26 Jul 2016

Global warming is thought to be making Indian monsoon rainfall more unpredictable, and prone to extremes—reason enough for cities to improve drainage and bolster flood defences. But new research suggests that another phenomen-on is also shaping rainfall over cities, making such measures even more urgent.

Several studies across the world, including in India, show that urbanization may be influencing precipitation, in many cases by intensifying extreme rainfall events.

The trend of increasingly intense rainfall events over urban areas in India was first identified in a 2010 study led by Isro's Chandra Kishtawal and US scientist Dev Nigoyi. A more recent study from IIT-Bombay looking at 42 regions across the country found that while urbanization does not play a big role in rainfall in north India, the effect was prominent in the western and central regions.

To confirm the role of urbanization, and eliminate other factors, IIT researchers Hiteshri Shastri, Subimal Ghosh, Subhankar Karmakar and Supantha Paul looked closely at the Mumbai urban area. They compared the city's rainfall records with the adjoining rural district of Alibaug for 1969-2005.

Both places are subject to the same weather systems, the same "circulation patterns and moisture levels", said Ghosh, an associate professor with the IIT's department of civil engineering. Yet, the researchers found, Mumbai saw a significant increase in extreme rainfall days. "When it comes to high rainfall, urbanization seems to play a factor in intensifying it," he said.

Globally, intense urban development was found to have aggravated a big storm in Beijing in 2006 and again in 2012. Urban sprawl is thought to have intensified a tornado in Atlanta, US, in 2009.

Some of this new research is being factored into the Intergovernmental Panel on Climate Change's models. The most recent IPCC report cites findings from North America showing how urbanization can enhance or reduce precipitation, depending on climate, location, and regional land use, and amplify climate impacts in other ways. In India, where temperature rise is likely changing rainfall pattern and intensity, the spatial variability in these changes is now thought to be influenced by local factors. "Some of the observed increase (in precipitation) could be from land use changes, especially urbanization," said Ghosh.

How can cities influence rainfall? Scientists think that urban heat islands play a role. Heat islands are warm areas formed by the concrete and asphalt of a city, aggravated by pollution. Warmer temperatures increase precipitation as well as atmospheric instability, which is conducive to thunderstorms. Increased temperatures may also push rain clouds upwards, or slow them down; so, rainfall may concentrate over one area.

Research on the effect of urbanization on local climate is relatively new and still poorly understood, said Dev Niyogi, assistant professor of agronomy at Purdue University and a state climatologist in Indiana, US. "As far back as the nineteenth century, people were talking about city being hotter than country," said Niyogi. "But while the urban heat island effect is well-known now, the effect of cities on circulation and rainfall is less well-understood."

He likens the factors that shape rainfall to a hammer and chisel. "The hammer is the large-scale circulation system that produces the rain system. Local perturbations are the chisel that help shape when, where and how much will it rain," he said. "We now think that urbanization is a dominant chisel."

Another study by Niyogi suggested that a city had to have a radius of 20km before it began to affect rainfall. "That's when maybe it builds up a sufficiently large heat bubble to change the flow of air coming to it. It's like attacking a heat dome."

In some studies, urbanization appeared to increase rainfall downwind of the city. Apart from temperature, polluting particles may also interact with and modify the rain system in an urban area. Urban form—the height of buildings for instance—might also modify wind patterns. Wind tunnels created by rows of tall buildings, for example, can potentially increase wind speeds and atmospheric instability.

The fact that human activity has an impact on short-term weather, not just long-term climate, is a good reason to rethink urban development and land-use policies, especially in newly booming cities, Niyogi said. "The things we are doing now, how we develop our cities have a much more short-term impact than we imagine."

Researchers recommend mitigating measures like green roofs as well as better drainage systems and flood forecasting. Ghosh and the others are working on a real time tidal forecasting system for the Mithi River that incorporates the urban feedback factor. Precipitation is difficult to forecast, compared to temperature, Ghosh notes. "It's like popcorn. We know when the whole handi heats up, but can't predict which corn will pop."

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