Abstract: Wuhan City Group is a regional economy association which consists of the center city Wuhan, as well as eight other cities within 100 kilometers. It dominates half population and more than sixty percent of GDP of Hubei Province, in sharp contrast to its limited area occupying only one third of the province. Such a concentration and the consequent rapid development of urbanization have caused a lot of environmental problems such damages on the original natural environment, changes in the nature of underlying surfaces and irreversible changes in the structure, process and function of the ecosystem. These further bring about the urban heat island effect which deteriorates the city environment for recent years and needs an urgent solution. Our study attempts to obtain a comprehensive insight into the spatiotemporal pattern of this effect in this region and provide a basis for the solution. Our study is established on the year-by-year MODIS land surface temperature data from2000 through 2010. As the reference, the classification map produced from multi-source and multi-temporal remote sensing images is also available. In order to remove the annual variation of temperature, we defined the heat island intensity with a normalization processing on the original temperature. We used the frequency image of the top-rank intensity to measure the severity of the heat island effect. Different spatial distributions of the frequency are recognized in day and night time respectively, since the night-time result is partly disturbed by the water. But generally, the night-time heat island effect is more significant than that in day time. By averaging the day-night image to remove the disturbing factors, the highest frequency almost concentrates in highly developed urban areas such as Wuhan, revealing that the heat island effect firmly occurs in the build-up area and the severity is proportional to its size. The finding supports the conclusion that Wuhan becomes the main heat source of the whole city group.In the following analysis on the evolution of urban heat island, we find that pixels of top-rank intensity decrease while those of third-rank intensity increase over the 11 years, whether the day-time data or the night-time data is considered. It reveals a shrinking urban-rural temperature gap and a spreading tendency of the heat island effect, which coincide with the urbanization process of the city group. An essential conclusion is drawn that the overall thermal environment undoubtedly deteriorates. As the central city and the main heat source of the city group, Wuhan is specifically studied. According to the year-by-year histogram of the heat island intensity, the thermal condition of major regions gradually moves to the warmer end. In fact, the hyperthermic area has dramatically expanded since 2000. The result is consistent with the former analysis on the whole city group. As illustrated by the detailed distribution of hyperthermic areas from every year, the development of the newly-developing city zone has increased the distribution of heat source, while the increase of artificial surface and decrease of natural surface result in the aggravation of the urban heat island effect.

Key words: MODIS, Wuhan, city group, heat island effect, urbanization