THE ERRATIC behaviour of monsoon rainfall, including the phenomenon of concentrated heavy rainfall on a small number of days, could, at least in part, be attributed to the rising air pollution, especially the increase in suspended particles in the atmosphere, a new study by scientists from IIT Kanpur has said.
The study, published in the prestigious journal, Nature Communications, on Friday has shown how excess aerosols, suspended solid particles like dust, smoke and industrial effluents, in the atmosphere is changing cloud patterns, its shape, size and other properties like temperature, which in turn is resulting in variability in rainfall over the Indian sub-continent during the monsoon season.
“Aerosols are extremely important for cloud formation. In fact, in the absence of aerosols, no clouds would be formed and consequently no rainfall will take place. But as we know, excess of everything is bad. That is what is happening here. An increase in the aerosol content in the atmosphere, a direct consequence of rising air pollution, is interfering with the stable cloud formation system and influencing rainfall patterns,” S N Tripathi, professor at the Centre for Environmental Science and Engineering at IIT Kanpur and one of the authors of the study, said.
Tripathi said in short term, these changes in cloud structure and cloud dynamics lead to sharp variability in rainfall, the kind of which is being witnessed very often in India in the last few years. In the long term, it is likely to lead to an overall suppression of rainfall during the monsoon season, he said.
“This year, Nagpur and Ujjain witnessed unusually heavy rainfall on certain days. Such rainfall events are not normal. We suspect that changing cloud dynamics could have a role to play in events like these. The high pollution levels are not just changing cloud shape and size and depth, but also its microstructure,” Tripathi said.
Tripathi’s student, Chandan Sarangi, is the lead author of the study. Other authors are Vijay Kanawade and Abin Thomas of University of Hyderabad, and Dilip Ganguly of IIT Kanpur.
The group analysed satellite data and data from atmospheric computer models from the last 16 years to make an assessment of the likely impacts of changes in cloud behaviour over land area of about 16 lakh square kilometres.
The linkage of air pollution to rainfall activity is not new and has been established in many earlier studies as well. Tripathi’s team, however, has, for the first time, given details of the exact changes that take place in the clouds over India as a result of an increase in aerosols, and how this was leading to a reduction in the difference in day and night temperatures, and also impacting rainfall activity.
Since the Uttarakhand tragedy in 2013, India has had an unusually extreme rainfall event every year. Long-term rainfall data also shows that rainfall activity is getting increasingly concentrated to a few extremely wet days during the season, while most of the other days remain relatively dry.