Impacts of urban dynamics and thermodynamics on convective rainfall across different urban forms

Abstract

Urban areas have been shown to impact convective rainfall over or near them. However, predicting the urban rainfall effect across different cities remains challenging because it is unclear which urban properties are the main drivers of rainfall modifications. Here, we simulate 11 summertime convective rainfall events with the Weather Research and Forecasting (WRF) model that occurred in Indianapolis, Indiana. To isolate the role of urban form, we replace the urban footprint of Indianapolis with those of eight other cities to assess links between different urban forms and urban rainfall effects. We also perform sensitivity tests to examine whether building density relates to the urban rainfall effect. Out of the simulated rainfall events, half intensified while passing over the cities, and half were suppressed. Rainfall intensification occurred when background wind speeds were low and the urban heat island effect was strong, resulting in increased vertical uplift and a deeper boundary layer, which enhanced convection. We find that urban rainfall intensification effects are the largest over the city cores and in their downwind boundaries. In contrast, convective rainfall was suppressed when background wind speeds were high, and the urban heat island effect was weak. In these cases, the strength of vertical uplift and the height of the boundary layer were reduced, inhibiting convection due to reduced moisture in the boundary layer and large deceleration effects at the surface due to the increased surface roughness of urban areas. While the magnitude of the changes in rainfall intensity tends to be related to city size, no relationship was found with its building density.