| Rapid urbanization in the past few decades has led to a significant increase of impervious surface area(ISA)at local,regional and global scale,which resulted in a series of adverse environmental and ecological issues,such as habitat loss,flooding,local climate changes,etc.,among the urban heat island(UHI)effect has been widely concerned.Previous studies demonstrated that the urban heat island effect is highly correlated with the impervious surface.However,whether are there co-evolutions between the spatial and temporal pattern of the UHI and ISA,in our best knowledge,has never been addressed.The impacts of urban expansion and urban renewal on the urban heat island has not been well addressed.Therefore,this study proposed a hypothesis that there are co-evolutions between the temporal and spatial pattern of impervious surface and urban heat island.To test this hypothesis,this study firstly extracted impervious surface area from time series Landsat images using an improved algorithm developed by myself,and retrieved land surface temperature from the thermal band of Landsat image to identify surface urban heat island.The temporal and spatial patterns of impervious surface,land surface temperature,and surface urban heat island in Shanghai from 1992 to 2019 were analyzed using various methods of landscape pattern analysis,gradient analysis,and geo-statistics.The main findings are as follows:(1)A novel algorithm was developed by integrating phenological information and Fisher transformation into the linear spectral mixture analysis(PF-LSMA)to improve the accuracy of impervious surface extraction.The PF-LSMA algorithm can greatly reduce the within-endmember spectral variability and further enlarge the betweenendmember spectral differences,therefore,can successfully distinguish impervious surfaces from soils,and enhance the accuracy of impervious surface extraction.The accuracy and spatial resolution of the impervious surface products extracted by PFLSMA(RMSE=0.11,R=0.95)in Shanghai is higher than the single-image Fisher transformed LSMA(F-LSMA)(RMSE=0.13,R=0.88)and existing global impervious surface products(GAIA: RMSE=0.14,R=0.93;NUACI: RMSE= 0.15,R=0.91).(2)During the study period,the impervious surface area(ISA)as an indicator of urban development can well describe the urban expansion and urban renewal process of Shanghai.The average ISA in Shanghai showed a trend of first rising and then decreasing,and 2013 was the turning point of the change.The ISA increases in the suburbs and rural areas,while the ISA in the urban center decreases,and the urban-rural difference in ISA gradually decreases.The process of urban expansion and urban renewal conforms to the law of "first development,first renewal",and the spatial orientation sequence is: from the city center to the northwest area,then to the eastern area,and finally to the southern area.(3)During the process of urbanization,the area of the high-temperature area in the city center gradually decreases,while the area of the sub-high-temperature area expands outward from the center.The temperature difference between urban and rural areas in Shanghai has a significant decreasing trend.Specifically,the normalized land surface temperature(LST)in the urban center decreases,while in rural areas and suburbs it increases,and the urban heat island gradually expands.The variation of surface temperature on the urban-rural gradient is anisotropic,especially in the northeast and southeast directions,whose temperature is significantly lower than that in other directions with the increase of the distance from the city center.(4)There is a co-evolution relationship between the impervious surface and the urban heat island in terms of area.On the time scale,as the ISA increases or decreases,the normalized LST also increases or decreases synchronously,and the inter-annual synergy between the two is greater than 70%.On the spatial scale,the correlation between the two decreased to a certain extent(67%)in the later stage of urbanization,which was mainly due to the cooling effect caused by the increased urban green space in urban renewal.The spatial extent changes on both the east-west and north-south transects show a consistent trend.(5)There is a co-evolution relationship between impervious surface and urban heat island in terms of intensity.On the east-west and north-south urban-rural transects,the ISA and normalized LST show relatively consistent urban-rural differences.Although the urban-rural difference in ISA was slightly larger than the normalized LST in the initial stage,with the advancement of urbanization,the intensity difference between the two gradually became uniform on the urban-rural transects and tended to fluctuate in concert.(6)With the urban expansion and urban renewal of Shanghai,the urban heat island effect has a trend of increasing first and then decreasing.The intensity of the surface urban heat island increased from 1992 to 2004,decreased from 2004 to 2013,and rebounded slightly from 2013 to 2019.Judging from the average temperature difference between non-urban and high-density cities along the urban-rural transects,the temperature difference increased rapidly from 1992 to 1998,decreased significantly from 1998 to 2013,and increased slightly from 2013 to 2019.The urban-rural temperature differences on the east-west and north-south transects show different trends over time.On the east-west transect,the urban-rural temperature difference gradually decreased from 1992 to 2013,and then increase from 2013 to 2019;on the north-south transect,the urban-rural temperature difference increased from 1992 to 1998,and then showed a decreasing trend from 1998 to 2019.In summary,in this study,an improved algorithm was developed to extract longterm urban impervious surface area(ISA)using time series Landsat images.The ISA were then used to quantify the urban expansion and urban renewal processes in Shanghai over the past 30 years.It was found that the urban heat island intensity in Shanghai showed a trend from sharp increase to slow decrease.It was revealed,for the first time in my knowledge,that there were spatial and temporal co-evolutions between urban impervious surface and urban heat island in the area and the intensity.These findings are helpful to enhance the understanding of the mechanism of urban heat island,to enrich the theories of urban heat island,and to provide insights for urban planning and landscape pattern optimization to alleviate the urban heat island effect. |
