In light of the rapid growth in cities, there is a pressing need to explore how urbanization affects extreme weather, especially short-duration convective storms that can potentially trigger urban floods. Here we use a high-resolution Weather Research and Forecasting convection-permitting model to simulate 23 summer convective storms over the subtropical city of Shanghai, China. We simulated these events in three different scenarios: present urban, no-urban, and urban-expansion settings. Results show contradictory findings of urbanization's influence on convective rainfall, which is associated with urban-surroundings hydrothermal differences and diurnal cycles. Urban expansion further intensifies daytime convective rainfall when inhomogeneous temperature and moisture conditions between the city and its surroundings are present, whereas other conditions may suppress rainfall intensity. The findings provide the potential mechanisms of rainfall modification by urban expansion in subtropical cities, offering useful insights for urban planning and flood management in Shanghai and other rapidly urbanizing cities.