| Urban thermal environment is formed by energy distribution and exchange.It works like a bridge which connects the physical,chemical and biological processes in an urban environment.In an environment the size of an urban agglomeration,the change of surface temperature shows the energy balance reached by the physical,chemical and biological processes,which result in different characteristics of thermal filed distribution and different spatial patterns.With the air temperature and the surface temperature of the underlying surface as the key parameters,urban thermal environment is also described by using factors including solar radiation,heat produced by people,and situation of the underlying surface(the type of underlying surface,the coefficient of thermal conductivity and the heat capacity)and the atmospheric condition which influences heat transfer.All of these factors together make a physical system which influences people and people's behaviors.Located in the droughty area of China,Urumqi is a city with a population of over 1 million.Due to the speed-up of urbanization in recent years,new buildings,tarmac roads and concrete roads are built intensively in the city.Due to high heat capacity and absorption as well as less heat consumed by evaporation,more heat is confined in the city and radiated to the surroundings and the atmosphere,therefore,the high temperature urban area is connected with the natural droughty areas as a whole area of high temperature,a phenomenon has a far-reaching influence on the urban development.A contradiction comes into being between urban sprawling and the deteriorating environment.This contradiction is the most significant problem for urban planning,environment and sustainable development.In this paper,Urumqi,the capital city of Xinjiang Uyghur Autonomous Region is used as a study area.The data of urban geology and environmental science from 1990 to 2010 are collected as the basis of the research.By using 3S technology and mathematics,remote-sensing images are used to show the inversion of urban temperatures.The urban thermal field pattern is analyzed in two aspects,namely space and time.The extent of impact to the urban environment and the underlying surface is obtained by a comprehensive analysis of the factors including the urban land utilization / coverage types,influential elements(vegetation index,water body index,elevation and heat produced by people)and thermal landscape pattern.By doing research on the correlation between urban thermal environment and underlying surface,the features of dynamic change in 20 years are obtained.These results can be used as reference for the sustainable development of the city.It is indicated by these results that:(1)In the research on the urban thermal environmental testing of a city in the droughty area,data of the Landsat Program and Single Window algorithm are used to do the inversion of surface temperatures,which indicates advantages such as high resolution,more files kept in record and easy access.The grass surface temperature actually measured at the moment at a weather station is the closest to the surface temperature produced by the inversion.The runner-up is the air temperature,with a range of deviation around ±2.6°C,±4.01°C,and ±8.48°C.The former result shows high accuracy.The temperature differences of each year are very differently distributed.The mountains,desert and the Gobi Desert surrounding the city always show high temperatures.Since the urban area is surrounded by barren mountains,the temperature of urban area is lower than that of the barren mountains and lands.Also with the expansion of urban scale,the heat island inside the city is showing a process of “concentrating-being scattered-expanding and warming up”.The tendency of warming up is enhanced instead of being weakened.And it goes to three directions namely the north,the east and the south.(2)In the research on urban underlying surface,a large area well covered by vegetation is the critical part of the low temperature area in the city,which shows a negative phase moderate correlation;a large area of water also has a minor effect,but it can only produce a low-grade temperature locally and a unobvious impact to the surroundings,which shows a negative phase low-grade correlation;elevation only has an impact in the mountain area,which shows a negative phase moderate correlation;temperature rises are caused by all of these obvious interactions between the thermal environment and the factors such as the growth of population density,the increase of other heat produced by people and the expansion of impervious water surface and unused lands(especially bare land).(3)During the period from 1990 to 2010,new urban area and Tou-tun-he district were developed in a state of a steady low temperature thermal field;in the second places are Shui-mo-gou District,Saybag District and Tian-shan District,where the low temperature thermal fields were fragmented and rational planning and development are required;the low temperature areas in Urumqi County,Mi-dong District and Da-ban-cheng District were reduced apparently and the range of high temperature was raised,therefore,attention shall be given to protection of green lands,reduction of emission and design of urban layout.(4)The thermal landscape has changed greatly in the past 20 years.The unused land is the major part of high temperature area while green land,the major part of the low temperature area,is continuously cut off and converted into construction land.Inside the city it shows a highly complicated aggregation and each patch is fragmented,therefore,the dominance is enhanced and the aggregation is reduced.The areas of super high temperature patch and sub-moderate temperature patch are relatively increased;he areas of moderate temperature patch and low temperature patch drops at first and then climbs up;and the areas of relatively low temperature patch and sub-low temperature patch are continuously dropping.The high temperature patch takes the largest percent and shows an absolute dominance.After the stage of being highly fragmented,it gradually evens out.The degree of urban thermal landscape as a whole being fragmented is increased and the scope of heat island inside the city is expanded,therefore,the current situation is that the patch of thermal landscape is continuously rising up and being complicated,which requires improvement of urban planning and management of human activities.Based on the research and analysis above and in accordance with the environmental features of a city in the droughty area,advice is given to Urumqi's urban layout and green land system,it is suggested that targets shall be set up for green and healthy city of sustainable development. |
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