Urbanization-Induced Spatial Modifications in Propagating Storms Implicate Urban Flooding Over Chennai, a Coastal Megacity of South Asia

Abstract

Urbanization alters the spatial distribution of rainfall and impacts urban hydrology, water management, and flood risk. This study examines spatial precipitation patterns and their implications on urban hydrology over Chennai (a coastal megacity in South India) using geostationary satellite data from 2014 to 2022. An object detection algorithm (YOLOv5) was employed to identify core rainfall regions of propagating coastal storms and track their motion, detecting 55 and 71 rainfall events in urban and rural regions, respectively. Results show that urban regions experience a maximum downwind shift of ∼20 km in peak rainfall accumulation from the coastline, compared to a ∼12 km downwind shift in rural regions. However, differences in rainfall intensity during these propagating storms over the urban and rural cases were not significant. The convection-permitting simulations of propagating storms revealed that the urban heat island effect drives warm-moist easterly winds inland (relative to the rural region), leading to convergence and rainfall closer to the urban-rural boundary on the downwind side. Further LULC sensitivity experiments indicate weakened penetration of easterlies, shifting convergence, and rainfall closer to the coastline when urbanization effects are absent. Hydrological model forced with simulated rainfall captured the observed reservoir inflows accurately. Specifically, the atmosphere-hydrological coupled simulations showed higher cumulative reservoir inflow (∼41.64%) and river inflow into the city (∼49.3%) due to the urban-induced spatial shift in the rainfall. These findings highlight that urban-induced shifts in rainfall patterns from heavy storms, rather than increased intensity, can significantly impact river flow and heighten riverine flooding risks in coastal megacities.