Comparing the weather pattern in urban Mumbai and relatively non-urban Alibaug during the periods 1969–88 and 1989–2008, researchers found that higher temperature (post-urbanization) in the atmosphere of Mumbai accumulates more low-level moisture, increasing in ‘heavy rainfall’ instances over the area.

To understand how urbanisation affects rainfall extremes, the researchers took up a specific urban case study— comparing Mumbai and Alibaug — both of which receive an annual precipitation around 2100 mm/year.

“In Mumbai, the (wind) circulation pattern in post urbanisation period results into high rainfall percentile as compared to those of pre–urbanization period. The same is not observed for non–urbanised Alibaug. We observe that association between extreme rainfall in Mumbai and corresponding circulation pattern has been changed with urbanisation, which is not visible in Alibaug. This reconfirms our conclusions on impacts of urbanisation on extreme rainfall in Mumbai,” stated the report titled ‘Impacts of Urbanization on Indian Summer Monsoon Rainfall Extremes’. The report has recently been accepted by Geophysical Research, published by American Geophysical Union.

Higher temperature in the atmosphere of Mumbai accumulates more moisture and at the same time, urbanization leads to atmospheric instability. Both result into intensification of rainfall extremes, which is not visible for non-urban Alibaug, the research found.

Researchers included IIT-B professors Hiteshri Shastri, Subimal Ghosh, Subhankar Karmakar and Supantha Paul from Chandubhai S. Patel Institute of Technology, Anand. The team found Mumbai has more turbulent heat flux than in Alibaug, which indicates high instability in atmosphere over Mumbai and results in extreme rainfall.

“We find the difference of temperature to be on higher side before extreme rainfall over Mumbai, as compared to that of Alibaug. High temperature results in greater vertical wind velocities. This enhanced circulation would process more low-level moisture and hence intensifies rainfall,” explained Ghosh, an associate professor in IIT-B’s civil engineering department and researcher on climate change.

The researchers also studied impact of urbanisation on extreme rainfall in 42 urban areas in the country. The study found that in north India, the impacts of urbanisation on rainfall are overwhelmed by the large scale decreasing trend. In Southern, Central and Western India, however, the impacts of urbanisation on rainfall extremes are clearly visible.