Atmospheric air pollution could play a significant role in improving rainfall and urban flood forecasting, according to an international study led by Indian Institute of Technology Madras (IIT Madras).
The study found that explicitly accounting for atmospheric aerosols, tiny particles suspended in the air that influence cloud formation and rainfall, can improve simulations of extreme rainfall and the resulting urban flooding.
The research examined Chennai’s December 2015 extreme rainfall and flooding event, using high-resolution atmospheric simulations incorporating aerosols and cloud condensation nuclei (CCN), the particles around which cloud droplets form.
Aerosols improve rainfall simulation
The researchers used the Weather Research and Forecasting (WRF) model to simulate atmospheric conditions during the 2015 Chennai event under different aerosol scenarios.
The resulting rainfall data was then fed into HEC-HMS for hydrological modelling and HEC-RAS for hydraulic and flood-inundation modelling across the Adyar basin.
The study found that explicitly representing aerosols produced an approximately 22% improvement in simulated rainfall over the Adyar basin compared with the control simulation.
The improvement was carried through the modelling chain, resulting in an approximately 50% improvement in the accuracy of simulated flood inundation compared with the control case.
Rainfall distribution key to flood forecasting
IIT Madras, Department of Civil Engineering, Corresponding Author, Prof. Chandan Sarangi said urban flood forecasting is often treated primarily as a rainfall-to-runoff problem, while the spatial and temporal distribution of rainfall is not always adequately captured by weather models.
He said atmospheric conditions before and during rainfall can significantly influence rainfall patterns, and explicitly representing aerosol-cloud interactions can improve the simulation of extreme rainfall and subsequent flood inundation.
IIT Madras, Department of Civil Engineering, Prof. S. N. Kuiry said rainfall forecasting for a megacity such as Chennai cannot be separated from water-resources management.
The timing and spatial distribution of extreme rainfall directly affect runoff, reservoir inflows and flood forecasting, he said, adding that a coupled atmosphere-hydrology-hydraulic framework could provide a more informed basis for these decisions.
Implications for Chennai reservoirs
The findings could have implications for water-resources management in Chennai, particularly when decisions on reservoir storage and releases have to be taken during extreme rainfall events.
For a rapidly urbanising coastal city, maintaining adequate reservoir storage during the monsoon requires accurate forecasts of intense rainfall at the urban-basin level.
The study brought together atmospheric science, hydrology, hydraulic modelling and Earth observation expertise from IIT Madras, GFZ Helmholtz Centre for Geosciences, Germany, Japan Aerospace Exploration Agency (JAXA) and Kathmandu University, Nepal.
The research was supported by the Asia-Pacific Network for Global Change Research (APN) and published in Natural Hazards and Earth System Sciences.
The researchers, however, stressed that the framework is not yet an operational flood-forecasting system. Further studies are needed to determine whether the improvements seen in the 2015 Chennai event can be replicated across other extreme rainfall events and cities.