## 4 Discussion
### Analysis of the Possible Influence of Urbanization on Precipitation  
The time series of the annual precipitation amount from 1979 to 2010 and the percentage of the built-up area from 1990 to 2010 for Xujiahui and five coastal sites are shown in Figure 15. The results show that there are remarkably different patterns for the percentage of built-up area. For Xujiahui station, the percentage of built-up area in the surrounding 5 km buffer zones exceeded 90% in the period from 1990 to 2010, and an obvious increasing trend at a rate of 4.75 mm/year was observed at this station from 1979 to 2010 (statistically significant at the \(\alpha=0.05\) level). Among coastal stations, Cixi station also exhibits an increasing trend at a rate of 1.30 mm/year. However, a decreasing trend in annual precipitation amount was observed for the Pinghu and Dinghai stations, and a relatively weak increasing trend was observed for the Lvsi and Shengsi stations (not statistically significant at the \(\alpha=0.05\) level). We found that the amount of precipitation increased significantly in the Shanghai urban areas and suggest that urbanization has a profound impact on precipitation at a regional level. Moreover, the effects of regional and nearby oceanic influences on precipitation trends in Shanghai should be considered in detail. Firstly, a brief statement of sea-land breezes is appropriate because rain in coastal regions is usually initiated by the convergence associated with land-sea breezes [93]. The different thermal conductivity of the land surface and sea surface results in the land-sea temperature difference and causes horizontal pressure gradients which can drive mesoscale circulations, daytime sea breeze and nighttime land breeze [81]. The subsequent sea breeze may affect the diurnal cycle and intensity of local precipitation by modifying low-level convergence [94,95]. However, urban-induced circulation changes can cause enhanced convergence leading to moisture transport from the ocean [96,97]. [PERSON] et al. [98] found that even with the active-phase monsoon urbanization  
Figure 14: Scatter plots of temperature and change rates of built-up area percentage for: the (**a**) Ur; (**b**) OUr; and (**c**) OS regions. also contributes to local heavy precipitation and mesoscale precipitation distribution, and the upward trends of precipitation region in Mumbai city was increased due to the feedback of the sea breeze-urban landscape convergence. In Shanghai, previous studies have documented that the urban heat island effect is most prominent during the late afternoon to early evening, which combines the influence of urban effects and the basic local circulation associated sea breeze. Thus, urban heavy precipitation is much more intensive than that in rural areas [55]. The decadal and interdecadal variabilities of precipitation in China may be associated with the large-scale East Asia Summer Monsoon (EASM) circulation features [99; 100]. The prevailing winds in summer controlled by the EASM are the southeast winds that blow from adjacent seas [101]. The variation of precipitation is closely related to the northward airflow, with an abundance of water vapor and humidity. It has been proposed in many studies that the large-scale anomaly of summer precipitation has a complex relationship at intraseasonal, interannual and interdecadal timescale [102; 103; 104]. In addition, after the late 1970s, EASM exhibited a low northward movement with a northern edge located south of its normal position. Reduced summer precipitation occurred in East China except for the Yangtze River basin [99; 105]. Recently, numerical model simulations have shown that the expansion of the urban surface is an important factor that weakened the EASM system after the 1990s via changes in thermal states [106]. It is noted that human-induced land use changes in East China could affect the climate [67; 107].