Remote Sensing Research On Thermal Environment Of Xi’an Metropolitan Circle

As an artificial ecosystem created by human, a city is centered on human settlement. There is a large quantity of matter and energy exchange between the human world and the objective world, producing plenty of waste, so a city is the most centralized place breeding with various contradictions of human and nature. Urbanization process interacts with and dependents on urban climate and environment temporally and spatially. Urban diseases such as air pollution, urban heat islands, and disasters caused by urbanization, has become increasingly serious, so how to build cities developing in an ecological and sustainable approach has caught wide attention.This paper, taking Xi’an metropolitan circle as an example, with the support of GIS technology, Landsat5 TM, Landsat8 OLI/TIRS, MODLT1 M remote sensing image as data sources, supplemented by the basic geographic data and other related information of Xi’an metropolitan circle and Xi’an central urban area, obtains a set of surface-temperature data of Xi’an central urban area by the inversion of different models. Classification network algorithm is employed to extract the range of urban heat island of Xi’an City, based on which the space distribution pattern and its change characteristics of the urban thermal environment in the central area of Xi’an city is analyzed; the spectral unmxing model is adopted to disclose the different underlying surface components of Xi’an. The relationship between different underlying surface factors and the urban land surface temperature is explored. Ten parks are selected as typical examples to illuminate the relationship between the internal surface temperature and characteristics of patches in those parks as well as the cooling range of those parks; On the basis of MODLT1 M data products, the method of average-standard deviation is adopted to divide the range of urban heat island and reveal the features of the seasonal and diurnal changes for the thermal environment of Xi’an metropolitan circle. The main conclusions are as follows:(1) Dynamic characteristics of the thermal environment in Xi’an central urban areaThe average surface temperature in Xi’an central urban area goes up in successive years and the rise is obvious. With the expansion of the city, the space distribution of the high temperature area together with the relatively high temperature area enlarges. In 1988-2006, the proportion of areas in high and relatively high temperature in Xi’an central urban area rises while in 2006-2014 the proportion of areas in high and the highest temperature goes down. Heat island and secondary heat island area was expanding before 2006, but diminishing after that. During 26 years, the area of middle zone, secondary heat island and heat island regions increases, while the area of cold island and secondary cold island regions reduces. The absolute value of area change is: cold island area > the middle zone> secondary heat island > secondary cold island > heat island.High temperature areas in Xi’an city mainly distribute at the central urban districts with a dense population, thickly residential quarters and prosperous business, the economic and technological development zones concentrated with enterprises, large demolition areas and areas with poor vegetation covering; Low and sub-low temperature area presents a zonal distribution along the Wei River, Ba river, and Hancheng Lake and also distributes in parks and green land such as the ruin reserves of Great Wall of Han Dynasty, Qujiang Relics Park, Datang Furong Garden, Xingqing Palace, as well as Chang’an District and Lintong District which have been incorporated into urban planning but still have large tracts of farmland.(2) The thermal environment effect of the underlying surface of Xi’an central urban areaExtract information of the underlying surface of Xi’an by adopting the spectral unmixing model, and analyze the relationship between all kinds of city underlying surface and the surface temperature; Build a model to reflect the influence of parks on the surrounding environment and explore the cooling effect of urban parks on its surrounding areas. The study finds that the vegetation coverage of Xi’an central urban area is logarithmically and negatively related with the surface temperature, while the impervious surface and the surface temperature positively and logarithmically correlates with each other. Increase vegetation coverage and green area in urban construction land, roads, and on the top of houses reasonably and effectively to reduce surface temperature, and thus easing urban heat island effects.The analysis of the correlation between park patch characteristic parameters and the average surface temperature indicates that the internal temperature of a park has a significantly negative correlation with the perimeter and the area of the park, and a significantly positive correlation with the perimeter-to-area ratio. As the distance away from the park increases, the temperature rises up accordingly. However, when it reaches a certain distance, it shows an irregular change; the farther the distance away from park patches, the smaller of the cooling rate. The logarithmic relationship between temperature difference and distance is not obvious. The cooling range of a park is affected by park patch characteristics and the surrounding environment. All elements such as park area, perimeter and shape index presents a linear positive correlation with the cooling range of the park, but the correlation is not very high, so increasing park area and perimeter will be conducive to achieving certain cooling effects.(3) The seasonal and day-and-night characteristics of heat island effects in Xi’an metropolitan circleBased on MODI’s monthly temperature product, the study conducts a comparative analysis of the seasonal and day-and-night changes with respect to the urban heat island effect of Xi’an metropolitan circle. The study indicates a larger temperature difference between urban and rural areas as well as obvious seasonal and day-and-night differences. In terms of daytime, the urban-heat-island ratio index(URI) manifests the feature: summer>autumn> winter>spring; in terms of nighttime, the URI reflects the tendency: summer> winter>autumn>spring. Observed from each month, on the whole, apart from August and September when the URI of daytime is lower than that of nighttime, the urban-heat-island intensity of daytime for the rest months in Xi’an metropolitan circle is larger than that of nighttime.