An intra-urban thermal breeze in Göteborg, Sweden

Summary ?The existence and character of an intra-urban thermal breeze (IUTB) in the city centre of G?teborg, Sweden was examined. The study was carried out in a 10 ha open area covered with gravel, asphalt and grass, surrounded by narrow street canyons. Measurements included an array of fixed wind and temperature sensors and smoke in the open area; temperature of the surrounding neighbourhood from mobile thermal mapping and fixed temperature sensors; standard climate measurements from four automatic weather stations in and around G?teborg. The results based on sixteen field surveys performed between December 1998 and March 2000 showed that the open urban area might generate a special type of IUTB under certain conditions. The IUTB was shown to be a rare nocturnal winter phenomenon developing during clear and calm weather conditions, with snow covered ground, strong ground based inversions and high stability (Ri _b  > 1). The most important driving forces for the generation of an IUTB were large intra-urban horizontal air temperature differences and strong atmospheric stability which led to decoupling of the canopy layer atmosphere from wind flow aloft. In order to estimate the physical principles underlying an IUTB of this type a simple 1.5-dimensional model was devised. Results indicated that the horizontal intra-urban air temperature difference recorded in G?teborg was enough to develop an IUTB. Received May 6, 2002; revised November 20, 2002; accepted January 7, 2003 Published online May 20, 2003     

城市短时强降雨中尺度观测特征分析

通过对2009年5月9日、2007年7月18日和2006年8月28日邢台市3次城市短时强降雨过程的对比分析,应用高时空分辨率的加密自动站资料、雷达风廓线资料,研究其中尺度特征.强降水天气产生在大尺度有利的环流背景下,低层切变线系统、强中低空垂直风切变、地面低压倒槽提供了有利的辐合上升动力条件,低层西南暖湿气流与西风带冷...     

Investigation of temporal-spatial parameters of an urban heat island on the basis of passive microwave remote sensing

Summary Quantitative measurements of the impact of an urban environment on the thermal state of the atmospheric boundary layer are presented. Temperature profiles up to the height of 600 m were obtained in a continuous series of measurements by three microwave profilers MTP-5 located in different areas of Moscow. The influence of this large city on urban heat island (UHI) parameters was estimated on occasions with stationary atmospheric processes and during cases with frontal passage. Two types of UHI were identified: one with a dome of urban warmth at all levels, and another with a low warm dome in combination with a lens of cold air above.     

下垫面对郑州城市边界层风的影响

根据１９９３年１月和７月郑州市中心测点和南郊郑州气象观测站同步观测的地面和边界层气象资料,分析了郑州城市下垫面对边界层风的影响。结果表明,在地面气压梯度比较小且天气晴朗条件下,郑州存在城市热岛环流。受城市热岛环流影响,郊区地面风向指向市区;边界层８００ｍ以下,城市上空吹偏西风时同一高度上南郊风向偏于城市风向左侧,吹偏东风时同一高度上南郊风向偏于市区风向右侧;１２００ｍ以上,城市上空吹偏西风时,同一高度上南郊风向偏于城市风向右侧     

达州市2016年元旦期间持续重污染天气气象条件分析

利用空气质量监测资料、高空和地面气象观测资料、NCEP再分析资料,对达州市2016年元旦节期间重污染天气过程特征及气象条件进行分析。结果表明:达州市此次重污染天气过程为长时间无冷空气活动,无降雨,大气污染物不断积聚形成。AQI日变化受污染源排放情况影响更大,早上低,白天逐渐增加,天黑后达到峰值。大气污染物的积累一般发生大气稳定度为中性或以上。AQI与08时和17时混合层厚度负相关,但日平均混合层厚度与AQI没有通过相关性检验。重污染时近地面有逆温层且逆温层较厚。AQI与逐日最高气温、日平均风速和日最大风速正相关,降雨对大气污染物稀释作用明显,特别是降雨持续时间长,雨量大效果更为显著。AQI逐时变化与温度正相关,与风速负相关。      

A Comparative Study and Evaluation of Mixing-Height Estimation Based on Sodar-RASS, Ceilometer Data and Numerical Model Simulations

The unmanned aerial system SUMO (Small Unmanned Meteorological Observer) has been used for the observation of the structure and behaviour of the atmospheric boundary layer above the Advent Valley, Svalbard during a two-week period in early spring 2009. Temperature, humidity and wind profiles measured by the SUMO system have been compared with measurements of a small tethered balloon system that was operated simultaneously. It is shown that both systems complement each other. Above 200?m, the SUMO system outperforms the tethered balloon in terms of flexibility and the ability to penetrate strong inversion layers of the Arctic boundary layer. Below that level, the tethered balloon system provides atmospheric profiles with higher accuracy, mainly due to its ability to operate at very low vertical velocities. For the observational period, a numerical mesoscale model has been run at high resolution and evaluated with SUMO profiles reaching up to a height of 1500?m above the ground. The sensitivity to the choice of atmospheric boundary-layer schemes and horizontal resolution has been investigated. A new scheme especially suited for stable conditions slightly improves the temperature forecast in stable conditions, although all schemes show a warm bias close to the surface and a cold bias above the atmospheric boundary layer. During one cold and cloudless night, the SUMO system could be operated nearly continuously (every 30?C45?minutes). This allowed for a detailed case study addressing the structure and behaviour of the air column within and above Advent Valley and its interaction with the local topography. The SUMO measurements in conjunction with a 10-m meteorological mast enabled the identification of a very stable nocturnal surface layer adjacent to the valley bottom, a stable air column in the valley and a strong inversion layer above the summit height. The results indicate the presence of inertial-gravity waves during the night, a feature not captured by the model.     

Meteorological prerequisites of formation of severe wintertime air pollution episodes in Moscow

Large-scale and local weather conditions during severe wintertime air pollution episodes in the Moscow megalopolis are analyzed. Concentrations of CO, NO, and NO₂ obtained from the automated network of the atmosphere pollution control are used as tracers for atmospheric processes in the urban atmospheric boundary layer. It is shown that a high surface air pollution level in the city is formed at a weak wind in the lower atmosphere and only in the presence of a surface or low elevated temperature inversion. Temperature contrasts in the urban heat island generate the circulation that promotes air pollution in megapolis regions remote from large emission sources. It is supposed that in case of severe frosts the amount of anthropogenic heat in the megapolis sharply increases, promoting active turbulent mixing, thus preventing pollution accumulation in the surface air.     

北京奥运会开幕式期间的中尺度天气系统研究

利用北京地区稠密的地面观测网资料以及北京市观象台、海淀、上甸子3部风廓线仪的观测资料,通过分析2008年8月8日北京奥运会开幕式期间发生在北京地区的降水过程,讨论了此次过程中在环境风场、地形和城市热岛作用下,中尺度系统发展或减弱的可能机制及对城区降水的影响。结果表明:城市热岛和地形作用形成的次级环流圈对城区南北两侧的影响不同,在城区南侧,次级环流圈使南风减弱,同时受次级环流圈下沉气流影响,中尺度系统北上时会减弱;在城区北侧,次级环流圈使南风加强,中尺度系统南下时会使气流辐合增强,有利于中尺度系统发展。当环境风场是较弱的偏南风时,城市热岛-地形次级环流圈在城区1500 m以下形成辐合,以上辐散,在3000 m左右辐散最强,不利于北上的中尺度系统向城区发展形成降水。     

基于激光雷达资料的气溶胶辐射效应研究

利用新型激光雷达气溶胶探测资料及综合数值模式,以地形复杂的兰州市及周边地区冬季典型天气形势下的大气边界层为研究对象,通过理想试验模拟研究了城市气溶胶辐射效应与大气边界层的相互作用。结果表明:夜间,低空(50~600 m)气溶胶所在气层冷却效应明显,温度降低0.13~0.18℃,600 m高度以上,气溶胶浓度较低,其冷却效应较小,温度降低不足0.1℃;白天,受气溶胶短波辐射效应影响,边界层内增温明显,增温最大值位于低层脱地逆温层顶300 m高度附近,600 m以上由于气溶胶浓度减小,加热率亦降低,增温由0.2℃减至0.1℃。此外,气溶胶的存在使得所在层的风速降低。可见,激光雷达探测资料在边界层模式中有很好的应用价值,对于研究气溶胶辐射效应的大气边界层响应有重要意义。     

Thermal Structure of a Coastal–Urban Boundary Layer

We use various temperature profilers located in and around New York City to observe the structure and evolution of the thermal boundary layer. The primary focus is to highlight the spatial variability of potential-temperature profiles due to heterogeneous surface forcing in an urban environment during different flow conditions. Overall, the observations during the summer period reveal the presence of thermal internal boundary layers due to the interaction between the marine atmospheric boundary layer and the convective urban environment. The summer daytime potential-temperature profiles within the city indicate a superadiabatic layer is present near the surface beneath a mildly stable layer. Large spatial variability in the near-surface (0–300 m) potential temperature is detected, with the thermal profile in the lower atmosphere uniquely determined by the underlying surface forcing and the distance from the coast. The summer and winter average night-time potential-temperature profiles show that the atmosphere is still convective near the surface. The seasonal averages of mixing ratio show large variability in the vertical direction.     

Quantifying the influence of land-use and surface characteristics on spatial variability in the urban heat island

The urban thermal environment varies not only from its rural surroundings but also within the urban area due to intra-urban differences in land-use and surface characteristics. Understanding the causes of this intra-urban variability is a first step in improving urban planning and development. Toward this end, a method for quantifying causes of spatial variability in the urban heat island has been developed. This paper presents the method as applied to a specific test case of Portland, Oregon. Vehicle temperature traverses were used to determine spatial differences in summertime ~2 m air temperature across the metropolitan area in the afternoon. A tree-structured regression model was used to quantify the land-use and surface characteristics that have the greatest influence on daytime UHI intensity. The most important urban characteristic separating warmer from cooler regions of the Portland metropolitan area was canopy cover. Roadway area density was also an important determinant of local UHI magnitudes. Specifically, the air above major arterial roads was found to be warmer on weekdays than weekends, possibly due to increased anthropogenic activity from the vehicle sector on weekdays. In general, warmer regions of the city were associated with industrial and commercial land-use. The downtown core, whilst warmer than the rural surroundings, was not the warmest part of the Portland metropolitan area. This is thought to be due in large part to local shading effects in the urban canyons.     

北京市郊区及城区边缘的大气湍流结构特征

为了比较城市边缘地区和郊区大气边界层中湍流结构的差异,本文分析了1986年春、夏两季在北京市城市边缘地区和郊区两处大气湍流观测结果.结果表明,由于城市边缘地区的下垫表面具有较高数值的粗糙度,使其近地面层大气受到较强的动力影响,在其湍流速度分量谱曲线;无量纲化湍流速度分量标准偏差,σ_u/u*,σ_v/u*和σ_w/u*;以及无量纲化湍流耗散率φ_ε等湍流特征量在近中性层结条件下显示出与大气稳定度的关系比较微弱.     

Five years of thermal intra-urban monitoring in Florence (Italy) and application of climatological indices

This paper presents 5?years of detailed intra-urban thermal measurements carried out in Florence (Italy) by a network of 25 air temperature stations. Daily, hourly, and degree-day indices were applied to hourly data to evaluate the difference within the complex urban environment of Florence. Stations were grouped in four clusters with similar thermal regimes in each season. Our results evidence a mean difference of almost 2°C between the hottest and the coolest cluster in all seasons. Furthermore, the coolest cluster had on average more than ten frost days in winter and the hottest cluster more than 12 summer days in summer. The intra-urban difference of tropical nights was even more evident, with values of 42 versus 10?days between the hottest and the coolest cluster during the summer period. The results of this study contribute to quantify the thermal intra-urban differences in the city of Florence, suggesting important applications in plant phenology, aerobiology, human health, urban planning, and biometeorology.     

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.     

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.     

Observations of the barotropic Ekman layer over an urban terrain

Data from low-level soundings over Cambridge, U.S.A. were selected on the basis of an Ekman-like variation of the wind vector with altitude combined with evidence of a barotropic atmosphere. The method of geostrophic departure was used to determine the shear-stress distribution. The analysis yields the dimensionless properties of the barotropic Ekman layer under neutral and stable stratification. Some important results include: the geostrophic drag coefficient displays no dependence on the degree of static stability; the dimensionless height of the boundary layer decreases with increasing stability in agreement with the prediction of Zilitinkevich; the properties of the urban surface layer, where the roughness elements are multistory buildings, show no dependence on atmospheric stability under the moderate wind conditions which display the Ekman-like wind profile; and the directions of the horizontal shear stress and the vertical derivative of the velocity vector usually tend to be parallel only near the surface layer. Values of the two constants of the Rossby number similarity theory are found for the neutral barotropic Ekman layer at a surface Rossby number equal to 2 × 10⁵. The implications of the work with respect to wind-tunnel simulation of the flow over models of urban areas are discussed.     

BOUNDARY LAYER GROWTH AND LAPSE RATE CHANGES DETERMINED BY LIDAR AND SURFACE HEAT FLUX IN SOFIA*

In this study the results from a boundary layer experiment,conducted in autumn 1991 over a flat,build-up urban area in Southeast Sofia,together with some models for mixed layer growth rates are used to investigate the layered structure of the vertical atmospheric stability distribution in the Sofia Valley.Lidar measurements of aerosol layer heights and morning boundary layer development are combined with surface eddy correlation measurements of kinematic heat and moisture fluxes,profiles of temperature and humidity,wind speed and wind direction.A diagnostic method is presented for determining vertical lapse rates using surface meteorological measurements and lidar returns observed during the transition from nighttime stable stratification to daytime convective boundary layer after the sunrise.     

BOUNDARY LAYER GROWTH AND LAPSE RATE CHANGES DETERMINED BY LIDAR AND SURFACE HEAT FLUX IN SOFIA

In this study the results from a boundary layer experiment,conducted in autumn 1991 over a flat,build-up urbanarea in Southeast Sofia,together with some models for mixed layer growth rates are used to investigate the layered struc-ture of the vertical atmospheric stability distribution in the Sofia Valley.Lidar measurements of aerosol layer heightsand morning boundary layer development are combined with surface eddy correlation measurements of kinematic heatand moisture fluxes,profiles of temperature and humidity,wind speed and wind direction.A diagnostic method is pres-ented for determining vertical lapse rates using surface meteorological measurements and lidar returns observed duringthe transition from nighttime stable stratification to daytime convective boundary layer after the sunrise.     

城市气溶胶对太阳辐射的影响及其在边界层温度变化中的反映

本文利用冬季观测的兰州市区、邻近山顶以及郊区皋兰县的辐射资料和温度廓线资料,分析了兰州城市气溶胶对太阳辐射影响;并用二流近似累加法计算了低层大气吸收太阳辐射的加热率,其结果与低层大气上部的实际增温在数值上比较接近。分析了城市气溶胶短波辐射效应对边界层温度廓线分布的影响。表明城市烟雾层削弱了地面热通量但增加了低层大气中上部的增温,从而增加了城市低层大气的稳定度。     

BECAPEX科学试验城市建筑群落边界层大气环境特征及其影响

20 0 1～ 2 0 0 3年在北京实施了大气边界层动力、热力、化学综合观测试验 (BECAPEX ,BeijingCityAtmosphericPollutionObservationFieldExperiment) ,获取了北京城市大气动力和大气化学三维结构图像。综合观测试验分析研究发现 ,城市区域呈非均匀次生尺度热岛分布 ,并伴随着城市次生尺度环流 ,影响了局地空气污染物分布特征。MODIS卫星遥感 地面观测资料经过变分分析 ,可发现北京城市空气污染与周边区域影响源有密切关系 ,并影响城市群落环境气候特征 ,导致该区域日照、雾日、低云量和能见度呈显著年代际变化趋势。