The urbanization process alters the radiative, thermal, moisture and aerodynamic characteristics of a surface, which significantly affect the surface energy balance within the atmospheric boundary layer. Such modifications can lead to the so-called urban warming phenomenon, where the extent and rate of urban surface and air temperature is substantially higher than the corresponding rural region, which has caused serious concern in recent decades. However, the understanding of the causes of urban warming is incomplete, and the same applies to lack of effective mitigation strategies. Therefore, in order to have a complete understanding of the formation of urban warming, the present thesis focusses on the estimation of temporal variations of urban surface and air temperatures by using numerical simulations, analytical methods and field measurements. To better understand the mechanism(s) of urban surface temperature variation, a three-dimensional model that incorporates the energy exchange processes is first developed for a realistically complex city. In order to reduce the computational effort for the radiation heat transfer calculation, the compressed row storage scheme is applied, which permits the rigorous consideration of multiple reflections in a realistic urban area with hundreds of buildings. The developed surface energy balance model is then used to investigate the effects of the urban canopy geometry on urban albedo and surface temperature. The average urban albedo is less for a moderately compact city having high rise buildings with varying building heights than other cases. A cooler urban street surface temperature with smaller amplitude and earlier occurrence of the daily maximum temperature is observed in a high rise compact city than a low-rise sparse city. In order to understand quantitatively the causes of urban air warming, a new analytical zero-dimensional urban air temperature model is also developed, which is able to capture the features of the urban temperature variation. Results show that solar heat gain, evapotranspiration and the anthropogenic heat affect the mean air temperature, while heat storage and thermal convection affect the amplitude and phase shift of the daily cycle. A high-rise, high-density city generates low surface temperature, resulting in low air temperature during the day. The main conclusion of this study is that on the condition rural air temperature cycle is given, the mean temperature of the urban air and surface temperature is determined by the net heat gain and ventilation rate, and the amplitude and phase can be obtained from thermal storage and ventilation rate. Essentially, the net heat gain, thermal storage and ventilation are affected by urban morphology, and hence a city thermal environment can be designed. (419 words) / published_or_final_version / Mechanical Engineering / Doctoral / Doctor of Philosophy

Thesis (Ph. D.)--University of Hong Kong, 2009. / Includes bibliographical references (p. 133-142). Also available in print.

Thesis (M. Phil.)--University of Hong Kong, 2009. / Includes bibliographical references (leaves 118-122). Also available in print.

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Contrasting the day-night variation in thermal landscape, the higher local variation and irregularity on urban surface temperature pattern during daytime was observed and identified by landscape metrics compared with those of nighttime pattern. The diversity and fragmentation metrics had revealed the influence of urban development on the overall urban landscape pattern. Along with urban development, daytime pattern of urban thermal landscape presented more fragmentation, less diversity and uneven texture distribution within daytime observations. / During the process of worldwide urbanization along with high rise and high density housing development in large cities, urban warming has received growing concern among many environmental issues related to urban landscape change. Due to the complicated interplay between urban environment and local climate, it is far from certain about the urban effects on local warming. In this literature, a systematic monitoring and analysis of the spatial dependency and heterogeneity of urban thermal landscape at city scale remains inadequate. The goal of this doctoral research is to develop a research framework incorporating geospatial statistics, thermal infrared remote sensing and landscape ecology to study the urban effect on local surface thermal landscape regarding both the pattern and process. / GWR analysis offered an in-depth investigation of local effect on surface temperature variation which had been proven to be spatially varying and influenced by local weather condition with local environmental setting quantified with the referred site specific environmental factors. The local dominant factor accounted for most to the site specific surface temperature variation which varied significantly in space and time and prevented a general delineation of the relative association among environmental factors to surface temperature disparities. The effective adaptive measures could be devised locally with reference to day-night needs in the identification of this feature. / In summary, global regression analysis confirmed the relationship between environmental factors and surface temperature and gave a general overview of urban effect on local surface warming. The distinctive mechanism of dominating day-night surface warming was uncovered by regression analysis. Vegetation played the most important role which could be referred as surface cooling in average to local surface temperature variation as compared with other measures of local environment during both daytime and nighttime. Besides the dominant role of local solar radiation on surface warming, building square footage demonstrated the second important influence on local surface temperature elevation during daytime. During nighttime, population density played a dominant role on nighttime surface warming among different parameters, with the second important contribution of nighttime surface warming coming from road density. While elevation and distance from coast demonstrated obvious cooling effect on surface temperature within most nighttime models. / Located in a subtropical region, Hong Kong's development with high rise and high density housing made it a suitable site for studying urban effect on local warming. This research chose Hong Kong as the case study which hopes to enrich our knowledge regarding urban local thermal performance and add to our understanding of urban microclimate in hot-humid weather area. / Xue, Yucai. / Adviser: Tung Fung. / Source: Dissertation Abstracts International, Volume: 71-01, Section: B, page: 0185. / Thesis submitted in: September 2008. / Thesis (Ph.D.)--Chinese University of Hong Kong, 2009. / Includes bibliographical references (leaves 201-228). / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Electronic reproduction. Ann Arbor, MI : ProQuest Information and Learning Company, [200-] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstracts in English and Chinese.

近幾十年來，全球氣候變化──特別是城市氣候變化──影響城市環境及居民生活質素的程度已引起公眾廣泛的討論。然而，過去研究一般採用之低空間解析度並不足夠為城市規劃及設計提供完善的資訊，引致對於氣候變化缺乏充分的考慮。高密度的城市環境（如香港）需要高時間解析度的氣候數據以制定有效的適應和減緩策略來應對未來氣候的變化。 / 本研究採用線性迴歸技術，以預測未來香港市區和郊區的氣溫。本研究利用氣象站和統計延伸得出之基線氣溫數據來建立統計降尺度模型，以預測未來香港市區和郊區之平均氣溫、最高氣溫和最低氣溫。 / 根據結果顯示，統計降尺度模型能夠有效建立大氣氣象參數和本港氣溫兩者之間的關係，尤其春季、秋季和冬季之氣溫預測表現理想。另外，冬季氣溫的上升趨勢則出現較大的升幅。研究結果亦顯示夜間氣溫的上升趨勢一般比日間為高。在未來的日子，郊區的溫度上升亦將會比市區為高。隨著城市化的影響納入預測溫度因素之中，預計郊區的氣溫將超過城市核心（天文台總部之氣象站），而郊區暖化的速度亦比市區和近郊為高。 / 本研究發現統計降尺度方法能有助利用全球氣候模型（GCM）提供之數據，以預測未來氣候之變化。城市規劃與設計過程是需要大量的數據進行評估氣候變化對城市環境的影響之研究，儘管統計降尺度方法有一定程度的局限性，它仍然是一個低成本而有效的方法。根據未來預測之氣溫，本研究具體指出未來之氣候變化對於城市規劃和設計過程的影響，亦提出了一系列於不同規劃層面適用之適應和減緩措施的建議。 / The effects of global climate change on urban environment have been widely discussed in recent decades. In particular, changes in urban climate have received much attention as they affect the living quality of urban dwellers. However, the coarse spatial scales employed in recent climate change studies were found to be insufficient in the context of urban planning and design. It leads to the lack of information on the changing urban climate and insufficient consideration of climate change in urban planning and design processes. In high-density cities like Hong Kong, the complex urban environment requires climatic data at very fine temporal resolution in order to formulate effective adaptation and mitigation strategies for future climate change. / The present study employed regression techniques to establish empirical relationship between large-scale predictor variables and local predictands in order to obtain future air temperature of urban and rural areas of Hong Kong. 40-year baseline conditions of local air temperature were obtained from both the observational and statistically extended temperature record. Monthly means of daily mean, maximum, and minimum air temperatures for both daytime and night-time were calculated for establishing statistical downscaling (SD) models to project future air temperature of urban and rural areas of Hong Kong. / The results suggest that regression-based downscaling techniques are able to capture the relationship between large-scale atmospheric conditions and station-scale meteorological parameters. The SD models performed particularly well in winter and considerably satisfactory results were obtained in spring and autumn. Night-time temperature trends generally exhibited greater increases than daytime trends. Seasonal variations were present with greatest increases observed in winter. Rural areas would likely experience greater warming than the urban areas in the future. With urbanization effect incorporated into the projected temperature series, it was found that air temperature projected for suburban stations would exceed that for the urban core. Rural warming also exhibited a higher rate than those observed in suburban and urban stations. / The present study shows that statistical downscaling approach provides a method to obtain information about future climatic conditions at local scale by using GCM outputs which are widely accepted to be useful tools to assist climate change studies. Despite of the limitations that historical climate would persist in projected climatic series, it allows a low-cost but effective measure for climate impact assessments, particularly in the context of urban planning and design, which requires extensive data for a wide range of studies. Based on the projected air temperature, implications of future climate change on urban planning and design of potential development were discussed and recommendations on potential adaptation and mitigation measures at different planning levels were also presented. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Lau, Ka Lun. / Thesis (Ph.D.) Chinese University of Hong Kong, 2013. / Includes bibliographical references (leaves 159-173). / Abstracts also in Chinese.

全球變暖現象在最近幾年的高漲，引發了城市中不斷發生的嚴峻局部炎熱氣候，也令戶外熱舒適度課題不斷得到更多關注。這篇論文中作者從城市設計角度考察熱舒適度的分佈，並且著眼于通過定量模型研究城市形態因子與熱舒適度之間的關係，以模型作爲客觀數據實測和人主觀感受之間的橋梁。文中以荷蘭城市鹿特丹為典型代表研究了輻射爲主情況下城市設計對熱能動力學的影響，用平均輻射溫度為關鍵變量基於SOLWEIG軟件模擬探討這個變量的時空分佈。同時爲了在更長時間段中對熱舒適度作校準，輻射模擬的結果也通過RayMan軟件和氣象學觀測結果結合得出更加符合人體感受的熱環境評判結論。 / 文中主要考慮三种城市形態因子：高寬比，街道方向，和建築物外形。街道被分成中間區域和兩邊行人區域，以力圖得到最接近事實上熱舒適度感覺的結果。通過分析模型輸出和模擬結果，我們確認了城市形態因子對熱舒適度有顯著影響。從不同角度進一步研究不同城市因子的變化和熱舒適度變化之間的定量關係，我們發現所考慮的三种形態因子都會改變熱舒適度的分佈，並且各個因子的影響之間也有相關性。這些影響和相關性讓我們可以從結果中總結出更深層的機理，其中最重要的分別是日間太陽短波輻射的阻擋陰影效果和夜間長波散熱的束縛效果。在這兩個原理的作用下模型結果可以得到統一的解釋歸納，並且給出基於這些結果的城市設計方面的建議，包括在日照最充足的區域減小天空開闊度，通過調整街道方向減少日光直接照射等等。 / 文中所給出的結果和研究方法展現了城市形態對熱舒適度的動態影響，並可以作爲進一步研究的基礎。我們給出了一些可行的擴展，討論了更多可以研究的重要效應，以進一步加深對這些影響的理解和拓寬改善城市熱環境的手段。經由定量建模和多變量分析，城市設計對熱舒適度的影響可以得到更好理解和利用。 / The outdoor thermal comfort is becoming an increasingly important topic in recent years because of the global warming effect and the severe heat events experienced by cities. In this thesis we address the thermal comfort from the urban design perspective and aim at quantitatively modeling the relationship between urban form factors and thermal comfort output from a data-based human-oriented approach. Selecting Rotterdam of the Netherlands as a target city, we focus upon the radiation component of thermal environment when connecting urban design to thermal equilibrium dynamics. The radiation is represented by the Mean Radiant Temperature (Tmrt) variable, and simulated using the Solar Long Wave Environmental Irradiance Geometry (SOLWEIG) model. To evaluate the long-term calibrated thermal comfort using data over longer periods the RayMan model is also used to combine the radiation simulation output with meteorological parameters for a more human-oriented comparison. / The urban form factors considered include the height-to-width ratio (h/w), the street orientations, and the urban forms. To enable a more realistic evaluation of pedestrian thermal comfort we also divide the street into three separate zones so that the center and two side pavement zones can be individually studied and placed together for a comparison. The results confirm the impact of urban forms on thermal comfort and quantitatively evaluate such effects from all different perspectives. We find that all the urban form factors impose variations of the simulated thermal variables, and they also have correlated interactions revealing deeper connections and principles governing such relations. The most significant principles found for the scenario studies are the shading of solar short-wave radiation during the day and the trapping of long-wave emission at night, summarizing all the individual effects into a coherent body of knowledge. Utilizing these discoveries we can make design recommendations toward a better thermal environment, including reducing the sky view factor at the locations with the most solar radiation exposure, orienting the streets to utilize the building shades, etc. / As a precursor into the dynamic nature of urban form impact over thermal comfort, the results and methodology presented in this thesis serve as a foundation of more research in the same direction. Several possible extensions are proposed and more important effects are mentioned, revealing the potential and strength of our approach. Both the current research and possible extensions contribute to the field of urban thermal comfort and provide unique perspectives for rigorous quantitative investigations into a realistic urban thermal comfort model. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Dai, Qun. / Thesis (Ph.D.) Chinese University of Hong Kong, 2014. / Includes bibliographical references (leaves 76-80). / Abstracts also in Chinese.

published_or_final_version / Mechanical Engineering / Doctoral / Doctor of Philosophy

published_or_final_version / Architecture / Master / Master of Philosophy