Different Turbulent Regimes and Vertical Turbulence Structures of the Urban Nocturnal Stable Boundary Layer

Turbulence in the nocturnal boundary layer(NBL)is still not well characterized,especially over complex underlying surfaces.Herein,gradient tower data and eddy covariance data collected by the Beijing 325-m tower were used to better understand the differentiating characteristics of turbulence regimes and vertical turbulence structure of urban the NBL.As for heights above the urban canopy layer(UCL),the relationship between turbulence velocity scale(VTKE)and wind speed(V)was consistent with the“HOckey-Stick”(HOST)theory proposed for a relatively flat area.Four regimes have been identified according to urban nocturnal stable boundary layer.Regime 1 occurs where local shear plays a leading role for weak turbulence under the constraint that the wind speed V<VT(threshold wind speed).Regime 2 is determined by the existence of strong turbulence that occurs when V>VT and is mainly driven by bulk shear.Regime 3 is identified by the existence of moderate turbulence when upside-down turbulence sporadic bursts occur in the presence of otherwise weak turbulence.Regime 4 is identified as buoyancy turbulence,when V>VT,and the turbulence regime is affected by a combination of local wind shear,bulk shear and buoyancy turbulence.The turbulence activities demonstrated a weak thermal stratification dependency in regime 1,for which within the UCL,the turbulence intensity was strongly affected by local wind shear when V<VT.This study further showed typical examples of different stable boundary layers and the variations between turbulence regimes by analyzing the evolution of wind vectors.Partly because of the influence of large-scale motions,the power spectral density of vertical velocity for upsidedown structure showed an increase at low frequencies.The upside-down structures were also characterized by the highest frequency of the stable stratifications in the higher layer.

Urban Heat Island and Boundary Layer Structures under Hot Weather Synoptic Conditions:A Case Study of Suzhou City,China

A strong urban heat island （UHI） appeared in a hot weather episode in Suzhou City during the period from 25 July to 1 August 2007. This paper analyzes the urban heat island characteristics of Suzhou City under this hot weather episode. Both meteorological station observations and MODIS satellite observations show a strong urban heat island in this area. The maximum UHI intensity in this hot weather episode is 2.2℃, which is much greater than the summer average of 1.0℃ in this year and the 37–year （from 1970 to 2006） average of 0.35℃. The Weather Research and Forecasting （WRF） model simulation results demonstrate that the rapid urbanization processes in this area will enhance the UHI in intensity, horizontal distribution, and vertical extension. The UHI spatial distribution expands as the urban size increases. The vertical extension of UHI in the afternoon increases about 50 m higher under the year 2006 urban land cover than that under the 1986 urban land cover. The conversion from rural land use to urban land type also strengthens the local lake-land breeze circulations in this area and modifies the vertical wind speed field.

Large Eddy Simulation and Study of the Urban Boundary Layer

Based on a pseudo-spectral large eddy simulation (LES) model, an LES model with an anisotropy turbulent kinetic energy (TKE) closure model and an explicit multi-stage third-order Runge-Kutta scheme is established. The modeling and analysis show that the LES model can simulate the planetary boundary layer (PBL) with a uniform underlying surface under various stratifications very well. Then, similar to the description of a forest canopy, the drag term on momentum and the production term of TKE by subgrid city buildings are introduced into the LES equations to account for the area-averaged effect of the subgrid urban canopy elements and to simulate the meteorological fields of the urban boundary layer (UBL). Numerical experiments and comparison analysis show that: (1) the result from the LES of the UBL with a proposed formula for the drag coefficient is consistent and comparable with that from wind tunnel experiments and an urban subdomain scale model; (2) due to the effect of urban buildings, the wind velocity near the canopy is decreased, turbulence is intensified, TKE, variance, and momentum flux are increased, the momentum and heat flux at the top of the PBL are increased, and the development of the PBL is quickened; (3) the height of the roughness sublayer (RS) of the actual city buildings is the maximum building height (1.5-3 times the mean building height), and a constant flux layer (CFL) exists in the lower part of the UBL.

Urban Boundary-Layer Stability and Turbulent Exchange during Consecutive Episodes of Particle Air Pollution in Beijing,China

Based on measurements at the Beijing 325-m Meteorological Tower,this study reports an analysis of atmospheric stability conditions and turbulent exchange during consecutive episodes of particle air pollution in Beijing(China),primarily due to haze and dust events(15–30 April 2012).Of particular interest were relevant vertical variations within the lower urban boundary layer(UBL).First,the haze and dust events were characterized by different atmospheric conditions,as quite low wind speed and high humidity are typically observed during haze events.In addition,for the description of stability conditions,the bulk Richardson number(RiB) was calculated for three different height intervals: 8–47,47–140,and 140–280 m.The values of RiB indicated an apparent increase in the occurrence frequency of stably-stratified air layers in the upper height interval—for the 140–280-m height interval,positive values of RiB occurred for about 85% of the time.The downward turbulent exchange of sensible heat was observed at 280 m for the full diurnal cycle,which,by contrast,was rarely seen at 140 m during daytime.These results reinforce the importance of implementing high-resolution UBL profile observations and addressing issues related to stably-stratified flows.

Comparison and Verification of Coherent Doppler Wind Lidar and Radiosonde Data in the Beijing Urban Area

As a new type of wind field detection equipment, coherent Doppler wind lidar(CDWL) still needs more relevant observation experiments to compare and verify whether it can achieve the accuracy and precision of traditional observation equipment in urban areas. In this experiment, a self-developed CDWL provided four months of observations in the southern Beijing area. After the data acquisition time and height match, the wind profile data obtained based on a Doppler beam swinging(DBS) five-beam inversion algorithm were compared with radiosonde data released from the same location. The standard deviation(SD) of wind speed is 0.8 m s^(–1), and the coefficient of determination R~2 is 0.95. The SD of the wind direction is 17.7° with an R~2 of 0.96. Below the height of the roughness sublayer(about 400 m), the error in wind speed and wind direction is significantly greater than the error above the height of the boundary layer(about 1500 m). For the case of wind speeds less than 4 m s^(–1), the error of wind direction is more significant and is affected by the distribution of surrounding buildings. Averaging at different height levels using suitable time windows can effectively reduce the effects of turbulence and thus reduce the error caused by the different measurement methods of the two devices.

An Analytical Study on the Urban Boundary Layer

A two-layer model based on the linearized time-independent atmospheric dynamical equations is proposed in this paper. The analytical solutions of the vertical, the horizontal motions and the potential temperature field induced by the anthropogenic source of urban surface heating are obtained, therefore the heat island circulation existing in unstable boundary layer is verified theoretically. From the analytical solutions, some conclusions can be drawn. (1) The vertical motion induced by urban heat island consists of two parts, namely, the cross-hill wave and the lee wave; (2) The cross-hill wave only exists in the unstable boundary layer, and varies with height according to exponential function law; (3) The vertical motion induced by heat island reaches the maximum at the top of the unstable boundary layer; (4) The wave generated by heat island not only propogates to the downwind district but also travels to the upwind area; (5) γ≠0 is not the necessary condition of the lee wave generation.

On the Influences of Urbanization on the Extreme Rainfall over Zhengzhou on 20 July 2021: A Convection-Permitting Ensemble Modeling Study

This study investigates the influences of urban land cover on the extreme rainfall event over the Zhengzhou city in central China on 20 July 2021 using the Weather Research and Forecasting model at a convection-permitting scale[1-km resolution in the innermost domain(d3)].Two ensembles of simulation(CTRL,NURB),each consisting of 11 members with a multi-layer urban canopy model and various combinations of physics schemes,were conducted using different land cover scenarios:(i)the real urban land cover,(ii)all cities in d3 being replaced with natural land cover.The results suggest that CTRL reasonably reproduces the spatiotemporal evolution of rainstorms and the 24-h rainfall accumulation over the key region,although the maximum hourly rainfall is underestimated and displaced to the west or southwest by most members.The ensemble mean 24-h rainfall accumulation over the key region of heavy rainfall is reduced by 13%,and the maximum hourly rainfall simulated by each member is reduced by 15–70 mm in CTRL relative to NURB.The reduction in the simulated rainfall by urbanization is closely associated with numerous cities/towns to the south,southeast,and east of Zhengzhou.Their heating effects jointly lead to formation of anomalous upward motions in and above the planetary boundary layer(PBL),which exaggerates the PBL drying effect due to reduced evapotranspiration and also enhances the wind stilling effect due to increased surface friction in urban areas.As a result,the lateral inflows of moisture and high-θe(equivalent potential temperature)air from south and east to Zhengzhou are reduced.

国家气候观象台多维度观测能力建设及科学研究——以深圳为例

国家气候观象台是中国国家级气象观测的重要节点,发展国家气候观象台的观测能力,并依托其开展有特色的科学研究,有助于推动我国大气科学发展。深圳国家气候观象台是我国珠三角经济圈环境综合观测区唯一的国家级气候观象台,经过20余年的持续不断建设,已具备了城市—郊区的对比监测、气象—环境的联合监测、地面—高空的立体监测和陆地—海洋的协同监测等多方面的监测能力。基于深圳国家气候观象台的观测数据,大气科学领域的学者们已在粤港澳大湾区城市气候、边界层风特性及致灾机理、大气环境与环境气象、气候资源、地闪雷电特征等多个领域开展研究并取得了丰硕的成果,进一步丰富了对这些领域的科学认识。此外,深圳国家气候观象台还支撑了深圳本地的防灾减灾、城市管理、城市规划以及生态文明建设等工作,已逐渐成为发挥气象观测社会效益的典范。

基于CNN与GAN深度学习模型近壁面湍流场超分辨率重构的精细化研究

由城市抗风减灾的目标出发,城市边界层的高保真再现是工程界亟待解决的关键问题.基于高精度的近地风场,有望实现真实环境下城市建筑风致效应的准确预测.传统的基于气象模型的城市风场模拟方法存在预测耗时长、成本昂贵、求解尺度过高等缺陷.为更准确、高效地预测边界层的空间变化,研究利用超精度卷积神经网络(SRCNN)与生成对抗神经网络(SRGAN),在空间上将低精度的近壁面湍流场超精度重构成高精度的风场.利用近壁面湍流直接数值模拟的公共数据库训练模型并评价模型的重构性能.为寻求合适的超精度模型生成方式,研究围绕训练样本量及网络深度,开展详细的敏感性分析,确定合适的训练网络及其较优的训练参数设置.同时,基于经不同下采样因子处理的低精度流场输入,分析模型在近壁面湍流重构中的适用范围.研究发现,对比于SRCNN模型,SRGAN模型对近壁面湍流内小尺度结构的重现效果更佳.当基于4层卷积残差块、300样本量开展训练时,所生成的SRGAN模型可在较低的训练代价下实现较优的重构效果.当进行10倍超精度重构时,SRGAN模型可保证较理想的预测精度.研究成果为边界层风场的准确重构提供技术支撑,为城区建筑物风致效应的高效预测提供精确的入流条件.

基于北京325m气象铁塔的城市冠层以上湍流强度垂直分布特征分析与推算方法评估

为了研究与风载荷密切相关的城市大气边界层湍流的垂直分布特征,利用北京325m气象铁塔的不同高度一年四季的湍流观测数据进行统计分析,重点对比分析了湍流强度剖面实测值和推算值。结果表明:采用《建筑结构荷载规范》(GB 50009-2012)推荐的10m湍流强度和风廓线幂指数得到的湍流强度剖面推算值明显小于考虑了小风情况的实测值,荷载规范推荐参数不适用于发达城市边界层湍流强度剖面推算;采用实测风廓线幂指数和城市冠层内8m湍流强度实测值得到的湍流强度剖面推算值与实测值的相对误差达20%以上;采用实测风廓线幂指数和城市冠层之上的47m湍流强度实测值得到的湍流强度剖面推算值与实测值的相对误差小于9%。因此,以城市冠层之上的湍流强度实测值和实测风廓线幂指数为基础推算湍流强度剖面的方法能够得到比较接近于实测值的结果,此推算方法具有可行性与可靠性。

基于WRF模拟的中国西北河谷城市夏季的大气边界层特征

为提高河谷地形气象场的模拟效果,利用第5代再分析资料(ERA5)和全球再分析资料(FNL)作为初始场,以天水市为研究对象,驱动中尺度天气预报模式比较对西北河谷城市边界层模拟的适用性,分析西北河谷城市夏季的大气边界层特征.结果表明, ERA5模拟的天水市主城区近地面温度、近地面风速、风向以及相对湿度与观测值的相关性更好,尤其是近地面风速和风向,分别比FNL模拟的结果提升25.4%和70.0%.天水市主城区的气象场空间分布呈明显的城市热岛效应和山谷风环流,相对开阔的麦积区城市热岛效应更强;白天发生的降水会弱化谷风环流和热岛效应,河谷内及周边风速均较小.天水市主城区夏季近地面温度与风速呈正相关,与相对湿度呈负相关,大气边界层高度呈现明显的日变化,大气层结稳定.

基于耦合湖模式的城市边界层模式研究太湖的气象与环境效应

本文发展了一个耦合的城市边界层—湖泊模式(RBLM-Chem-Lake),通过离线和在线两种方式对模式的模拟效果进行了检验。并通过敏感性实验分析了太湖冬季和夏季对周边地区气象特征和空气污染的影响。结果表明:(1)白天太湖对周边气象环境产生了明显的降温效应,而夜晚出现了明显的升温。冬季夜晚周边地区升温达到0.4~0.6℃,白天可以使得周边气温降低1~2℃。夏季夜晚使周边地区升温可达1~1.5℃,白天使得周边气温降低1~2℃。(2)太湖对周边地区气温的影响范围夏季远高于冬季,夏季影响范围可达到80 km,冬季影响范围平均为5 km。此外,太湖使得湖区平均边界层高度下降,湿度上升,使得苏州市区垂直风速减弱,这种影响同样在夏季更为明显。(3)太湖的存在总体上使湖区和周边地区PM_(2.5)和臭氧的质量浓度增加。在冬季PM_(2.5)污染个例中,太湖导致区域夜晚PM_(2.5)质量浓度下降,下降幅度达到4~5μg·m^(-3)。而在白天,太湖区域500 m高度以下PM_(2.5)质量浓度增加,幅度达到25~30μg·m^(-3),在500 m高度以上质量浓度下降约5~10μg·m^(-3)。夏季的臭氧污染日个例中,白天,在太湖的东岸以及苏州市区臭氧的质量浓度出现了明显的增加,可以达到10~12μg·m^(-3)。

东北城市群大气污染立体观测与研究进展

基于国家重点研发计划专项课题“哈长城市群大气复合污染立体监测技术体系”研究成果,回顾了近年来在我国东北城市群大气污染立体监测、污染特征及成因机制研究方面取得的主要进展;介绍了东北地区大气污染立体观测网的布局和建设,总结了大气污染物时空分布特征和大气气溶胶光化学特性,分析了大气边界层结构特征及其物理过程对大气污染生消的影响。最后指出东北城市群还需要加强PM_(2.5)和臭氧污染的协同控制以及气溶胶—边界层反馈机制对大气污染影响等方面的研究。

Vertical dynamic and thermodynamic characteristics of urban lower boundary layer and its relationship with aerosol concentration over Beijing

By utilizing observational data from a 325 m tower of the Institute of Atmospheric Physics (IAP), Chinese Academy of Sciences (CAS) on March 19-29, 2001 and August 11-25, 2003, a comprehensive study was conducted on the vertical dynamical and thermodynamic characteristics of the urban lower boundary layer (ULBL) and its relationship with aerosol concentration over Beijing. Firstly, a comparative analysis was made on the gradient data (wind, temperature and humidity), ultrasonic data (atmospheric turbulences) and air-quality observations at different tower heights (47, 120 and 280 m). Secondly, a diagnosis was made to reveal the major features of normalized variances of velocity and temperature, turbulence kinetic energy as well as their relationship with aerosol concentrations. Furthermore, the characteristics of the ULBL vertical structure and the TSP concentration/distribution variations during a sand/dust weather process were also analyzed. The outcome of the study showed that under unstable stratification, the normalized variances of velocity (σu/u*, σv/u*, σw/u*) and temperature (σT/T*) at 47 and 120 m heights fit the Monin-Obukhov similarity (MOS) framework and the fitting formulas were given out accordingly. According to the stratification parameter (z′/L), the stable ULBL could be divided into 2 zones. With z′/L<0.1, it was a weakly stable zone and MOS framework was applicable. The other was a highly stable zone with z′/L>0.1 and the normalized velocity variances tended to increase along with higher stability, but it remained constant for normalized temperature variances. At daytime, the near-surface layer includes two heights of 47 and 120 m, while 280 m has been above it. The ULBL analysis in conjunction with a sand/dust weather process in Beijing in March 2001 indicated that the maximum concentration of Total Suspended Particulates (TSP) at 320 m reached 913.3 μg/m3 and the particles were transported from the upper to lower ULBL, which was apparently related to the development process of a low-level jet and its concomitant strong sinking motion.

温度层结对建筑风环境定量影响的大涡模拟

基于局地气候区分类,选取了北京门头沟地区4种局地气候区(高层开阔、高层密集、中层密集、稀疏建筑)作为研究对象,利用高分辨率大涡模拟方法,数值研究温度层结效应对不同局地气候区风和湍流特性的影响。2019年11月7日晴天小风个例模拟结果表明:1)温度层结对湍涡的形状和范围有显著影响。在近地面水平剖面上,稳定层结下涡旋数量较中性层结情况减少,但涡旋的纵向延伸范围可增大67%;不稳定层结下涡旋数量较中性层结增加,涡旋的纵向范围可缩小60%。在垂直剖面上,相较于中性层结,稳定层结下环流结构减弱且涡旋的纵向范围可缩小40%,不稳定层结下环流结构增强且涡旋的纵向范围可增大20%,该现象在高层密集型地块最为明显。2)4种局地气候区的风速的高值区主要位于平行于盛行风方向的建筑物两侧及屋顶附近,热力作用对总风速有增益作用,近地面风速较入流风速可增加1.27~2.18倍。3)4种局地气候区湍动能的高值区主要位于建筑物底部拐角处和屋顶,不稳定层结下近地面的湍动能是中性层结的1.2~1.5倍,而稳定层结下是中性层结的0.5~0.8倍,即不稳定层结条件下浮力引起的热力湍流增强混合效率,而稳定层结条件下湍流运动受到抑制。4)相较于其他局地气候区,高层密集区域的建筑物底部风速较大,在不稳定层结下易形成较强的狭管效应,其街区峡谷最大风速是中层密集的1.5倍。

Numerical simulation of the influence of aerosol radiation effect on urban boundary layer

With the intensification of pollution and urbanization, the aerosol radiation effect continues to play an important role in the urban boundary layer. In this paper, a winter pollution process in Beijing has been taken as an example, and a new aerosol vertical profile in the radiative parameterization scheme within the Weather Forecast Research and Forecasting(WRF) model has been updated to study the effect of aerosols on radiation and the boundary layer. Furthermore, the interactions among aerosols,urbanization, and planetary boundary layer(PBL) meteorology were discussed through a series of numerical experiments. The results show the following:(1) The optimization improves the performance of the model in simulating the distribution features of air temperature, humidity, and wind in Beijing.(2) The aerosols reduce the surface temperature by reducing solar radiation and increasing the temperature in the upper layer by absorbing or backscattering solar radiation. The changes in the PBL temperature lead to more stable atmospheric stratification, reducing the energy transfer from the surface and the height of the boundary layer.(3) With the increase in the aerosol optical depth, the atmospheric stratification most likely becomes stable over rural areas, most likely becomes stable over suburb areas, and has great difficultly becoming stable over urban areas. Aerosol radiative forcing,underlying urban surfaces, and the interaction between them are the main factors that affect the changes in the meteorological elements in the PBL.

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 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.

Numerical Study of Winter Urban Boundary Layer Structure over Beijing Area

Based on the successful simulation of a typical winter urban boundary layer(UBL) process over Beijing area during the Beijing City Air Pollution Experiment (BECAPEX) in 2001by the use of MM5 coupled with urban canopy parameterization, a series of simulation experiments areperformed to investigate the effects of urban influence, surrounding terrain, and different extentof urbanization on urban boundary layer structures over Beijing area. The results of factorseparation experiments of urban influence indicate that the total effect of urban influence, whichis the synthetic effect of urban infrastructure on thermal and dynamic structures of atmosphere, isresponsible for the formation of main UBL features over Beijing area. Meanwhile, the relativeimportance of thermal and mechanical factors of urban infrastructure and interaction between thermaland mechanical factors for the formation and evolution of UBL over the Beijing area are alsoexplored. The results show that, during nighttime, mechanical factors are responsible for maincharacteristics of nocturnal urban boundary layer such as elevated inversion layer over downtownarea, smaller wind speed and stronger turbulent kinetic energy (TKE) and its behavior with peak atthe top of canopy layer, whereas in the daytime, thermal factors play dominant role in the structureof UBL, such as the intensity of mixed layer and temperature in the lower atmosphere in urban area.The interaction between mechanical and thermal factors plays an important role in the formation andevolution of UBL, but its specific characteristics of mechanisms are complex. The results ofsurrounding terrain experiment show that terrain surrounding Beijing area not only determines thecharacteristic of prevailing airflow over Beijing area, but also has obvious effect on thermalstructure of UBL, such as the distribution of elevated inversion and urban heat island, and makesthem with special localization feature. The results of different extent urbanization experiment showthat with the increase in the density and height of buildings in Beijing area, wind speed woulddecrease and TKE increase. Meanwhile, the bottom of nocturnal elevated inversion would increase indowntown area, and the intensity of urban heat island would strengthen, and even probably is obviousin the daytime.

The vertical distribution characteristics of integral turbulence statistics in the atmospheric boundary layer over an urban area in Beijing

Turbulence data(2008–2012) from a 325 m meteorological tower in Beijing, which consisted of three layers(47,140, and 280 m), was used to analyze the vertical distribution characteristics of turbulent transfer over Beijing city according to similarity theory. The conclusions were as follows.(1) Normalized standard deviations of wind speeds/ui * were plotted as a function only of a local stability parameter. The values under near-neutral conditions were 2.15, 1.61, and 1.19 at 47 m, 2.39, 1.75,and 1.21 at 140 m, and 2.51, 1.77, and 1.30 at 280 m, showing a clear increase with height. The normalized standard deviation of wind components fitted the 1/3 law under unstable stratification conditions and decreased with height under both stable and unstable conditions.(2) The normalized standard deviation of temperature fitted the.1/3 law in the free convection limit, but was quite scattered with different characteristics under near-neutral conditions. The normalized standard deviations of humidity and the CO2 concentration fitted the.1/3 law under unstable conditions, and remained constant under near-neutral and stable stratification. The normalized standard deviation of scalars, i.e., temperature, humidity, and CO2 concentration, all increased with height.(3) Compared with momentum, and the water vapor and CO2 concentrations, the turbulence correlation coefficient for heat was smaller under near-neutral conditions, but larger under both stable and unstable conditions. A dissimilarity between heat, and the water vapor and CO2 concentrations was observed in urban areas. The relative correlation coefficients between heat and each of momentum, humidity, and CO2 concentration(|rwT/ruw|, |rwT/rwc| and |rwT/ruq|) in the lower layers were always larger than in higher layers, except for the relative correlation coefficient between heat and humidity in an unstable stratification. Therefore, the ratio between heat and each of momentum, humidity, and CO2 concentration decreased with height.

Seasonal variation of the temperature profile and its characteristics within urban roughness sublayer

By using conventional micro-meteorological observation data of Beijing Yuetan Park Tower (180 m), the temperature profile of urban boundary layer (UBL), its characteristics and seasonal variation are analyzed. The main results are as follows: (1) In winter, the interdiurnal surface air temperature varia- tion at the surface is not synchronized with that of the upper levels, other than in summer and other seasons, which illuminates the impacts of Beijing’s geographical location, sky view factor and stably stratified nocturnal inversion. (2) Except that the stratification is unstable around noon, the stratifica- tion in the roof layer or above-roof layer is of seasonal variability, which is weak unstable or weak sta- ble in winter or summer respectively. This weak stable stratification possibly inhibits urban pollutant dispersion upwards in summer season. (3) The effect of urban building rooftop on the UBL thermal state has seasonal difference, that is, the rooftop plays the role of heating or cooling the urban roughness sublayer, in summer or winter respectively, which is similar to the effects of the Qing- hai-Tibet Plateau on the atmospheric thermal state.