The exacerbated intensity and duration of the heat waves events over Shanghai as caused by urban clusters expansion

Shanghai urban clusters, the most prosperous areas in China, are experiencing the rapidly spatial growth of circle-shape expansion surrounding the urban center and clusters over suburban areas since 1981. As a result, the urban constructed area proportion about 47.9% was very close to the ecological threshold of 50% in 2013. A regional heat island covering 40 km around the Xuhui district has been induced by the rapid urban clustering expansion, and has affected the spatial distribution of heat waves. An urbanization index, represented by the remote sensing data from the Operational Linescan System instruments of the Defense Meteorological Satellite Program (DMSP/OLS), is proposed to depict the urbanization of Shanghai, which shows stronger clustering urbanization over the Jiading, Qingpu and Pudong districts. Using the Chow test method based the optimal piecewise modeling to detect the structural change in urban heat island intensities, it is found that the urban heat island intensities will occur structural change when the DMSP/OLS urbanization index exceeding 60%. Urban expansion will further induce stronger and wider regional urban heat islands effects and enhanced heat waves after the DMSP/OLS urbanization index exceeding 60%. In the period of 1977-2000, the DMSP/OLS urbanization index was lower than 60%, in which the urban heat island intensities increased significantly over all the suburban areas. However, this tendency has changed since 2001 when the DMSP/OLS urbanization index exceeded 60% in 2000 over the western suburban areas. Urban heat island intensities were decreasing gradually over the western suburban areas during 2001-2013. Compared with the difference in the climate background between western suburban areas and the reference stations of Jiangshan and Nanhui during 1959-1978, the urban clusters expansion exacerbated the difference between the western suburban areas and the reference stations about 1.6 times for heat wave days, enhanced 34.4% for mean temperature and 41.7% for mean maximum temperature. On the basis of the surface temperature data from the Moderate-resolution Imaging Spectroradiometer (MODIS), the land surface warming cores have expanded westward about 32.8% and thus strengthened local heat waves. All the above results are conducive to assess the climate effects of the urban planning pattern with one center and four sub-centers over Shanghai in the near future.