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.     

An urban canopy-layer climate model

Summary This paper outlines a computer simulation model designed to assess the thermal characteristics of the urban canopy layer (UCL). In contrast to other UCL models, the layer simulated here includes both closed volumes (buildings) and open volumes (canyons). The purpose of the model is to allow the comparison of the climate impacts of different building group configurations. Traditional boundary-layer theory is applied to the surface urban boundary layer (UBL) which lies above the UCL and the derived relations are used to parameterize exchanges of momentum and heat across the UBL/UCL interface. The exterior energy budgets of the roof, walls and floor of the canopy are solved using an equilibrium surface temperature method. The open canopy and interior building air temperatures are found which are in agreement with the surface exchanges. Using measured data for Los Angeles in June, the output of the model is examined. The results show some agreement with measurement studies and suggest that the density of structures can have a substantial impact on UCL/UBL interaction.With 6 Figures     

城市下垫面对河谷城市兰州冬季热岛效应及边界层结构的影响

利用中尺度数值模式WRF耦合单层城市冠层模块UCM,引入2005年MODIS土地利用类型资料,在对2005年1月25—28日兰州市热岛现象进行高分辨率数值模拟的基础上,设计了去除城市下垫面敏感性试验,探讨了城市下垫面对城市边界层的影响程度。结果表明,城市下垫面能使近地层大气温度升高而风速减小,并且,在夜间表现更明显。由城市热岛强度日变化分析可知,城市下垫面对兰州市热岛强度的贡献率为44%。夜间,城市上空200 m以下的近地层大气保持了白天的混合层特征,热岛环流的上升运动促进了山风环流,使得上升气流到达地面以上600 m左右;白天,由于山峰加热效应,城市上空400—600 m存在一个脱地逆温层,城市热岛环流使得11—15时（北京时）市区近地层出现弱上升气流,抑制了谷风环流的形成及发展。城市下垫面的低反照率特性和建筑物的多次反射作用导致城市下垫面的净辐射通量大于非城市下垫面;城市下垫面由于建筑材料的不透水性,导致潜热通量远小于感热通量,而储热项所占比重明显增大。     

苏州—无锡—常州城市带热岛效应个例研究

应用WRF(weather research and forecasting)及其耦合的多层城市冠层模式BEP(building energy parameterization),对2013年8月13日长江三角洲地区一次高温天气过程进行了模拟。此次过程盛行东南风,风向与苏州—无锡—常州城市带走向一致。模拟结果表明:苏州—无锡—常州城市带热岛效应明显,热岛强度向下游城市逐渐增加;在东南风作用下,三座城市的热岛连成一片,形成了一个更强大的热岛环流。夜晚,边界层逐渐趋于稳定,热岛环流减弱,有利于热岛温度向下游地区输送。热岛效应导致城市边界层高度明显上升。白天太湖产生强盛的湖风对其周边城市影响显著,来自太湖的冷气团导致无锡和常州边界层内热岛强度明显下降,抑制城市热岛向上发展,削弱了无锡与常州两城市热岛间的联系。白天太湖使得无锡和常州边界层高度明显下降。     

Simulation of urban climate with high-resolution WRF model: A case study in Nanjing, China

In this study, urban climate in Nanjing of eastern China is simulated using 1-km resolution Weather Research and Forecasting (WRF) model coupled with a single-layer Urban Canopy Model. Based on the 10-summer simulation results from 2000 to 2009 we find that the WRF model is capable of capturing the high-resolution features of urban climate over Nanjing area. Although WRF underestimates the total precipitation amount, the model performs well in simulating the surface air temperature, relative humidity, and precipitation frequency and inter-annual variability. We find that extremely hot events occur most frequently in urban area, with daily maximum (minimum) temperature exceeding 36°C (28°C) in around 40% (32%) of days. Urban Heat Island (UHI) effect at surface is more evident during nighttime than daytime, with 20% of cases the UHI intensity above 2.5°C at night. However, The UHI affects the vertical structure of Planet Boundary Layer (PBL) more deeply during daytime than nighttime. Net gain for latent heat and net radiation is larger over urban than rural surface during daytime. Correspondingly, net loss of sensible heat and ground heat are larger over urban surface resulting from warmer urban skin. Because of different diurnal characteristics of urban-rural differences in the latent heat, ground heat and other energy fluxes, the near surface UHI intensity exhibits a very complex diurnal feature. UHI effect is stronger in days with less cloud or lower wind speed. Model results reveal a larger precipitation frequency over urban area, mainly contributed by the light rain events (< 10 mm d^?1). Consistent with satellite dataset, around 10?C20% more precipitation occurs in urban than rural area at afternoon induced by more unstable urban PBL, which induces a strong vertical atmospheric mixing and upward moisture transport. A significant enhancement of precipitation is found in the downwind region of urban in our simulations in the afternoon.     

苏州-无锡-常州城市带热岛效应个例研究

应用WRF（weather research and forecasting）及其耦合的多层城市冠层模式BEP（building energy parameterization）,对2013年8月13日长江三角洲地区一次高温天气过程进行了模拟.此次过程盛行东南风,风向与苏州-无锡-常州城市带走向一致.模拟结果表明：苏州-无锡-常州城市带热岛效应明显,热岛强度向下游城市逐渐增加;在东南风作用下,三座城市的热岛连成一片,形成了一个更强大的热岛环流.夜晚,边界层逐渐趋于稳定,热岛环流减弱,有利于热岛温度向下游地区输送.热岛效应导致城市边界层高度明显上升.白天太湖产生强盛的湖风对其周边城市影响显著,来自太湖的冷气团导致无锡和常州边界层内热岛强度明显下降,抑制城市热岛向上发展,削弱了无锡与常州两城市热岛间的联系.白天太湖使得无锡和常州边界层高度明显下降.     

Estimation of urban heat island intensity using biases in surface air temperature simulated by a nonhydrostatic regional climate model

This study demonstrates that urban heat island (UHI) intensity can be estimated by comparing observational data and the outputs of a well-developed high-resolution regional climate model. Such an estimate is possible because the observations include the effects of UHI, whereas the model used does not include urban effects. Therefore, the errors in the simulated surface air temperature, defined as the difference between simulated and observed temperatures (simulated minus observed), are negative in urban areas but 0 in rural areas. UHI intensity is estimated by calculating the difference in temperature error between urban and rural areas. Our results indicate that overall UHI intensity in Japan is 1.5 K and that the intensity is greater in nighttime than in daytime, consistent with the previous studies. This study also shows that root mean square error and the magnitude of systematic error for the annual mean temperature are small (within 1.0 K).     

The urban boundary-layer field campaign in marseille (ubl/clu-escompte): set-up and first results

The UBL/CLU (urban boundary layer/couche limite urbaine) observation and modelling campaign is a side-project of the regional photochemistry campaign ESCOMPTE. UBL/CLU focuses on the dynamics and thermodynamics of the urban boundary layer of Marseille, on the Mediterranean coast of France. The objective of UBL/CLU is to document the four-dimensional structure of the urban boundary layer and its relation to the heat and moisture exchanges between the urban canopy and the atmosphere during periods of low wind conditions, from June 4 to July 16, 2001. The project took advantage of the comprehensive observational set-up of the ESCOMPTE campaign over the Berre–Marseille area, especially the ground-based remote sensing, airborne measurements, and the intensive documentation of the regional meteorology. Additional instrumentation was installed as part of UBL/CLU. Analysis objectives focus on (i) validation of several energy balance computational schemes such as LUMPS, TEB and SM2-U, (ii) ground truth and urban canopy signatures suitable for the estimation of urban albedos and aerodynamic surface temperatures from satellite data, (iii) high resolution mapping of urban land cover, land-use and aerodynamic parameters used in UBL models, and (iv) testing the ability of high resolution atmospheric models to simulate the structure of the UBL during land and sea breezes, and the related transport and diffusion of pollutants over different districts of the city. This paper presents initial results from such analyses and details of the overall experimental set-up.     

人为热源对城市热岛效应影响的数值模拟试验

利用耦合了单层城市冠层模型UCM的中尺度模式WRF,研究了人为热源对上海区域城市气候的影响。冬季地表温度的模拟结果表明,使用新陆面资料的试验效果要好于使用旧的陆面资料,加入人为热源的试验效果要优于没有加入人为热源的试验,这些反映了热岛效应是不断增加的城市面积和人为热源共同决定的。不同试验模拟的2 m高度上气温的模拟情况时,使用新陆面资料并且加入合理人为热源的试验模拟值明显大于其他试验的模拟值。     

应用城市地表能量平衡方案研究城市冠层结构对城市热岛的热力影响

用南京大学区域边界层模式NJU-RBLM, 通过对一组理想试验的模拟, 研究了TEB方案 (town energy balance) 和SVAT方案 (soil－vegetation－atmosphere transfer) 模拟城市热岛现象的差异及本质原因, 发现TEB方案对城市热岛 (UHI) 尤其是夜间UHI模拟效果更优, 这是由于TEB方案具备较强模拟城市储热项的能力形成的。此外, 深入探讨UHI对大气边界层热力结构的影响, 发现UHI现象使城市和郊区的近地层位温廓线在清晨和傍晚都存在明显差异, 同时使城市区域气温全天高于郊区, 且日间城乡温差能达到的高度明显高于夜间。分析人为热源和建筑物冠层对UHI的影响时发现: 人为热源对UHI的影响在夜间强于白天, 而建筑物对白天城市湍能的影响强于人为热源的作用。     

Temporal characteristics of the Beijing urban heat island

Summary This paper describes the inter-annual trend, and the seasonal and hourly variation of the near surface urban heat island (UHI) in Beijing. The surface air temperature data (mean, maximum, and minimum) from one urban (downtown Beijing) and one rural (70 km from downtown Beijing) station were used for the period 1977 and 2000. It is found that the temperatures in both urban and rural stations show an increasing tendency. Specifically, minimum temperature shows the greatest tendency at the urban station whereas maximum temperature shows the greatest increase at the rural station. The UHI intensity obtained by calculating the difference in temperatures between the two stations identifies that the intensity is greatest and has the greatest increasing trend for minimum temperature, while the UHI intensity of maximum temperature shows a slow decrease over time. UHI intensity for minimum temperature has a strong positive correlation with the increase in the urban population and the expansion of the yearly construction area. Seasonal analyses showed the UHI intensity is strongest in winter. This seasonal UHI variation tends to be negatively correlated with the seasonal variation of relative humidity and vapor pressure. Hourly variation reveals that the strongest UHI intensity is observed in the late nighttime or evening, while the weakest is observed during the day.     

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。     

不同气象条件下廊坊城市热岛效应变化特征

利用2005年9月—2008年8月廊坊市区域加密自动站逐时气温资料,采用城、郊气温对比法研究了不同气象条件对廊坊城市热岛效应的影响。结果表明：廊坊城市热岛强度夜间大于白天,但变化幅度白天大于夜间;在四季不同时段存在“城市冷岛”现象。不同气象条件下,廊坊城市热岛强度及变化存在明显差异,晴朗无风时城市平均热岛强度最大,平均强度达1.25℃,阴雨气象条件下城市平均热岛强度最小,平均强度仅有0.10℃。     

Statistical and dynamical characteristics of the urban heat island intensity in Seoul

The statistical and dynamical characteristics of the urban heat island (UHI) intensity in Seoul are investigated for non-precipitation days and precipitation days using 4-year surface meteorological data with 1-h time intervals. Furthermore, the quantitative influence of synoptic pressure pattern on the UHI intensity is examined using a synoptic condition clustering method. The statistical analysis shows that the daily maximum UHI intensity in Seoul for non-precipitation days is strongest in autumn (4.8°C) and weakest in summer (3.5°C). The daily maximum UHI intensity is observed around midnight in all seasons except in winter when the maximum occurrence frequency is found around 08 LST. This implies that anthropogenic heating contributes to the UHI in the cold season. The occurrence frequency of the UHI intensity has a negatively skewed distribution for non-precipitation days but a positively skewed distribution for precipitation days. The amplitude of the heating/cooling rate and the difference in the heating/cooling rate between the urban and rural areas are smaller in all seasons for precipitation days than for non-precipitation days, resulting in weaker UHI intensities for precipitation days. The urban cool island occurs very often in the daytime, with an occurrence frequency being 77% of the total non-precipitation days in spring. The analysis of the impact of large-scale dynamical forcing shows that the daily maximum UHI intensity varies with synoptic pressure pattern, ranging from ?22% in spring to 28% in summer relative to the seasonal mean daily maximum UHI intensity. Comparison of the UHI intensity calculated using station-averaged temperatures to that based on the conventional two-station approach indicates that local effects on the UHI intensity are minimized by using multiple-station data. Accordingly, an estimation of the UHI intensity using station-averaged temperatures for both urban and rural areas is suggested.     

城市热岛效应的研究进展与展望

随着世界各国城市化的进展,城市热岛效应已经成为一个跨学科领域的问题,受到包括大气环境、区域气候、水文和生态等多学科科学家的关注。在过去半个多世纪中,城市热岛问题的研究获得了相当丰富的研究成果,通过对这些成果的综合分析,归纳出城市热岛研究中采用的3类主要方法——观测(外场试验和遥感技术)、数值模拟以及实验室仿真法。系统地回顾了城市热岛效应的研究历史,重点对与城市热岛关系最密切的城市边界层、热岛环流与复杂地形的相互作用以及能量平衡研究所取得的成果进行了总结和评述。最后对城市热岛问题未来8个可能的研究方向进行了探讨,其中,包括沿海和复杂地形附近的城市热岛问题、城市群间热岛环流的相互作用、城市化与空气污染问题、城市热岛效应对平均降水的影响、城市化对雾和闪电的影响、城市天气预报的精细化、城市气候变化预测以及城市热岛效应减缓方案的制定,并对其发展前景进行了粗略的展望。     

Interactions of an urban heat island and sea-breeze circulations during winter over the metropolitan area of São Paulo,Brazil

The Town Energy Budget (TEB) model, a detailed urban parameterisation using a generalised canyon geometry, coupled with the Regional Atmospheric Modelling System (RAMS) is used to simulate the wintertime local circulation in the megacity environment of the metropolitan area of Sao Paulo (MASP) in Brazil. Model simulations are performed using actual topography and land-use fields. Comparison with a simple urban parameterisation based on the LEAF-2 scheme is also shown. Validation is based on comparison between model simulations and observations. Sensitivity tests with TEB reveal an important interaction between the sea breeze and the MASP heat island circulation. Even though topography is known to play an important role in the MASP region’s weather, in these tests the simulations were performed without topography in order to unambiguously identify the interaction between the two local circulations. The urban heat island (UHI) forms a strong convergence zone in the centre of the city and thereby accelerates the sea-breeze front toward the centre of the city. The presence of the urban region increases the sea-breeze front propagation mean speed by about 0.32 m s⁻¹ when compared with the situation of no city. After its arrival in the urban region, the sea-breeze front stalls over the centre of the city for about 2 h. Subsequently, the sea breeze progresses beyond the city when the heat island dissipates. Thereafter, the sea breeze propagates beyond the urban area at a decelerated rate compared to a simulation without an UHI.     

空调系统对城市大气温度影响的模拟研究

人为热排放对城市边界层气候效应的影响起着举足轻重的作用。在人为热源的组成中,建筑物内部的能源消耗、热量产生以及室内外热量传输与交换,对于城市热环境的改变有着非常重要的作用。因此,为研究室内空调系统的产热和通过屋顶、墙面、地面进行室内外热量传输扩散,并对室外热环境产生的影响,运用WRF（Weather Research and Forecasting Model）模式,选取BEP+BEM城市冠层参数化方案,即基于多层城市冠层方案BEP（Building Effect Parameterization）增加室内空调系统影响的建筑物能量模式BEM（Building Energy Model）的方案,以南京2010年8月2—3日,夏季三伏天晴天小风天气作为背景天气进行模拟研究。结果表明,采用WRF模式考虑空调系统室内、外能量交换的BEP+BEM参数化方案,能够更好地模拟出夏季晴天城市近地层气温。当假设空调全天开启时,白天模拟值与观测值吻合较好。夜间温度模拟值高于观测值,在22时—次日00时（北京时）,存在1℃左右的偏差。白天空调系统开启对于城市近地层气温的影响不明显,而夜间使得城市气温普遍升高0.6℃,尤其是在居民区密集的地方,22—23时最大有2℃左右的升温。当调整室内空调目标温度从25℃调至27℃时,空调系统能量总释放量减少12.66%,13—16时温度下降最大,平均约为1℃,建筑物越是密集,温度下降幅度越大。      

Urban boundary layer analysis in the complex coastal terrain of Bilbao using Enviro-HIRLAM

This study analyses the atmospheric boundary layer over the Bilbao metropolitan area during summer (13–18 Jul 2009) and winter (20–29 Jan 2010) episodes using the Environment–High Resolution Limited Area Model (Enviro-HIRLAM) coupled with the building effect parameterisation (BEP). The main objectives of this study are: to evaluate the performance of the model to simulate the land–sea breezes over this complex terrain; to assess the simulations with the integration of an urban parameterisation in Enviro-HIRLAM and finally; and to analyse the urban–atmosphere interactions. Even if the hydrostraticity of the model is a limitation to simulate atmospheric flows over complex terrain, sensibility tests demonstrate that 2.4 km is the optimal horizontal resolution over Bilbao that allows at the same time: to obtain satisfactory reproducibility of the large-scale processes and to explore the urban effects at local scale. During the summer episode, a typical regime of diurnal sea breeze from the NW-N-NE direction and nocturnal valley breezes from the SE direction are observed over Bilbao. The urban heat island (UHI) phenomenon is developed in the city centre expanding to the suburbs from 22 to 10 local time (LT), covering an area of 130 km². The maximum UHI intensity, 1 °C, is reached at the end of the night (5 LT), and it is advected 12 km towards the sea by the land breezes. The urban boundary layer (UBL) height amplitude varies from 100 (night time) to 1,360 m (at 14 LT). During the winter episode, the land breeze dominates the atmospheric diffusion during the day and night time. The maximum UHI intensity, 1.7 °C, is observed at 01 LT. It is spread and remained over the city covering an area of 160 km², with a vertical extension of 33 m. The UBL reaches 780 m height at 16 LT the following day.     

不同下垫面数据和城市冠层参数化方案对江苏气温影响的个例分析

利用中尺度数值模式WRF,分别选用新旧两种下垫面资料和不同城市冠层模型设计试验,以江苏一次秋末高温天气个例(2014年11月20—21日)为背景,研究城市化进程对气温的影响和可能机制。将模式结果与江苏国家气象观测站和地面加密区域自动站观测资料进行对比,并分析3组试验结果发现:(1)采用BEP城市方案对2 m气温、2 m相对湿度和10 m风速等物理量的日变化模拟最优。(2)相比USGS数据,MODIS较新地表覆盖变化数据能更真实反映研究区域当前地表类型分布情况,且能提高近地面风温湿要素空间分布的模拟。(3)分析不同试验模拟的地表能量平衡过程差异,发现相比UCM单层城市冠层方案,BEP多层城市冠层方案在白天能更好模拟出城市地区的温度升高以及相对应的地表感热通量和地面热通量的增加。