Urbanization Intensifies Heavy Hourly Rainfall Preconditioned by Heatwaves

Heatwave preconditioned-heavy rainfall (HW_HR), a preconditioned compound event, can cause more damage than a single heatwave or rainstorm. Both heatwaves and rainstorms can be exacerbated by the presence of cities, but the response of their compounding to urbanization remains unclear especially at the hourly scale. Here, we investigate the spatial and temporal responses of hourly HW_HR to a typical urban agglomeration, the Pearl River Delta, using observations and scenario-based numerical simulations. Compared to rural areas, the observations show that HW_HR in urban areas has a higher probability of occurrence and mean intensity, and its diurnal cycle of frequency is narrower, peaking in the afternoon. The temporal and spatial response of HW_HR intensity to urbanization effects is the most significant, with the urbanization-induced increase in HW_HR being five times that in non-heatwave preconditioned-heavy rainfall (noHW_HR). Our simulations support the observations and suggest that urbanization-induced changes are intense and spatially heterogeneous in HW_HR but relatively weak and spatially continuous in noHW_HR. The simulations also suggest that heatwave preconditioning not only amplifies urbanization-induced changes in key variables that alter atmospheric conditions but also provides a pre-storm unstable environment for the urban-induced warm-dry surface to trigger and enhance convection. The sub-daily pre-storm environment suggests that the preconditioning-induced thermodynamic changes gradually decrease, whereas the dynamic changes gradually increase as the event approaches. Our study highlights the importance of understanding urbanization effects on preconditioned events, providing new insights into the role of preconditions in the urban water cycle.