Intensification and Changing Spatial Extent of Heavy Rainfall in Urban Areas

Herminia Torelló-Sentelles; Francesco Marra; Marika Koukoula; Gabriele Villarini; Nadav Peleg

doi:10.1029/2024EF004505

Abstract Urban areas have been shown to impact rainfall by altering both its intensity and spatial structure at sub‐hourly and sub‐kilometer scales. However, there is currently no clear understanding of the precise pattern of change and the mechanisms that drive these changes. Since the hydrological response in urban areas is highly sensitive to such rainfall properties, understanding these changes is critical to improving our ability to assess urban flood risk. We use 7 years of high‐resolution weather radar data (4‐ or 5‐min and 1 km) to analyze changes in patterns of rainfall intensity, spatial structure, and storm evolution across eight urban areas within Europe and the United States. The use of the same methodology across the different cities enables a consistent comparison among them. We track convective rainfall events using a storm tracking algorithm and assess changes in rainfall properties in the upwind, center, and downwind regions of each city. We also investigate changes in the frequency of storm initiations, terminations, splitting, and merging. Our results show that urban areas act to intensify rainfall—mostly over them, but sometimes on their peripheries. Overall, larger cities tend to show the largest rainfall enhancements. Our findings highlight that rainfall spatial structure is altered over the urban core; usually resulting in more spatially concentrated rainfall. We also observe increased storm initiations over most cities and increased storm splitting over one. Given that demographic projections show that future urban population will increase, our results point toward an increased future flood risk in growing cities. Plain Language Summary Urban flood risk is highly sensitive to the intensity and patterns of precipitation. At the same time, urban areas have been shown to impact rainfall by altering these properties. It is important to improve our understanding of how rainfall is modified by cities to better anticipate future flood risk. Here, we use weather radar observations to analyze changes in rainfall intensity and patterns across eight urban areas within Europe and the United States. Our results show that rainfall becomes more intense over cities compared to their surroundings, and sometimes on the city edges. Larger cities show larger increases in intensity. Our findings also show that rainfall usually becomes more spatially concentrated when passing the cities. Finally, we observe that more storms emerge over the cities in comparison to their surroundings, mostly in the late afternoon to evening hours. Given that we expect that cities will become larger and denser in the future, our results indicate that future flood risk may also increase in growing cities as urban rainfall trends toward increasing extremes. Key Points Heavy rainfall is intensified over urban areas and the effect is related to city size Urban areas usually alter rainfall spatial structure by increasing its spatial heterogeneity and reducing its area There are increased storm initiations over most urban areas in the late afternoon and evening hours

Intensification and Changing Spatial Extent of Heavy Rainfall in Urban Areas

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