Geospatial Analysis of Urban Heat Island Effects and Tree Equity

In recent decades, Urban Heat Island Effects have become more pronounced and more widely examined. Despite great technological advances, our current societies still experience great spatial disparity in urban forest access. Urban Heat Island Effects are measurable phenomenon that are being experienced by the world’s most urbanized areas, including increased summer high temperatures and lower evapotranspiration from having impervious surfaces instead of vegetation and trees. Tree canopy cover is our natural mitigation tool that absorbs sunlight for photosynthesis, protects humans from incoming radiation, and releases cooling moisture into the air. Unfortunately, urban areas typically have low levels of vegetation. Vulnerable urban communities are lower-income areas of inner cities with less access to heat protection like air conditioners. This study uses mean evapotranspiration levels to assess the variability of urban heat island effects across the state of Tennessee. Results show that increased developed land surface cover in Tennessee creates measurable changes in atmospheric evapotranspiration. As a result, the mean evapotranspiration levels in areas with less tree vegetation are significantly lower than the surrounding forested areas. Central areas of urban cities in Tennessee had lower mean evapotranspiration recordings than surrounding areas with less development. This work demonstrates the need for increased tree canopy coverage.

Analysis of Urban Heat Island Effect Using an Improved CTTC and STTC Model

An improved cluster thermal time constant(CTTC) and surface thermal time constant(STTC) numerical model was introduced,which took into account the effect of vegetation coverage and modified the expression of net longwave radiation of the canyon layer.In the case study the model was used to calculate the air temperature variation at downtown of Tianjin City.The relative error between the calculated and measured air temperatures was less than 3%.The tendency of air temperature variation was predicted when the building aspect ratio,vegetation rate,and wind speed changed respectively.It is demonstrated that when the aspect ratio of a building with south-north orientation increased,the heat island intensity at day time was mitigated;however,it became worse after sunset.The vegetation coverage rate and wind speed both had negative relationship with the urban heat island intensity.

Improving the WRF/urban modeling system in China by developing a national urban dataset

Accurate modeling of urban climate is essential to predict potential environmental risks in cities.Urban datasets,such as urban land use and urban canopy parameters(UCPs),are key input data for urban climate models and largely affect their performance.However,access to reliable urban datasets is a challenge,especially in fast urbanizing countries.In this study,we developed a high-resolution national urban dataset in China(NUDC)for the WRF/urban modeling system and evaluated its effect on urban climate modeling.Specifically,an optimization method based on building morphology was proposed to classify urban land use types.The key UCPs,including building height and width,street width,surface imperviousness,and anthropogenic heat flux,were calculated for both single-layer Urban Canopy Model(UCM)and multiple-layer Building Energy Parameterization(BEP).The results show that the derived morphological-based urban land use classification could better reflect the urban characteristics,compared to the socioeconomic-function-based classification.The UCPs varied largely in spatial within and across the cities.The integration of the developed urban land use and UCPs datasets significantly improved the representation of urban canopy characteristics,contributing to a more accurate modeling of near-surface air temperature,humidity,and wind in urban areas.The UCM performed better in the modeling of air temperature and humidity,while the BEP performed better in the modeling of wind speed.The newly developed NUDC can advance the study of urban climate and improve the prediction of potential urban environmental risks in China.

城市森林结构多样性预测冠下地面温度的潜力研究

城市森林冠层具有调控城市森林微气候的能力,但现有研究尚未阐明冠层结构对冠下地面温度的影响及其预测潜力。文章基于无人机机载激光雷达(UAV-LiDAR)提取哈尔滨林业示范基地的城市森林冠层结构多样性特征指标,探究单一结构多样性特征对冠下地面温度的影响,以及结构多样性多因子组合对温度的预测潜力。结果表明:1)城市森林结构多样性的8个特征因子与冠下地面温度呈显著相关关系(P<0.05),其中深间隙(DG)、深间隙分数(DGF)、覆盖分数(CF)、间隙分数分布(GFP)表征了结构多样性的覆盖/开放度特征;冠层高度标准差(H_(std))、冠层高度最大值(H_(max))、95%分位点高度(ZQ_(95))表征了高度特征;垂直复杂指数(VCI)表征了异质性特征。2)城市森林冠层结构多样性的覆盖/开放度特征对冠下地面温度的响应更强(R^(2)为0.15~0.5),强于高度指标(R^(2)为0.14~0.19)以及异质性指标(R^(2)=0.14)。3)结合高度指标、覆盖/开放度指标以及异质性指标的多因子预测模型2(R^(2)=0.61,RMSE=0.51,MSE=0.26,AIC=62.74),对于冠下地面温度的预测性能更优。研究明晰了城市森林结构多样性的多因子变量及其特征组合预测冠下地面温度的潜力,为城市森林冠层结构调控内部小气候环境研究提供了科学参考。

APPLICATION OF WRF/UCM IN THE SIMULATION OF A HEAT WAVE EVENT AND URBAN HEAT ISLAND AROUND GUANGZHOU

This paper evaluated the performance of a coupled modeling system,Weather Research and Forecasting(WRF)/Urban Canopy Model(UCM),in the simulation of a heat wave event which occurred around Guangzhou during late June through early July,2004.Results from three experiments reveal that the UCM with new land data(hereafter referred to as E-UCM)reproduces the best 2-m temperature evolution and the smallest minimum absolute average error as compared with the other two experiments,the BPA-Bulk Parameterization Approach with new land data(E-BPA)and the UCM with original U.S. Geological Survey land data(E-NOU).The E-UCM is more useful in capturing the temporal and spatial distribution of the nighttime Urban Heat Island(UHI).Differences in surface energy balance between the urban and suburban areas show that low daytime albedo causes more absorption of solar radiation by urban areas.Due to the lack of vegetation which inhibits cooling by evapotranspiration,most of the incoming energy over urban areas is partitioned into sensible heat flux and therefore heats the surface and enhances the heat wave.During nighttime,the energy in the urban area is mainly from soil heat flux.Although some energy is partitioned as outgoing long wave radiation,most of the soil heat flux is partitioned into sensible heat flux due to the small latent heat flux at night.This leads to the development of nighttime UHI and the increase of the magnitude and duration of heat waves within the municipality.

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.

Enhanced modeling of latent heat flux from urban surfaces in the Noah/single-layer urban canopy coupled model

The numerical modeling of the impacts of urban buildings in mesoscale meteorological models has gradually improved in recent years. Correctly representing the latent heat flux from urban surfaces is a key issue in urban land-atmosphere coupling studies but is a common weakness in current urban canopy models. Using the surface energy balance data at a height of 140 m from a 325 m meteorological tower in Beijing, we conducted a 1-year continuous off-line simulation by using a coupled land surface model and a single-layer urban canopy model and found that this model has a relatively large systematic error for simulated latent heat flux. To improve the numerical method for modeling latent heat flux from urban surfaces, we combined observational analysis and urban land surface model to derive an oasis effect coefficient for urban green areas; to develop a temporal variation formula for water availability in urban impervious surfaces; and to specify a diurnal profile and the maximum values of anthropogenic latent heat release for four seasons. These results are directly incorporated into the urban land surface model to improve model performance. In addition, this method serves as a reference for studies in other urban areas.

Impacts of Roof/Ground Mitigation Strategies on Improving the Urban Thermal Environment and Human Comfort over the Yangtze River Delta, China

The combined effects of global warming and the urban heat islands exacerbate the risk of urban heat stress. It is crucial to implement effective cooling measures in urban areas to improve the comfort of the thermal environment. In this study, the Weather Research and Forecasting Model(WRF), coupled with a single-layer Urban Canopy Model(UCM), was used to study the impact of heat mitigation strategies. In addition, a 5-km resolution land-cover dataset for China(ChinaLC), which is based on satellite remote sensing data, was adjusted and used, and 18 groups of numerical experiments were designed, to increase the albedo and vegetation fraction of roof/ground parameters. The experiments were conducted for four heatwave events that occurred in the summer of 2013 in the Yangtze River Delta urban agglomeration of China. The simulated results demonstrated that, for the single roof/ground schemes, the mitigation effects were directly proportional to the albedo and greening. Among all the experimental schemes, the superposed schemes presented better cooling effects. For the ground greening scheme, with similar net radiation flux and latent heat flux, its storage heat was lower than that of the roof greening scheme, resulting in more energy flux into the atmosphere, and its daytime cooling effect was not as good as that of the roof greening scheme. In terms of human thermal comfort(HTC), the improvement achieved by the ground greening scheme was better than any other single roof/ground schemes, because the increase in the relative humidity was small. The comprehensive evaluation of the mitigation effects of different schemes on the thermal environment presented in this paper provides a theoretical basis for improving the urban environment through rational urban planning and construction.

北京夏季晴天边界层特征及城市下垫面对海风影响的数值模拟

对耦合了Noah陆面模式和单层城市冠层模式的WRF(Weather Research and Forecasting)模式系统进行了改进和优化,通过对2010年8月6-7日北京地区晴天个例的模拟试验,检验了优化前后模式系统的模拟能力,分析研究了该个例中城市边界层的特征及日变化.另外,使用优化后的模拟系统通过两组敏感性试验研究了京津城市下垫面对海风的影响.结果表明,优化方案能够显著提高模式系统对该个例的模拟性能,模式系统基本能够模拟出北京夏季边界层的日变化特征,精确的地表使用类型分类等地理信息数据对提高模式预报的准确度有着至关重要的作用,京津城市对海风的发展和推进过程有明显影响,能够阻碍海风的推进、加强风场的水平辐合和垂直上升气流,北京城市下垫面还能在海风到达前增加其强度和推进速度,并在海风经过后延缓其消亡、增加其推进距离.

城市冠层过程的研究与进展

介绍了城市冠层过程研究的概况和历史,概括了城市冠层的特征及其作用于局地气候和中尺度系统的主要方式,总结了城市冠层参数化方案或模式的研究进展。比较了几种参数化方法和模式的特点,提出目前存在的一些问题。认为:现有的参数化方案和模式尚未能完整体现城市下垫面特征、准确反映人类活动影响,二者的发展有赖于对城市下垫面建筑特征更合理、细致地描述,对人为热量、水汽影响更准确的估计与刻划。与局地因素密切相关的城市冠层模式还需要更多地在模式中引入局地差异影响。同时指出,要提高耦合城市过程的中尺度模式预报水平,需要相应改进包括云、降水、次网格地形及边界层动力学等的参数化。

WRF/UCM在广州高温天气及城市热岛模拟研究中的应用

应用WRF及其耦合的城市冠层模式(UCM,Urban Canopy Model),对2004年6月底—7月初受副热带高压和台风外围气流影响下发生在广州地区的一次高温天气过程进行了数值模拟。考察了WRF/UCM对"城市热岛"及城市高温天气模拟的应用效果。三个不同设计的模拟试验表明,E-UCM试验(新土地利用资料,耦合UCM模式)比E-BPA试验(新土地利用资料,BPA方法)和E-NOU试验(旧土地利用资料,耦合UCM模式)更好地模拟出了城区2 m高度温度的演变,平均绝对误差最小。尤其在夜间,E-UCM试验成功地再现了夜间热岛的形成及分布。城区及郊区地表能量平衡差异的分析表明,日间城区高温与低反射率引起的短波辐射吸收增加有关,由于城区缺少水汽蒸发蒸腾冷却过程,大部分能量收入被分配为感热加热大气。夜间,地表能量收入来自土壤热通量的向上输送,收入能量除部分用于长波辐射之外,由于城区潜热通量小,其余部分仍主要以感热形式加热大气。夜间热岛的形成与感热加热的持续有关,有利于夜间高温的维持。

南京地区城市冠层效应的模拟试验研究

利用Weather Research and Forecasting Model(WRF)模式分城市下垫面(wrf-ucm)、城市冠层下垫面(wrf+ucm)和自然下垫面(wrf-no urban)三种情况模拟了南京地区2005年7月17-18日的2 d天气过程.当模拟城市冠层效应时,WRF模式与Urban Canopy Model(UCM)模式耦合.结合同期外场试验观测结果和模式模拟结果,分析城市冠层效应对局地天气的影响.结果发现:考虑冠层效应时,城市地区的2 m温度比不考虑冠层效应时略低一些,但都高于自然下垫面;白天地面感热通量与不考虑冠层时基本相当,而在夜间前者高于后者,两者都明显高于自然下垫面情形;地面潜热通量比不考虑冠层低,均远低于自然下垫面.考虑冠层效应的地面通量模拟结果较为接近观测结果.冠层效应使得城市地区近地层水平风速明显减小;考虑冠层时城市地区气流垂直运动强于自然下垫面,冠层对气流的影响夜间强于白天.

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。

城市冠层结构热力效应对城市热岛形成及强度影响的模拟研究

本文在城市边界层预报模式中耦合了一个单层冠层模式,此模式能够体现城市冠层结构和人为热源对城市热岛的共同作用.通过传统平板模式和城市冠层模式的模拟结果与自动气象站观测资料对比发现,耦合了城市冠层模式的模拟结果与观测资料更为吻合,尤其能够较好地模拟出城市地区夜间地面的气温变化情况.对北京城市区域的模拟结果进行分析,白家庄地区冠层建筑物使得城市地区气温白天下降,夜晚上升,不考虑人为热源作用时,城市冠层使得白家庄站地面气温白天最低下降2.5℃,夜间气温最大升高为4.7℃.针对模拟区域较小的理想算例模拟结果分析表明,城市冠层模式能够很好地模拟城市地区地表能量平衡关系,体现城市冠层对长短波辐射的封截以及热量存储能力,全天平均净辐射通量由传统模式的43.38W/m2变为84.19W/m2,热存储通量白天最大值为278.04W/m2,夜晚最大释放热存储通量为160.35W/m2.冠层建筑物和人为热源对夜间城市热岛强度的贡献分别为70.65%和29.35%.城市冠层建筑物对夜间城市热岛的形成起决定性作用.

城市地表特征对京津冀地区夏季降水的影响研究

本文利用京津冀地区24个气象站的日降水资料和耦合有单层城市冠层模式（SLUCM）的中尺度数值模式 WRF 的模拟结果，研究了城市地表特征对京津冀地区夏季降水的影响。结果表明，在京津冀城市面积迅速增长的近三十年（1981～2010），该地区大部分站点的降水量都呈现减少的趋势，减少最明显的站点主要集中在京津唐城市区域，其中≥50 mm的降水量减少趋势占总降水量减少趋势的50%以上。城市扩张可能是造成京津冀降水时空格局改变的因素之一。通过对比分析控制试验与敏感性试验的模拟结果，发现城市化引起的地表特征的改变使北京、天津、唐山主要城市地区的降水量和降水频次都有明显减少，而城市群下风向的降水量和降水强度则明显增加和增强，其中50 mm以上等级的降水量变化最为显著，贡献率在60%以上。城市地表特征使北京、天津和唐山地区50 mm以上等级降水量的百分比下降了6%～20%，下风向地区增加了8%。城市地表特征也影响了主要城市和城市群下风向地区降水量的日变化结构，使北京和唐山几乎所有时段的降水量都有所减少，而城市群下风向降水量的增加主要发生在白天。研究发现城市地表特征对深对流的抑制（加强）可能是造成京津冀地区降水减少（增多）的重要原因，而由于城市地表蒸发量的改变引起的潜热通量和对流有效位能的改变则可能是引起深对流变化的重要因素。

城市化对河南省极端降水空间分布的影响

针对城市化对极端降水空间分布的影响问题,利用城市冠层模型结合WRF(weather research and forecasting)中尺度天气预报模式对河南省2021年"7·20"特大暴雨进行数值模拟,并结合国家级地面观测站实测降水数据对该模型的模拟精度进行了验证。结果表明,考虑城市冠层影响的耦合模型能更好地模拟出区域极端降水的强度和落区,与实测降水的空间分布更为接近,耦合模拟得到的平均降水量比未考虑城市冠层影响的模拟结果高12.1 mm;人工耗水改变了区域的水热耦合平衡,促进了城市区域对流性降水的形成。

北京城市下垫面对雾影响的数值模拟研究

为了探究北京城市下垫面对雾天气过程的影响,为北京地区雾数值预报水平的提高提供理论基础和科学依据,选取2011年10月29日北京地区雾天个例进行了数值模拟试验,通过对WRF/Noah/UCM模式系统中城市冠层参数的调整,显著改善了模式对此次雾天气过程的模拟效果.使用参数调整后的模式系统通过敏感性试验分析研究了北京城市下垫面对雾发生、发展和消散过程的影响.结果表明:参数调整后的WRF/Noah/UCM模式系统能够与实际观测较相符地模拟此次发生在北京地区的雾天气过程,北京城市下垫面主要通过对温度的改变对雾的形成、发展和消散产生显著影响,使雾不易在城市及其附近形成和发展,延后城市地区雾的形成,但城市的存在也使得城市地区及其附近雾不易消散,相较于没有城市时消散时间延后.

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。

南京复杂下垫面条件下的三维城市热环境模拟

运用WRF模式,选取考虑城市冠层结构(UCM算例)及不考虑城市冠层(NOUCM算例)两种城市下垫面参数化方案,对南京2010年夏季晴天小风典型天气条件下的城市热环境以及不同下垫面的边界层特征进行了模拟研究。结果表明:1)UCM方案模拟结果与实际情况较为吻合。其中2 m气温的模拟有较大的改进,模拟结果明显高于NOUCM方案,与观测更为吻合,同时更好地模拟出了冠层建筑物对于近地层风速的拖曳,10 m风速的模拟有非常明显地提高。2)UCM方案较好地模拟出了城市的三维热岛分布。由于建筑物地表对辐射的截留,白天14时(北京时间,下同)热岛较强,地面2 m高度处热岛范围较大,热岛面积大约为120 km2,强度为2℃。同时建筑物的存在使得城市湍流动能更大,向上的垂直扩散增加,距地面20 m时,依然能看出明显的热岛效应,热岛强度为1.5℃。距地面55 m处,UCM模拟所得的热岛范围缩小,热岛强度为1.1℃。UCM模拟所得的白天地表热量的扩散影响可达143 m,02:00 2 m处热岛最强为2℃,热岛影响也可达70 m以上。3)不同下垫面呈现出了不同的边界层特征,城市冠层结构对周边下垫面边界层结构存在程度不等的影响,14:00城市区域的湍流混合更强,城市边界层高度升高100 m左右,02:00,城市冠层结构的存在,导致近郊庄稼下垫面及紫金山植被下垫面的稳定逆温层结明显减弱。

大气数值模式中城市效应参数化方案研究进展

介绍了对城市效应影响大气运动的物理过程,以及在不同尺度大气模式中如何合理考虑这些物理过程,对进一步发展完善城市效应参数化方案中亟待解决的问题及困难作了论述,并着重强调了不同城市冠层参数化方案的优缺点。结论认为,随着城市的扩张,城市效应对大气运动的影响是数值模拟研究中一个不可或缺的物理过程,城市冠层模式的引入可以有效地提高数值模拟效果。对于不同尺度的数值模拟研究工作,应该采取不同的城市效应影响方案。目前,由于冠层模式中涉及的物理过程十分复杂,冠层模式还有待进一步完善,此外冠层方案的验证工作,尤其是应用城市地表能量平衡观测资料进行的验证工作还应该进一步深入开展。