Variations in the distribution of solar radiation on the earth surface can fundamentally alter characteristics of temperature, humidity, precipitation and atmospheric circulation, and are thus considered important indicators in the evaluation of climate change. All Intergovernmental Panel on Climate Change (IPCC) Assessment Reports contain special chapters that illustrate how solar radiation variations have been used to indicate or deduce global climate change. Given the current global trend of energy intensity triggered by over-exploitation of fossil fuels and global warming caused by excessive greenhouse gas emissions, energy from solar radiation represents the cleanest form of energy that can be used directly by humans. Therefore, solar radiation is a priority within the area of research on natural energy resources. Xi'an is the political, economical and cultural center of Shaanxi Province. The city is located at the center of the Central Shaanxi Plain. This location has the Weihe River to the north and the Qinling Mountains to the south. It has a total area of 10,108 square kilometers, an urban area of 369 square kilometers, and a population of over 8.4346 million. Xi'an was one of the four great capital cities of the ancient Chinese civilization. Research on solar radiation in Xi'an and its impact involves data from only a single point, which were analyzed in the 1990s. This research was on natural resources in Northwest China. Therefore, there has been no systematic and in-depth investigation of solar radiation characteristics in the area around Xi'an.
According to data on global solar radiation from Xi'an radiation stations along with percentage of sunshine data from 23 nearby national meteorological stations, this paper presents time series ofglobal solar radiation data from the period 1961 to 2009. We apply fuzzy cluster analysis methods to divide the city of Xi'an into three sections-middle (section I), eastern (section II), and western (section III). We use the sliding T method to conduct mutation testing and phase analysis, and we study temporal and spatial distributions as well as variation of the global radiation. We also perform correlation analysis of the global urban solar radiation and urban development, with reference to Xi'an urban population data over the years. The results show that Xi'an global solar radiation had the greatest distribution features in the eastern section, less in the western section, and least in the middle section. The time series from the period 1961 to 2009 reveal a band decrease trend, which has a linear rate of decrease at -2.01%/10a. There was a period of reduction from 1977 to 1991. In this period, the greatest band reduction extent was found in the middle section. There were two years of mutation, 1977 and 1992. With urban development, there was an increase in the contamination of the atmosphere. This in turn caused urban global solar radiation in Xi'an to have a relatively large impact on such development. This was demonstrated by the seasonal patterns. The impact was more evident in winter and summer seasons, compared with the spring and autumn seasons.