Pollution triggering bursts of heavy rain

Aerosols and particulate matter in polluted air are inducing the formation of larger clouds that trigger bursts of heavy rain in the Indo-Gangetic plains, according to the findings of a study by researchers at the Indian Institute of Technology (IIT)- Kanpur and the Pacific Northwest National Laboratory in the US.

For the study, the researchers simulated a storm over Kanpur city and found that the urban core and areas downwind from the city received more rainfall compared to upwind areas.

Using a weather research and forecasting model, they found similar patterns in Delhi, Agra, the Durgapur-Asansol region, and Kolkata, all located in the Indo-Gangetic plain where particulate matter pollution is at severely hazardous levels.

“The increased releases of latent heat in a polluted environment invigorate the cloud systems to generate more ice hydro meteors (ice particles in the atmosphere) and eventually more rain,” the researchers wrote in a paper accepted by the Journal of Geophysical Research, the flagship publication of the American Geophysical Union.

The large agrarian population in the Indo-Gangetic plains is heavily dependent on the monsoon to irrigate farm holdings. Extreme rainfall interferes with sowing and harvesting patterns, adversely affecting crop productivity and the storage of harvest.

Aerosols are solid particles or liquid droplets suspended in the atmosphere. A higher aerosol load means there are more nuclei for water vapour to condense over, delaying the onset of warm rain (or rain caused when water particles coalesce or come together).

“These aerosols modify clouds and once they cross a certain threshold, they delay and redistribute rainfall in cities ,” said SN Tripathi, a co-author and professor at IIT-Kanpur. Heavy rain is a severe weather hazard that is associated with flooding and loss of property and life.

The effect of aerosol loading on cloud formation is well understood, according to Vijay Kanawade, a scientist at the University of Hyderabad. “If you pump in more aerosol in the monsoon time, it will suppress warm rain and delay the onset of monsoon, but it will enhance cold rain.” Cold rain is rain caused by rapid formation of ice crystals in some clouds, with the crystals falling to the ground once they are heavy enough, and melting if the lower temperatures are warm as they usually are in the Indo-Gangetic plain. The extent to which higher aerosol loading is impacting the monsoon rains in India is a subject of debate. “The pollutants are likely to impact the early rains but not after that because they settle down,” DS Pai, a scientist at the India Meteorological Department, said.

In urban centres, the problem is compounded by the heat-island effect, where large built-up areas absorb heat intead of reflecting it. “The urban heat-island effect caused convergence of winds and moisture in the lower troposphere, which enhances convection over urban regions and induces more rainfall over the urban core compared to the upwind region,” Tripathi said. The impact of the heat island effect on rainfall is still being studied. “Not all scientists believe that it leads to increase in rainfall in the affected area,” Kanawade said.