When storms slow down: urban effects on rainfall accumulation and flood hazard

Changes to convective storm motion over urban areas may have important implications on rainfall accumulation and flood risk. Here, we investigate speed changes in storms passing over cities using weather radar data and convection-permitting numerical simulations. The observational analysis consists of tracking individual rain cells across eight cities and comparing movement speeds near the cities relative to a control upwind region. Second, we simulate ten heavy rainfall events crossing Indianapolis, Indiana, and compare cloud-layer horizontal wind speeds from two scenarios: one with and one without the city. We find that the speed of the observed rain cells decreases over and downwind of five urban areas, and seven simulations reveal dampened cloud-layer wind speeds over Indianapolis. Stronger updrafts induce horizontal wind slowing, driven by the warm urban surface. We conclude that rainfall intensification is the primary driver of enhanced urban rainfall accumulation, yet storm slowing contributes to more frequent and stronger enhancements.