的城市边界层模式系统，来研究城市边界层在中尺度背景场作用下的精细结构。通过个例模拟，模式能够较准确地模拟城市边界层的风温场分布情况，可以用来对楔形绿地规划进行模拟试验。通过对规划后的气象场在特定的气象条件下进行模拟，结果显示，建造大型的楔形绿地后，绿地区域及绿地周围约1 km以内的地区温度有所降低，降低的程度由规划前后的地表类型改变的剧烈程度、风速大小及与绿地的距离决定，但是这种规划方案却会因城市的下风方向的风速减小而导致通风不畅。