Aerosols can sharpen Chennai flood forecasts, finds IIT-Madras study

CHENNAI: Atmospheric aerosols could hold the key to improving extreme-rainfall and urban-flood forecasts, with an IIT Madras-led international study finding that explicitly accounting for these tiny airborne particles improved simulated rainfall over Adyar basin by about 22 per cent and flood-inundation accuracy by up to 51 per cent.

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The study, published in Natural Hazards and Earth System Sciences, examined the extreme rainfall and flooding event in Chennai on December 1, 2015, to assess how aerosol-cloud interactions influence rainfall and how those changes propagate through runoff, reservoir inflows and flood inundation.

The research brought together scientists from IIT-Madras; GFZ Helmholtz Centre for Geosciences, Germany; Japan Aerospace Exploration Agency (JAXA); and Kathmandu University, Nepal. The team included Oscar Paul, N Nithila Devi, Rakesh Teja Konduru, Soumendra Nath Kuiry, Kundan Lal Shrestha, and Chandan Sarangi.

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The researchers used the high-resolution Weather Research and Forecasting (WRF) model, configured in large-eddy simulation mode, to reproduce atmospheric processes during the 2015 event under different aerosol conditions. The simulated rainfall was subsequently coupled with hydrological and hydraulic models to assess runoff, reservoir inflows, and inundation across the Adyar basin.

The 2015 event, considered a 1-in-100-year flood, produced nearly 500 mm of maximum daily rainfall over the basin and was among Chennai's most catastrophic urban flooding events. The estimates put the death toll at about 500 and economic losses at USD 3 billion.

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"Urban flood forecasting is often treated primarily as a rainfall-to-runoff problem. The problem in urban flood forecasting is that rainfall spatial and time distribution is not well captured by our weather models," said Chandan Sarangi, corresponding author and faculty member in the IIT-Madras's Department of Civil Engineering.

"What happens in the atmosphere before and during rainfall can significantly influence the spatial rainfall pattern. By explicitly representing aerosol-cloud interactions, we can better capture the spatial and temporal characteristics of extreme rainfall within urban regions and consequently improve flood inundation simulations," he said.

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Soumendra Nath Kuiry said rainfall forecasting in a megacity such as Chennai should be viewed alongside water resources management. "Timing and spatial distribution of extreme rainfall directly influence runoff, reservoir inflows and flood forecasting. A coupled atmosphere-hydrology-hydraulic framework can provide a more physically informed basis for these interconnected decisions," he said.

The study found that realistic, lower CCN concentrations during the event favoured warm-rain processes and helped reproduce the observed rainfall pattern more accurately. The resulting improvements also reduced errors in simulated reservoir inflows and improved the estimated extent of flooding.

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The findings have a direct water management dimension. The study noted that about one-third of the peak inflow during the 2015 event resulted from Chembarambakkam reservoir releases and cited earlier research indicating that timely regulation of initial reservoir storage could have reduced reservoir outflows by 30 per cent while delaying the flood peak.

However, the researchers cautioned against treating the findings as an operational forecasting breakthrough. The study covers a single extreme event, while the high-resolution modelling is computationally demanding. Further testing across different extreme rainfall events and cities is needed to establish whether the approach can be applied reliably in operational flood forecasting, they added.