Study On Heat Island Of Typical Cities In East China Based On Plant Phenology

Based on the background of global climate change,the paper uses remote sensing technology to extract the critical phase of plant phenology in cities.The differences in plant phenology in urban and rural areas were revealed,and the temporal and spatial features and intensity of heat islands were studied.Using plant phenology as an alternative indicator.This study of the heat island effect not only has important theoretical significance,but also provides a scientific basis for in-depth understanding of the city's cold and heat distribution laws,and provides scientific basis for adopting measures to reduce heat islands and improving the urban human settlements.Under this background,this paper takes the eastern North Subtropical middle climatic zone as the research area,and chooses Harbin,Changchun,Shenyang,Beijing,Shijiazhuang,Zhengzhou,Hefei and Wuhan as the research objects.In this paper,the MODIS EVI data of 2009-2016 was collected as the main data source.The critical phase of urban plant phenology were extracted by dynamic threshold method.By monitoring the start of the growing season(SOS)and the end of the season(EOS),the spatial and temporal features of the start and end dates of the growing season are revealed,and the differences between urban and rural areas are compared.The correlation between the extracted phenological parameters and the surface temperature during the same period was analyzed.It was found that the surface temperature was highly correlated with the phenological period of the plants.Based on plant phenotypic SOS,a new heat island intensity index is proposed,namely the SOS Urban-Rural Difference Index.The difference between urban SOS and rural SOS is used as an index of the intensity of heat islands.This index is then used to assess the intensity of heat islands in large cities in eastern China.Through the above analysis,the main conclusions of this paper are as follows:(1)The MODIS EVI time series dataset was reconstructed.The dynamic threshold method was used to obtain the key parameters of plant phenology in 8 typical cities,and the spatial and temporal distribution features of SOS and EOS in major cities were further analyzed.According to the phenological data measured on the surface,it is found that the phenological trend of the remote sensing image is consistent with the phenological trend of the measured data.It indicates that it is reliable to study the plantphenology of the city by remote sensing images.(2)The spatial distribution of the average annual SOS and EOS in each city shows similar distribution characteristics,and the average annual SOS and EOS spatial distribution are complementary.The temporal and spatial distribution of plant phenology is almost identical with the latitudinal zonal law of plants.The spatial distribution of plant phenology is highly consistent with the distribution of land use /cover.As the distance from the center of the city gets closer,the earlier SOS and the later EOS in each city.The change is more obvious.(3)The annual variation in the phenological period in regions with different levels of urbanization have certain similarities.Each city's SOS showed an advance trend,and the urban and rural SOS inter-annual trends remain the same.Among the study periods in the subtropical and warm temperate regions,2011 was the coldest year,while the coldest year in the temperate zone appeared in 2012.However,EOS in major cities does not show these regularities.(4)The shape,direction and location of the spatial distribution of annual average surface temperature are basically consistent with the spatial distribution of urban phenology critical period.There is a significant correlation between the average annual land surface temperature and the corresponding key phenological periods of the major cities.SOS has a significant negative correlation with land surface temperature,and EOS is highly positively correlated with land surface temperature.(5)The spatial distribution of urban heat islands is basically the same as the contours of urban built-up areas.With the development of the city,there is a tendency to expand outward.As the city's phenology is ahead of the surrounding things.The urban area is the center of high temperature,forming the phenomenon of heat island.Since the annual variation in the urban and rural areas of each city in SOS are unanimously advanced,it can be inferred that the heat island effect in each city has been increasing year by year.There are differences in the intensity of heat islands in different climatic zones,of which the heat islands in the middle temperate zone have the highest intensity.