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When storms slow down: urban effects on rainfall accumulation and flood hazard
Herminia Torelló-Sentelles1, Marika Koukoula1, Gabriele Villarini2,3
1Institute of Earth Surface Dynamics, University of Lausanne, Lausanne, Switzerland.
Npj Natural Hazards
|December 23, 2025
Summary
Urban areas slow down storms, increasing flood risk. This study used radar data and simulations to show how city heat impacts storm speed and rainfall.
Area of Science:
- Atmospheric Science
- Meteorology
- Urban Climatology
Background:
- Convective storm motion influences rainfall accumulation and flood risk.
- Urban areas can alter local weather patterns, including storm behavior.
Purpose of the Study:
- To investigate how urban areas affect the speed of convective storms.
- To understand the implications of storm slowing on urban rainfall and flood risk.
Main Methods:
- Observational analysis of rain cell speeds across eight cities using weather radar data.
- Convection-permitting numerical simulations of heavy rainfall events over Indianapolis, Indiana.
Main Results:
- Observed rain cell speeds decreased over and downwind of five urban areas.
- Simulations showed dampened cloud-layer wind speeds over Indianapolis due to urban surface warming.
- Stronger updrafts, induced by urban heat, were identified as a cause of horizontal wind slowing.
Conclusions:
- Rainfall intensification is the main driver of enhanced urban rainfall.
- Storm slowing over urban areas contributes to more frequent and stronger rainfall enhancements.
- Understanding these effects is crucial for urban flood risk management.
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