Can Aerosols improve Urban Flood Prediction? A case study of 2015 Chennai extreme event

Abstract. In December 2015, Chennai, a coastal megacity in India, faced an extreme precipitation-flooding event (EPF) that triggered a devastating 1-in-100-year flood. Several previous attempts failed to accurately simulate the spatiotemporal variability of this EPF at the urban basin scale. Even though incorporating aerosols into operational weather models can improve the accuracy of EPF simulations, it is often ignored due to its computational cost. To address this, we conducted ensemble experiments to highlight the significance of aerosol-cloud interactions in simulating this EPF. In that regard, we use a computationally intensive, high-resolution WRF model configured in large-eddy simulation (LES) mode to represent the interactions in the complex urban microphysics. The results indicate that explicit aerosol representation significantly influenced the microphysics-dynamics interaction during the 2015 EPF and produced rainfall patterns in closer agreement with satellite and rain gauge observations, with basin-scale improvement of ~22 %. Further, employing simulated rainfall in a coupled hydrologic-hydraulic modeling framework increased inundation accuracy by ~50 %. Thus, this study suggests that explicit aerosol representation can improve the space-time simulation of rainfall and flooding for the EPF in Chennai and potentially for similar coastal megacities.