Circular cities get more rainfall, finds study

Girdled by Outer Ring Road, Hyderabad is now a circular city

May 20, 2022 07:01 pm | Updated 07:01 pm IST - HYDERABAD

Circular cities get, and are likely to get, more intense rainfall compared to triangular or square cities, a new study has found. “Circular city shape shows potential risks of extreme rainfall and resultant flood risk,” says the study published in Earth’s Future journal.

The research used “idealised large eddy simulations coupled with the weather research and forecasting model to bridge the knowledge gap”.

Girdled by the Outer Ring Road, Hyderabad is now a circular city. For three centuries, the city was shaped like a triangle due to the encircling wall. Hyderabad has been seeing extreme variation in rainfall pattern over the last few years with some areas recording heavier rainfall.

On Sunday evening, parts of Jubilee Hills near the check-post were flooded while other areas remained dry. On May 4, Kala Pathar area in Old City was flooded after a one-hour spell. Other parts of the city however, remained dry.

The study found “increased rainfall rate over inland cities compared to their surrounding rural areas, and provide insights into urban rainfall modification within the context of large-scale urban-rural contrast.”

While urban planners have been talking about paved surfaces creating heat sinks and increased runoffs during rains, the study found that this is leading to heavier rainfall than in surrounding rural areas.

“Though cities themselves have negligible moisture at the surface, the uneven warming rate modulate urban-rural circulation by creating a convergent flow, and thus facilitate the transport of moist air from rural areas towards the city,” says the study authored by Jiachuan Yang of University of Hong Kong.

Balaji, who has been forecasting short-term Hyderabad weather with accuracy on social media, agrees with the pattern of rainfall. “Urbanisation (urban heat island) effect influences thunderstorms and rainfall patterns in the city. For example, sometimes it rains all around Hyderabad, but not in the core city. We are noticing this pattern over the last two days. But sometimes, the same effect causes sudden high-intensity downpour within a short span of time that leads to flooding,” says the amateur weatherman.

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