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2015-01-11T12:50:07.000000Z
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According to a report published by PRB(Population Reference Bureau,2008a), In 2008, more than half the world's people lived in urban areas. By 2030, urban dwellers will make up roughly 60 percent of the world's population.Most of the additional population will take place in developing countries, most of which are in the tropical world. Massive migrations were witnessed by metropolis which have to undertake a list of consequences caused by human activities, while enjoying great productivity.
Urban climate is a field of the consequences and has been well studied in
recent years. anthropogenic activities will influence urban synoptic situation, and then generate micro climate locally. urban climate generally affects global energy transferring process, changing turbulence flux, radiation buget, and air condition both locally and regionally.
In this review, post research on important questions of urban climate. We
are going to draw a simple comparison to current energy balance models;we'll talk about urban air condition and urban air polution; then we will foucus on urban heat island(UHI) effect, a popular phenomenon studied a lot. At last we draw a conclusion and have some brief remark on urban climate.
urban climate refers to climatic conditions in an urban area that differ
from neighboring rural areas, and are attributable to urban development.Progress in urban climatology over the three decades was emphasized that urban climatology during this period has benifit from conceptual advances made in microclimatology and boundary-layer climatology in general.
recent urban climatology researches are concerning about multiple scales of urban climate and their spatial heterogeneity.[添加文献标题案例]
Scientists are looking into physical process to understand latent climate patterns explaining present climate problems instead of rather lengthy description.[添加文献标题案例]
Actually people are seek for available and feasible data for climate modeling industriously, in order to build a global climate monitoring network.It's a career requiringglobal collaboration.
in a rather small scale, researchers paid much attention to a concept called the roughness sublayer. in this layer, flow consists of thw interacting wakes and plumes of heat, humidity and pollutants.The nature ofthe urban surface, with its rigid buildings of different heights and physical characteristics, separated by trees,canyons and open spaces, makes it particularly susceptible to the development of a roughness sublayer of significant depth, perhaps several times the average building height.
While the UHI remains a compelling foucus of climate research in built-up areas.the UHI is the most remarkable effect caused by human activity, which indicates urban regionally high-temperature anomalies compare to neighbor surburbs resulting from urban lande use change.Models are built to simulate UHI and analysize quantificationally.
Early researches on urban climate were limited by data lack, so phenomena can be described but not be clearly analysized. Recently with supprots of hyperspectral remote sensing data and high resolution data, people are able to interpret information about urban climate and build up convincing models for better urban climate pattern reconstruction. Researchers now go deep into process of multi-scale events of climate change, while field measurement is also a effecient method to study ueban micro climate.
2.2.1 Field Measurement
[Song Xiaocheng,etc] conducted a three-day field measurement in studies of the potential effects of urban river on urban thermal climate and thermal comfort.[引用]
Short term observation in a specified region has higher possibility to acquire detailed discoveries and make it easy to understanding some particular cliamte events. [引用]
[Ganbo Han] chose a typical block alongside the Yangtze River in Hankou,Wuhan city to perform the fleldmeasurement and relevant analysis about anomal temperature in Wuhan.They found that ground surface temperature is mostly affectedby solar radiation while it is about 14°C higher than air temperature above the river.Indeed, field measurement is relatively easily performed but it is sometimes of indeterminacy.
2.2.2 Remote Sensing Methods
Remote sensing imagery has been widly applied in climatology research, as it's high resolution, long period and multispectral reliable data. [Hanqiu Xu] used multispectral Landsat TMimages of 1987,1996, and 2006 to study the urban expansion and heat island dynamicsof Quanzhou region.they found that the great increase in urban area has resulted in the development of UHIs in the region.
[Benjamin Bethtel] used multitemporal thermal imagery to conduct a direct determination of thermal surface properties and thematic classification.In his paper Landsat thermal and NDVI data were related to a long-term urban heat island pattern for Hamburg.
deployed Inversion algorithms, Remote sensing data can be converted to parameters related to urban climate models, so that researchers can draw useful conclusions of higher quantitative feature.
2.2.3 Urban Climate Processes
Urban climate is dynamic, sensitive to condition changes, so study on climate processes is necessary and fruitful.
Urban climate process contains dynamic processes like atmospheric turbulence and exchanges of energy and water.Although the processes are of small scale, they are complicated and integrated questions for quite much related factors. Boundary-layer above urban city is chaotic system and heat and matter tranmission there are flowing turbulently.Oke remarked that 'eveidence of the structure of turbulence over cities is limited'. But the availability of turbulence measurements in urban environments has improved dramatically recently.Dramastic changes are occuring mainly in troposphere and we have little knowledge about it.
What's more, urban radiation balance is another main topic when talking about urban climate process.A case study of a desert city area shows that the hazy atmosphere observed during the summer period has affected the retrival of incoming shortwave radiation. In last century people had noticed additional attenuation by urban boundary-layer pollutants was believed to reduced solar irraiance, and later study found more decrease of solar irradiance in cities around the world.Recent years has witnessed huge changes in the beam ultraviolet portion of the spectrum, with compensating increase in diffuse UV.[引用]
Urban heat island is among the most evident aspects of
human impacts on the earth system. Generally, the intensity of UHI has a positive relation with the size of a city. There are researches indicating that UHI effect is most servere in winter, while it is not obvious in spring. As for meteorological condition, wind speed is the key factor.
Primarily, UHI performs in temperature amomalies between urban area and its suburbs. this often brings about city wind and heat flux disturbance.In a research scientists did monitoring across 419 big cities, they found that the average annual daytime surface urban heat island intensity(SUHII) is higher than the annual nighttima SUHII.The distribution of nighttime SUHII correlates positively with the difference in albedo and nighttime light between urban area and suburban area, while the distribution of daytime SUHII correlates negatively across cities with the difference of vegetation cover and activity between urban and suburban areas.
In our country,Zhou Shuzhen discussed UHI in Shanghai in 1982,and she proposed five-island effect of Shanghai at that time.Sun Jisong found Significant positive correlation between urban development and UHI using temperature records and files recorded urban development intensity.Wang Wenjie monitored urban land use,urban NDVI and urban heat area in Beijing based on remote sensing imageries of multiple period time since 1978, and found correlations of spatial distribution,urban grass land use and UHI.
This review has attempted to explain main problems and methods in urban climatology.As of the most importance, this paper emphasizes UHI effect and discusses UHI its concept and research content.The major clues are as follows:
1.Urban climate is definitly complicated and global related,while urban climatology has developed a lot benifit from rencent conceptual developments and available data deployment.
2.Models are springing out to simulate processes in urban climate. With more acurate data available and more perfect monitoring network being built,quantitive analysis can be conducted effectively and convincingly.
3.UHI represents significant temperature anormalies caused by human activities, and it's also a symbol of new heat flux balance, which is influenced by underlying surface roughness, air turbulence condition and huamn heat emissions.