## 4 Discussion  
The urbanization process is complex and includes continuous population concentration, industrialization and commercialization that trigger the conversion of underlying surface types, as well as changes in urban geometry. Changes involving wider impervious surface areas consisting of cement and asphalt dramatically alter the environment factors of urban boundary layer, including land surface albedo, roughness, surface latent fluxes, etc. [66; 67; 68]. In this process, decreases in albedo increase the radiative energy absorption by urban land surface [69; 70]; higher and denser urban buildings increase the roughness length, which exerts a stronger drag on the wind [71; 72; 73; 74]. Moreover, a lower canopy fraction results in reduced ground moisture content and surface latent fluxes [75; 76]. Urbanization is often associated with high emissions of artificial aerosols, which reflects and scatter solar radiation, thereby altering the energy budget and acting as cloud condensation nuclei [77]. In addition, anthropogenic heat production enhances sensible heat flux from the land surface to the atmosphere near cities [78; 79; 80]. The urban boundary layer is described as being unstable whenever the surface is warmer than the air and in a state of free convection in the case of vigorous thermal updrafts and downdrafts [81].  
The changes of the urban boundary layer thermodynamic stability induced by increased urbanization processes can affect the local microclimate [82; 83; 84]. In addition, enhanced convection over a city can cause a measurable increase in convective clouds and thunderstorms, and it may also produce modification of precipitation patterns [85]. It has been shown in previous studies that thermally induced changes in circulation play an important role in the simulated precipitation increase in the central urban domain [13; 29]. For instance, [PERSON], [PERSON], [PERSON] and [PERSON] [21] confirmed that the increase in the amount of precipitation over the Tokyo urban area was caused by anthropogenic heat and artificial land cover.  
In the current study, we considered changes in built-up area as one of the most important factors that influence the occurrence and intensity of urban precipitation [86; 87]. We also demonstrated that precipitation trends show different characteristics in the Ur, OUr and OS regions in Shanghai, during the period of rapid urbanization. However, further efforts are still needed to determine changes in other factors interlinked with urbanization in Shanghai, for example, the impact of urban heat island (UHI) effects on precipitation. In addition, apart from LULC changes, influences of natural external variability including extreme precipitation is also very important and is often assumed to scale with changes in atmospheric moisture and content [88] and large-scale circulation patterns [89]. As a coastal city, precipitation in Shanghai is not only influenced by anthropogenic activities but is also affected by adjacent oceans. Therefore, detailed analyses of observed trends in temperature change in Shanghai during urbanization and the effects of regional and nearby oceanic influences on precipitation, should be thoroughly investigated.