The urban heat island of a city in an arid zone: the case of Eilat, Israel

Summary This study presents the results of a preliminary research that was conducted in the city of Eilat, located in an extreme hot and arid zone on the northern coast of the Red Sea. The purpose was to analyse the characteristics of the local urban heat island (UHI). Diurnal pre-dawn and early-afternoon measurements were taken in winter and summer weather conditions on three separate occasions for two consecutive years. The results show the development of a moderate UHI located around the most intensive area of human activity; the city business centre and dense hotel belt. The UHI is more significant at midday during the summer period, while early morning inversions in winter have a weakening effect on the UHI intensity. It was found that the topography and wind regime have a dominant effect on the location and intensity of the UHI, while the sea has a very marginal effect. Due to the UHI influences on the spatial distribution of the heat stress in the city, it is suggested that further applied UHI research should be focused on the summer period.     

Maximum urban heat island intensity in a medium-sized coastal Mediterranean city

This paper studies the maximum intensity of the urban heat island (UHI) that develops in Volos urban area, a medium-sized coastal city in central Greece. The maximum temperature difference between the city center and a suburb is 3.4°C and 3.1°C during winter and summer, respectively, while during both seasons the average maximum UHI intensity is 2.0°C. The UHI usually starts developing after sunset during both seasons. It could be attributed to the different nocturnal radiative cooling rate and to the different anthropogenic heat emission rate that are observed at the city center and at the suburb, as well as to meteorological conditions. The analysis reveals that during both seasons the daily maximum hourly (DMH) UHI intensity is positively correlated with solar radiation and with previous day’s maximum hourly UHI intensity and negatively correlated with wind speed. It is also negatively correlated with relative humidity during winter but positively correlated with it during summer. This difference could be attributed to the different mechanisms that mainly drive humidity levels (i.e., evaporation in winter and sea breeze (SB) in summer). Moreover, it is found that SB development triggers a delay in UHI formation in summer. The impact of atmospheric pollution on maximum UHI intensity is also examined. An increase in PM10 concentration is associated with an increase in maximum UHI intensity during winter and with a decrease during summer. The impact of PM10 on UHI is caused by the attenuation of the incoming and the outgoing radiation. Additionally, this study shows that the weekly cycle of the city activities induces a weekly variation in maximum UHI intensity levels. The weekly range of DMH UHI intensity is not very large, being more pronounced during winter (0.4°C). Moreover, a first attempt is made to predict the DMH UHI intensity by applying regression models, whose success is rather promising.     

Estimating daily net radiation in the FAO Penman–Monteith method

Heatwave intensity and frequency are predicted to increase in the coming years, and this will bear adverse consequences to the environmental well-being and the socio-economic fabric in urbanized areas. The hazardous combination of increased heat storage and reduced water retention capacities of the land surface make the urban areas warmer than the surrounding rural areas in what is commonly known as the urban heat island (UHI) effect. The primary motives of this study are to quantify the interaction of this city-scale UHI with synoptic-scale heatwave episodes and to analyze the factors that mediate this interaction. A modified version of the Weather Research and Forecasting model (WRF) is utilized to simulate two heatwave episodes in New York City. The land surface scheme in the default WRF model is modified to better represent the surface to atmosphere exchanges over urban areas. Our results indicate that during the heatwave episodes, the daily-averaged UHI in NYC increased by 1.5 K. Furthermore, most of this amplification occurs in the mid-afternoon period when the temperatures peak. Wind direction and urban-rural contrasts in available energy and moisture availability are found to have significant and systematic effects on the UHI, but wind speed plays a secondary role.     

The extent and intensity of the urban heat island in Iași city,Romania

The study underlines the characteristics of the urban heat island of Ia?i (Ia?i’s UHI) on the basis of 3 years of air temperature measurements obtained by fixed-point observations. We focus on the identification of UHI development and intensity as it is expressed by the temperature differences between the city centre and the rural surroundings. Annual, seasonal and daily characteristics of Ia?i’s UHI are investigated at the level of the classical weather observation. In brief, an intensity of 0.8 °C of UHI and a spatial extension which corresponds to the densely built area of the city were delineated. The Ia?i UHI is stronger during summer calm nights—when the inner city is warmer with 2.5–3 °C than the surroundings—and is weaker during windy spring days. The specific features of Ia?i’s UHI bear a profound connection to the specificity of the urban structure, the high atmospheric stability in the region and the local topography. Also, the effects of Ia?i’s UHI upon some environmental aspects are presented as study cases. For instance, under the direct influence of UHI, we have observed that in the city centre, the apricot tree blossoms earlier (with up to 4 days) and the depth of the snow cover is significantly lower (with up to 10 cm for a rural snow depth of 30 cm) than in the surrounding areas.     

南京地区下垫面变化对城市热岛效应影响的数值模拟

利用新一代中尺度研究和预报模式(Weather Research and Forecasting Model,简称WRF)分别耦合多冠层、单冠层和平板模式三种情况进行南京地区2007年8月1日的天气过程模拟,分析不同城市冠层方案对南京气象场的模拟效果。在此基础上,结合模拟效果最好的城市冠层方案,研究南京城市下垫面的变化对其热岛的影响。结果表明:多冠层方案对近地面气温、10 m风场的模拟效果最好;城区的扩张使南京地区近地面气温升高,主要表现为城市区域夜间升温显著,并且导致热岛强度明显增强;城市扩张后,城区白天风速大范围地减小,同时热岛环流更加显著,且具有明显的城市热岛的"下游效应"。     

1954—2004年珠江三角洲大气能见度变化趋势

利用珠江三角洲5个城市气象观测站 (广州、深圳、台山、惠州、高要) 的长期能见度观测资料, 采用累积百分率、Ridit分析法和烟幕/霾日频率3种统计分析方法, 分析了1954—2004年珠江三角洲能见度变化趋势, 初步探讨了珠江三角洲大气环境的变化特征及其可能原因。结果表明:近50年来, 珠江三角洲城市能见度呈显著下降趋势。能见度的下降起始于20世纪70年代初, 并在经济高速发展的80—90年代初进一步恶化, 90年代中期之后虽然能见度变化趋于平缓, 但能见度下降的趋势仍未从根本上改变。珠江三角洲能见度与当地的人口增加和经济发展程度有密切的关系, 同时污染物的区域输送对西部下风向城市能见度也有很大影响。珠江三角洲能见度未得到有效改善的主要原因, 很可能是由于细粒子污染, 尤其是二次粒子前体物如SO₂和NO_X等的排放未得到有效控制。     

Modeling the relative roles of the foehn wind and urban expansion in the 2002 Beijing heat wave and possible mitigation by high reflective roofs

Rapid urbanization has intensified summer heat waves in recent decades in Beijing, China. In this study, effectiveness of applying high-reflectance roofs on mitigating the warming effects caused by urban expansion and foehn wind was simulated for a record-breaking heat wave occurred in Beijing during July 13–15, 2002. Simulation experiments were performed using the Weather Research and Forecast (WRF version 3.0) model coupled with an urban canopy model. The modeled diurnal air temperatures were compared well with station observations in the city and the wind convergence caused by urban heat island (UHI) effect could be simulated clearly. By increasing urban roof albedo, the simulated UHI effect was reduced due to decreased net radiation, and the simulated wind convergence in the urban area was weakened. Using WRF3.0 model, the warming effects caused by urban expansion and foehn wind were quantified separately, and were compared with the cooling effect due to the increased roof albedo. Results illustrated that the foehn warming effect under the northwesterly wind contributed greatly to this heat wave event in Beijing, while contribution from urban expansion accompanied by anthropogenic heating was secondary, and was mostly evident at night. Increasing roof albedo could reduce air temperature both in the day and at night, and could more than offset the urban expansion effect. The combined warming caused by the urban expansion and the foehn wind could be potentially offset with high-reflectance roofs by 58.8 % or cooled by 1.4 °C in the early afternoon on July 14, 2002, the hottest day during the heat wave.     

Mesoscale and macroscale aspects of the morning Urban Heat Island around Athens,Greece

Summary This paper examines the characteristics of the morning Urban Heat Island (UHI) in Athens basin, Greece. The study is performed by using and analyzing mesoscale and synoptic data covering the period 1990–2001. The UHI was estimated using the 0600 Local Time (LT) minimum temperature differences between rural and urban areas of the city. The analysis results in 7 UHI classes. A strong UHI was found for the 1/3 of days. The specific meteorological characteristics (surface and upper air, cooling rate) of each UHI class were revealed and examined. The spatial and temporal characteristics of the UHI were also identified. The UHI is largest on nights with clear skies and low relative humidity. In all seasons the UHI switches on rapidly in afternoon. During spring and summer, sea breeze commonly reduces and delays the UHI. Cases documenting the sensitivity and rapidity of changes of the UHI to changes in classes (cloud cover, wind) are also presented. The cooling rate is higher in the urban area under negative and lower under positive UHI conditions. Mesoscale and macroscale phenomena were examined during the different UHI classes through a weather type scheme. It was emerged that high UHI classes are associated with anticyclonic conditions or advection in the lower troposphere, while low UHI classes are associated with strong northeasterly winds. Anticyclonic conditions which frequently occur in spring and early summer, reduce or reverse the UHI to Urban Cooling Island.     

Climate Change driven evolution of hazards to Europe’s transport infrastructure throughout the twenty-first century

Air circulation due to the urban heat island (UHI) effect can influence the dispersion of air pollutants in a metropolis. This study focusses on the influence of the UHI effect on particulate matter (PM; including PM_2.5 and PM_2.5–10) between May and September 2010–2012 in the Taipei basin. Meteorological and PM data were obtained from the sites, owned by the governmental authorities. The analysis was carried out using t test, relative indices (RIs), Pearson product–moment correlation and stepwise regression. The results show that the RI values for PM were the highest at moderate UHI intensity (MUI; 2 °C ≤ UHI < 4 °C) rather than at strong UHI intensity (SUI; 4 °C ≤ UHI) during the peak time for anthropogenic emissions (20:00 LST). Neither the accumulation of PM nor the surface convergence occurred in the hot centre, as shown by the case study. At MUI, more than 89 % of the synoptic weather patterns showed that the weather was clear and hot or that the atmosphere was stable. The variation in PM was associated with horizontal and vertical air dispersion. Poor horizontal air dispersion, with subsidence, caused an increase in PM at MUI. However, the updraft motion diluted the PM at SUI. The stepwise regression models show that the cloud index and surface air pressure determined the variation in PM_2.5–10, while cloud index, wind speed and mixing height influenced the variation in PM_2.5. In conclusion, a direct relationship between UHI effect and PM was not obvious.     

Characterization and estimation of urban heat island at Toronto: impact of the choice of rural sites

In this study, the urban heat island of Toronto was characterized and estimated in order to examine the impact of the selection of rural sites on the estimation of urban heat island (UHI) intensity (?T _u-r). Three rural stations, King Smoke Tree (KST), Albion Hill, and Millgrove, were used for the analysis of UHI intensity for two urban stations, Toronto downtown (Toronto) and Toronto Pearson (Pearson) using data from 1970 to 2000. The UHI intensity was characterized as winter dominating and summer dominating, depending on the choice of the rural station. The analyses of annual and seasonal trends of ?T _u-r suggested that urban heat island clearly appears in winter at both Toronto and Pearson. However, due to the mitigating effect on temperature from Lake Ontario, the estimated trend of UHI intensity was found to be less at Toronto compared to that at Pearson which has no direct lake effect. In terms of the impacts of the rural stations, for both KST and Millgrove, the trends in UHI intensity were found to be statistically significant and also were in good agreement with the estimates of UHI intensities reported for other large cities in the USA. Depending on the choice of the rural station, the estimated trend for the UHI intensity at Toronto ranges from 0.01°C/decade to 0.02°C/decade, and that at Pearson ranges from 0.03°C/decade to 0.035°C/decade during 1970–2000. From the analysis of the seasonal distribution of ?T _u-r, the UHI intensity was found to be higher at Toronto in winter than that at Pearson for all three rural stations. This was likely accounted for by the lower amount of anthropogenic heat flux at Pearson. Considering the results from the statistical analysis with respect to the geographic and surface features for each rural station, KST was suggested to be a better choice to estimate UHI intensity at Toronto compared to the other rural stations. The analysis from the current study suggests that the selection of a unique urban–rural pair to estimate UHI intensity for a city like Toronto is a critical task, as it will be for any city, and it is imperative to consider some key features such as the physiography, surface characteristics of the urban and rural stations, the climatology such as the trends in annual and seasonal variation of UHI with respect to the physical characteristics of the stations, and also more importantly the objectives of a particular study in the context of UHI effect.     

Daily maximum urban heat island intensity in large cities of Korea

Summary This study investigates the characteristics of the daily maximum urban heat island (UHI) intensity in the six largest cities of South Korea (Seoul, Incheon, Daejeon, Daegu, Gwangju, and Busan) during the period 1973–2001. The annually-averaged daily maximum UHI intensity in all cities tends to increase with time, but the rate of increase differs. It is found that the average annual daily maximum UHI intensity tends to be smaller in coastal cities (Incheon and Busan) than in inland cities (Daejeon, Daegu, and Gwangju), even if a coastal city is larger than an inland city.A spectral analysis shows a prominent diurnal cycle in the UHI intensity in all cities and a prominent annual cycle in coastal cities. A multiple linear regression analysis is undertaken in order to relate the daily maximum UHI intensity to the maximum UHI intensity on the previous day (PER), wind speed (WS), cloudiness (CL), and relative humidity (RH). In all cities, the PER variable is positively correlated with the daily maximum UHI intensity, while WS, CL, and RH variables are negatively correlated with it. The most important variable in all cities is PER, but the relative importance of the other three variables differs depending on city. The total variance explained by the multiple linear regression equation ranges from 29.9% in Daejeon to 44.7% in Seoul. A multidimensional scaling analysis performed with a correlation matrix obtained using the daily maximum UHI intensity data appears to distinguish three city groups. These groupings are closely connected with distances between cities. A multidimensional scaling analysis undertaken using the normalized regression coefficients obtained from the multiple linear regression analysis distinguishes three city groups. Notably, Incheon and Busan form one group, whose points in the two-dimensional space are very close. The results of a cluster analysis performed using the multivariate data of PER, WS, RH, and CL are consistent with those of the multidimensional scaling analysis. The analysis results in this study indicate that the characteristics of the UHI intensity in a coastal city are in several aspects different from those in an inland city.     

城市热岛效应热点问题研究进展

概述了国内外城市热岛效应研究热点及不足,探讨了时空替代性、评价指标合理性、模拟空间尺度适宜性及理论与应用研究相结合等问题,指出未来城市热岛效应研究应注重新方法与新思路的集成、城市群热岛效应监测、城市热岛与全球异常环境要素间的生态关联性、缓解城市热岛效应的生态途径与技术方法。     

1月北大西洋东部-乌拉尔山阻塞高压频次年际尺度偶极子模态特征

基于观测和再分析资料,本文研究了近几十年来1月北大西洋东部-乌拉尔山阻塞高压频次年际变异主导模态特征及与其相联的大气背景场。结果表明,1980—2019年冬季(12月—次年2月)该地区阻塞高压频次年际变异的主导模态存在明显月际差异:12月表现为北大西洋东部-欧洲西部地区阻塞高压频次的显著同位相变化,1月为北大西洋东部-欧洲西部与乌拉尔山地区阻塞高压频次的具有显著的反位相变化即偶极子模态,2月则为北大西洋东部-乌拉尔山阻塞高压频次的显著同位相变化。进一步研究表明:1月北大西洋东部-乌拉尔山阻塞高压频次偶极子模态与同期局地纬向西风、纬向风垂直切变、经向位涡梯度等大气背景场异常偶极型变化相联系。当乌拉尔山地区关键大气背景场为负异常,而北大西洋东部-欧洲西部为正异常时,两个地区阻塞高压频次分别增加和减少,呈现偶极子模态;反之亦然。阻塞高压频次偶极子模态及相关的背景环流异常可通过调节水平温度平流、垂直运动和水汽输送等来影响1月欧亚北部气温和降水,以及它们的极端事件频次。当阻塞高压频次偶极子模态处于正位相时,乌拉尔山北部(南部)和欧洲南部增温(降温),极端暖、冷事件频次分别增加(减少)和减少(增加...     

基于MODIS的安徽省代表城市热岛效应时空特征

利用2001—2010年覆盖安徽省的MODIS数据,选取在气候、地理、城市化等方面具有代表性的合肥、芜湖、阜阳作为研究对象,并结合GIS技术,分析地表温度的日变化及季节变化特征,得到安徽省代表城市热岛效应的时空分布。结果表明:安徽省省会合肥的热岛效应最为显著,安徽省南部代表城市芜湖的热岛效应强于北部代表城市阜阳, 同时具有显著的日变化和季节变化特征。近10年来,安徽代表城市热岛面积和热岛强度均呈增加趋势,但合肥热岛强度大于3 ℃的极端热岛效应有一定缓解。白天大片水体对缓解城市的热岛效应作用明显,而夜晚则不明显,甚至成为地表温度的高值中心。夏季地表温度与归一化植被指数的负相关最显著,即提高城市植被覆盖度对降低地表温度和缓解城市热岛效应有重要影响。     

Urban heat island in a coastal urban area in northern Spain

This work examines the characteristics of the urban heat island (UHI) in a medium-sized city in northern Spain (Bilbao) using 5-year climate data (2005–2009) and the results of three specific measurement campaigns (2009–2010). Urban climate variables are not only compared with those in rural sites but also local climatic differences occurring inside the city are analysed. The findings presented in this paper show the influence of complex topography and sea/land breeze in the urban climate. Spatial characteristics and temporal evolution of UHI is presented. Hourly maximum temperature anomaly (ΔT _{u–r, max}) occurs just after sunrise and an urban cold island (UCI) is developed after midday. Along the year, mean UHI intensity is highest in autumn and the UCI effect increases in spring and summer in relation with sea breeze cooling potential. Diurnal and seasonal variation of air flow patterns appear to influence significantly on UHI intensity.     

Temporal variability of the Buenos Aires, Argentina, urban heat island

This paper describes the statistical characteristics and temporal variability of the urban heat island (UHI) intensity in Buenos Aires using 32-year surface meteorological data with 1-h time intervals. Seasonal analyses show that the UHI intensity is strongest during summer months and an “inverse” effect is found frequently during the afternoon hours of the same season. During winter, the UHI effect is in the minimal. The interannual trend and the seasonal variation of the UHI for the main synoptic hours for a longer record of 48?years are studied and associated to changes in meteorological factors as low-level circulation and cloud amount. Despite the population growth, it was found a negative trend in the nocturnal UHI intensity that could be explained by a decline of near clear-sky conditions, a negative trend in the calm frequencies and an increase in wind speed. Urban to rural temperature differences and rural temperatures are negatively correlated for diurnal and nocturnal hours both for annual and seasonal scales. This result is due to the lower interannual variability of urban temperatures in comparison to rural ones.     

基于MODIS资料分析近15年广州城市热岛特征及演变规律

利用MODIS地表温度数据,计算城市热岛强度指数,分析近15年广州市城市热岛的时空分布特征及演变规律,并结合气象观测数据、社会统计数据定性分析其主要影响因素。结果表明:广州市城市热岛的空间分布受地形地貌影响明显,负热岛区主要分布于森林密集的北部山区,无热岛区主要分布于中部低山丘陵区域,热岛区主要分布于高度城市化的中南部平原区。关于城市热岛的日变化规律,白天热岛区、负热岛区面积均小于夜间,但白天热岛区强度、负热岛区强度大于夜间。关于城市热岛的季节变化规律,冬季热岛区面积最大,热岛强度最小,夏季热岛区面积最小,热岛强度最大;冬季负热岛区面积最小,负热岛强度最小,夏季负热岛区面积最大,负热岛强度最大。对于城市热岛的年际变化规律,近15年来广州市的热岛区、负热岛区占全市总面积的百分比呈上升趋势,无热岛区所占百分比呈下降趋势,人为热排放在城市中心区域的持续增长,加上区内建筑物密度大、植被覆盖度低,导致了热岛区的增加,而北部山区至中部丘陵山区的植被的持续好转,加上地理特征限制了该区域的城市化发展,导致了负热岛区的增加。      

城市热岛效应热点问题研究进展

概述了国内外城市热岛效应研究热点及不足,探讨了时空替代性、评价指标合理性、模拟空间尺度适宜性及理论与应用研究相结合等问题,指出未来城市热岛效应研究应注重新方法与新思路的集成、城市群热岛效应监测、城市热岛与全球异常环境要素间的生态关联性、缓解城市热岛效应的生态途径与技术方法。     

不同天气条件下沈阳城市热岛特征

利用1992—2008年沈阳站和新城子站逐日4个时次的平均气温、平均风速、降水量、云量和能见度资料,对不同天气条件下沈阳的城市热岛效应进行研究。结果表明：除雾和浓雾天气条件下,沈阳城市热岛强度在08时最弱外,其他天气条件下均表现为20时最强,14时最弱;不同天气条件下,夜间城市热岛强度均高于白天;晴朗无风条件下昼夜城市热岛强度差最大,为0.73℃。四季相比,除雾条件下秋季城市热岛强度最强外,其他天气条件下均为冬季最强;除大雨条件下春季城市热岛强度最弱外,其他条件下均为夏季最弱。沈阳城市热岛强度随降水量的增加而减弱,随能见度的降低而减弱,随着风速的增加而减弱。白天和夜间两个时次的差值表现为,1～3级风夜间变化幅度大于白天,0级和4～5级风速有相反规律,其他天气条件下无明显规律。     

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。