| The rapid industrialization and urbanization have lead to dramatic changes in land use and land cover through the transformation of natural surfaces into dense,artificial buildings,resulting in the urban heat island(UHI),a phenomenon in which urban areas have higher temperatures than the surrounding environment.The UHI negatively affects energy consumption,air quality,runoff and human health,particularly for the poor and the old sections of society.The spatial structure of city can influence the observed urban heat island.As a result,a better understanding of the effect mechanism of the spatial structure of city on the urban thermal environment will be vital importance for providing a scientific basis for future climate mitigation and urban planning.Taking Changchun,Jilin,a snow climate city as a case study,series of Landsat images were used to extract the land use information in 1984,1992,2000,2007 and2014.High spatial resolution images GF-1 and GF-2 were employed to obtain the detailed land use information in the urban area with the help of object-oritented thought,and the shadows of buildings were used to estimate building height to build the database of city 3-D structure.Landsat TM6 and TIRS were applied to retrieve land surface temperature(LST)to characterize the interannual and monthly variations of the thermal environment,while hourly air temperatures collected from 195 meteorological stations were used to explore the hourly spatiotemporal pattern of UHI.Unmanned aerial vehicle(UAV)carried temperature sensor was used to measure the temperature differences in vertical direction.Based on the database of urban structure and thermal environment,GIS spatial analysis,3-D simulation,time series analysis,geostatistics,classic statistical regression and comparative analysis were used to investigate their relationships.Finally,both the composition and configuration landscape metrics were used to explore the cooling effect of urban green space to provide insights into UHI mitigation measures to benefit planners,decision makers and dwellers.The main conclusions are as follows:(1)In the past three decades,the pattern of land use and land cover(LUCC)had dramatic changes.The urban area of Changchun has increased more than four times from 143.15km~2 in 1984 to 577.45km~2 in 2014 with an annual increasing rate at14.46km~2 per year,while the dry land has decreased from 1130.12km~2 to 737.70km~2,which was the major contributor(76.82%)followed by rural settlements(17.81%)to the new emerging urban area.The proportion of the impervious surface area in the whole study area has continusely increased,but of which in the urban area increased initially but decreased after 2007.The detailed land use information in the urban area showed that residential land and green space were the two dominated land use types,and the sum area of the two land use accounted for about 46.98%.(2)Using the shadows could effectively estimate the heights of buildings.In total,there were 34714 builidings inside the fourth traffic ring in Changchun,most of which were lower than 16.8m with a proportion of about 69.85%,and there were not too much buildings whose heights were over 100m.Buildings with different heights were widely distributed in the urban area,and there was no obvious features of the spatial pattern of heights.Based on the city 3-D metrics,the building coverage of most area was lower than 0.35.The plot ratio was relatively small,most of which was below 2.0,and the plot ratio of the city center and southwest part were much higher than other area.(3)The spatial pattern of the thermal environment also had dramatic changes,and the urban heat region area has increased from 15.27%in 1984 to 29.62%in 2014.Similar to the changing trend of impervious surface area proportion in the urban area,the surface urban heat island intensity measured by NLST of 1984,1992,2000,2007and 2014 were 0.225,0.249,0.252,0.243 and 0.239,respectively;however,the inner urban heat island has been continuously increasing.For monthly variation,based on the results from LST,the temperatures of urban areas were much higher than that of rural area except April and October during the study period.The area of highest LST in July and August were much higher than other months.For seasonal variations,the urban heat island intensity was strongest in summer(4.98?),followed by autumn(1.76?),spring(1.71?)and winter(1.30?).Based on the results from air temperatures,the changing trends of air temperature and urban heat island were similar between summer and winter.On average,the urban heat island intensity in summer(0.75?)was smaller than winter(0.95?),and the daytime UHI intensity in summer(0.44?)was also weaker than winter(0.66?).However,its'nighttime intensity(1.72?)was stronger than winter time(1.50?).For hourly variations,the changing trends of air temperature and UHI intensity were quite diffrerent and could be divided into three and five stages,respectively.The hourly average,daytime,nighttime,hourly maximum and hourly minimum of UHI intensity during summer were 0.92?,0.49?,1.51?,1.64?and0.03?,respectively.(4)The changes of LUCC do have great influence on the thermal environment.In other words,the spatiotemporal changes in thermal environments were consistent with the process of urbanization.On average,the NLST can increase by 0.21 when the natural landscape was transformed to impervious surfaces.For monthly variation,the temperature differences among different land use tpyes was largest in August(3.55?)and smallest in winter(0.89?).For hourly variations,urban area had the largest average temperature(22.66?)and woodland had the smallest(21.77?).The differences in warming and cooling rates among different land use types were the main driving factors to result in the hourly variations of UHI intensity.Compared to building plot ratio,the influence of building coverage percentage on LST was much stonger,and the warming effect was stronger in summer than other seasons.In spring,summer,autumn and winter,for each 10%increase in the the bulding coverage,the LST can increase 0.30?,1.70?,0.46?and 0.12?,respectively.The vertical thermal environment was different among land use types.The air temperature differences between 1.5m and 100m was 2.20?in industrial construction land,while the value was 1.51?in city green space.(5)Urban green spaces have a significant cooling effect among all seasons except for winter,but the effects vary considerably among different seasons and green types.The lowest LST was found in park green space,while attached green space and road green space have the highest LST among different green patches.Compared to shape metrics such as PARA and SHAPE,the negative relationships between LST and the area and NDVI of urban green spaces were more significant.It was found that every10%increase in urban green spaces explained a 0.34?decrease in LST in summer at the 0.5×0.5 km scale.Rather than increasing the area of green patch,increasing the NDVI(more woodland area and denser vegetation)is a better and more practicable approach for urban green planning.Both the compositional features of urban green spaces and configuration can significantly affect LST.At certain spatial scale,for example,1×1 km,given a fixed area of urban green spaces,the number of green patches can increase or decrease the values of LST depending on whether the number is larger than a threshold or not,and this threshold changes for each given areas.For example,in our case,when the proportion of TA is between 20~40%in 100 ha,initially,the LST increases simultaneously with an increase NP.When NP is larger than 40,however,the LST decreases.(6)The urban parks showed a cooling island effect,called park cooling island(PCI),which varied significantly by month.The mean park cooling island intensity was approximately 4.0?between June and August,and 2.71?,1.93?and 1.10?for May,September and October,respectively.The strongest PCI(6.26?)was observed in parks with 200ha areas or greater in July,which was also the hottest month during the year.Urban park characteristics affected PCI considerably.PCI showed positive relationships with park size,park perimeter,and the percentage of water area.PCI also had negative relationships with patch density and area of impervious surfaces.Unexpectedly,the percent area of grass showed a positive relationship with park LST.Large urban parks tend to have greater PCI extents than small parks.For parks larger than 30ha,the cooling effect extended approximately 480m from the park edges between June and August.PCI extent was greatly reduced during the other months when temperatures were relatively low. |
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