SPATIAL AND TEMPORAL CHANGE ANALYSIS OF URBAN HEAT ISLAND EFFECT IN WUHAN CITY

China has experienced rapid urbanization and rapid development of economy in the past decades, resulting in severe damage to the urban ecological environment, causing changes in the urban thermal environment and triggering the urban heat island effect. Moreover, the heat island effect has become a hot topic for scholars. The urban heat island effect refers to the phenomenon that the urban surface temperature is significantly higher than that of surrounding suburbs due to the interaction of man-made and natural. The city is considered to be the largest man-made ecosystem. Its heat island effect will not only change the growth habit of urban vegetation, but also affect the outer environment of urban buildings, it further influences human life and has a great negative impact on human health. Therefore, the study of the spatial-temporal variation characteristics of urban heat island effect and its influencing factors can provide data support for the environmental quality control and urban planning of local government departments. Based on the surface temperature remote sensing product data, we studied the spatial distribution characteristics of urban heat island effect in Wuhan from 2001 to 2013, by calculating the temperature difference between the highest and lowest temperatures and the average interval method for heat island classification. We conducted a trend analysis of vegetation cover from 2001 to 2013 initially explore the effects of vegetation cover n heat island effect. The results showed that: (1) From 2001 to 2013, the intensity of heat island in Wuhan was strong in the city center, weaker surrounding city center and the weakest in the suburbs; From 2001 to 2011, the intensity of heat island in Wuhan city was significantly weaken, among which Huangpi, Xinzhou, Jiangxia, Hannan and Caidian district were weaken, and the urban heat island effect of the city center was enhanced; From 2011 to 2013, the intensity of heat island in Wuhan city presented an increasing trend, among which Huangpi district, Xinzhou district and Caidian district were the most obvious, and the urban heat island effect was slightly weaken. (2) Between 2001 and 2013, the vegetation cover in Huangpi district and Xinzhou district increased significantly, and the vegetation cover in the downtown, Jiangxia district and Dongxihu district decreased significantly, corresponding to the urban heat island effect of Wuhan increased volatility. Our results showed that the spatial distribution of urban heat island effect in Wuhan city fluctuated with time during the study period, and the vegetation cover had a significant influence on it..

become more and more obvious, which will change the growth habits of urban vegetation, local climate [3] , image air quality [4] , physical and chemical properties of soil [5] and many other urban ecological processes.As one of the driving factors affecting the heat island effect, the change of the way, type and degree of urban greening construction will lead to the continuous appearance of negative environmental effects such as surface temperature rise and heat island effect.For a long time, scholars have used meteorology, remote sensing, numerical simulation and other research methods to study the heat island effect,Alonso et al used meteorology to analyze the urban heat island effect in Salamanca, Spain, under different weather types [6] ,In 2013, when Manju Mohan and other scholars used remote sensing data to retrieve the surface temperature of New Delhi, they found that the high temperature centers of urban heat island effect usually appeared in commercial centers and buildings [7] ,Hu Xiaoming et al. simulated the boundary layer characteristics of Beijing on July 10 and 11, 2000 using a non-hydrostatic equilibrium moderate regime model [8] .
At present, many scholars focus their research on heat island effect on remote sensing data.According to the consistent correlation between surface temperature and meteorological temperature, the heat island grade is divided by normalizing the surface temperature, and the heat island grade is analyzed.Then, by means of the inversion algorithm of surface temperature, the surface temperature of a certain area can be accurately divided.It is more convenient and accurate than other research methods.This paper is based on MODIS LST data and MODIS NDVI data of Wuhan from 2002 to 2009,Using equal interval method to classify heat island, the spatial distribution pattern and temporal variation characteristics of surface temperature were studied.Combined with the influence of vegetation coverage on surface temperature, it provides scientific reference for future urban planning and construction in Wuhan.

Survey of Research Areas
Wuhan is located in the eastern part of Jianghan Plain and the middle reaches of Yangtze River. Its longitude and latitude range is 113˚ 41'-115˚ 05'in the East and The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XLII-3/W10, 2020 International Conference on Geomatics in the Big Data Era (ICGBD), 15-17 November 2019, Guilin, Guangxi, China spatial resolution of the data is 1000 meters, the temporal resolution is 16 days, the data format is Hdf, and the map projection is Sinusoidal (sinusoidal curve projection).

MRT (MODIS Reprojection Tools) Projection
Tools,The spatial resolution of the original MYD11A2 data is 930m, and the data is resampled by the method of near nature, which makes the data standard 1000m,The original format is converted to Geotiff, and the original projection is converted to Albertian Graded Conical Projection,finally, the data are joined together.In ENVI, the MYD11A2 data set after re-projection is clipped with the vector boundary of Wuhan as the mask.

Research methods
According to the time sequence of the administrative division of Wuhan, this paper chooses Jiangan District, Qingshan District, Jianghan District, Qiekou District,

Wuchang District, Hanyang District and Hongshan
District as the central urban area (Fig. 1), and the rest as the suburban area.

Fig. 1 Selection of urban area in Wuhan
In this paper, we use split window algorithm to

Intensity in July in Wuhan City
The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XLII-3/W10, 2020 International Conference on Geomatics in the Big Data Era (ICGBD), 15-17 November 2019, Guilin, Guangxi, China

Effects of vegetation on heat island effect in Wuhan
Referring to the standard for classification and gradation of soil erosion [9] ,and the actual vegetation

CONCLUSION
Based on MODIS synthetic products, this paper studies and analyses the spatial distribution characteristics, temporal variation characteristics of urban heat island intensity in Wuhan and the influence of vegetation coverage on it.Through the above research, we can draw the following conclusions: (1) The distribution of surface temperature field in Wuhan is mainly in the middle-temperature zone, followed by the middle-high temperature zone or the middle-low temperature zone.Evolution between each temperature region is usually gradual,the evolution direction of heat island enhancement is as follows,Low Temperature Zone→Medium and Low Temperature Zone→Medium Temperature Zone→Medium and High Temperature Zone→High Temperature Zone,on the contrary, the direction of heat island weakening.It's also possible to evolve across levels, but less likely.
(2) From 2002 to 2009, the area of high-temperature and sub-high-temperature zones in Wuhan showed fluctuating growth,The maximum heat island intensity also shows an increase in fluctuation.Therefore, the heat island effect of Wuhan has an increasing trend in the future.
(3) The vegetation coverage in Wuhan mainly consists of medium and high vegetation coverage and high vegetation coverage,the distribution of heat island is consistent with that of Wuhan,the characteristics of time variation are basically consistent with those of heat island intensity in Wuhan.It can be seen that vegetation coverage has an important influence on the change of heat island intensity,reasonable increase of vegetation cover helps to reduce urban heat island effect.
(4) In this paper, the relationship between vegetation cover and heat island intensity is studied. It is found that heat island intensity and vegetation cover may have bidirectional effects, and there may be other factors affecting the change of heat island intensity.Therefore, in the future study, more driving factors of heat island intensity will be further studied.