Urban dynamic and thermodynamic influences on short-duration heavy rainfall across different urban structures

With over half of the world's population living in cities and urbanization expected to increase, understanding how urban environments affect heavy rainfall is crucial due to its implications for flood risk. Urban areas have been shown to intensify convective heavy rainfall; however, the extent of this effect varies across cities worldwide, and the specific influence of urban form on rainfall modification remains unclear. We use the Weather Research and Forecasting (WRF) model to simulate 11 convective events that cross the city of Indianapolis, Indiana. The land cover of Indianapolis is then replaced with that of eight other cities to assess how different urban forms affect rainfall. We find differences in rainfall intensity when comparing simulations with and without a city present, and these differences are related to the size and structure of the city, specifically the proportion of buildings arranged in an open configuration. Half of the simulated rainfall events intensified over the urban areas. In these cases, convection was enhanced due to low background wind speeds and a strong urban heat island effect. The latter half of the storms were suppressed over the cities, when background wind speeds were high, and the urban heat island effect was weak. Here, convection was inhibited due to reduced boundary layer moisture and strong deceleration effects at the surface caused by increased urban surface roughness. Given the expected growth of cities, our results point towards further enhancements in rainfall implying that future flood risk may increase in growing cities.