Increased aerosol loading in the atmosphere due to pollution has led to changes in the number, shape and characteristics of the rain-bearing clouds. The increased cloud reflectance has led to 1 degree C drop in day time temperature during Indian summer monsoon. At the same time, night time temperature is increasing thus shrinking the diurnal temperature difference.

Increased emission of aerosols into the atmosphere due to pollution is beginning to have a definite cooling effect of 1 degree C during the Indian summer monsoon period, a study has found.

The increased cooling is seen during the day, while the night time temperature is increasing, thus shrinking the diurnal temperature difference. The diurnal temperature difference is what drives the convection process (where water evaporates and reaches the atmosphere as water vapour) and development of clouds.

As diurnal temperature difference decreases, the lower layer of the atmosphere will reduce in height and come closer to the earth surface. This will cause more aerosols to get into the atmosphere thus impacting the lower atmospheric turbulence which eventually may affect the moisture and rainfall distribution.

“Increased concentration of aerosols in the atmosphere also tends to change the shape and characteristics of the rain-bearing clouds leading to extreme rainfall events but weakened monsoon rainfall,” says Prof. Sachchida Tripathi from IIT Kanpur.

The study by a team of researchers led by Prof. Tripathi and Dr. Vijay Kanawade from University of Hyderabad found striking similarity between satellite data (2002-2016) and a global reanalysis modelled data that showed the cloud structure being modified with increased aerosol emission. The rain-bearing clouds were found to increase in number and height when aerosol emission is higher. The clouds also tend to have far more number of ice particles that are smaller in size when aerosol loading is higher, thus reducing the efficiency of water droplet growth.

“We found that when there is high aerosol loading, there will be more water droplets in the atmosphere. Once the droplets reach above the freezing level, ice formation begins. Heat is given off when ice formation processes begin. This acts as a fuel to make the cloud grow taller and thicker,” says Dr. Kanawade.

Normally, the clouds are below the tropopause (interface between the troposphere and the stratosphere). But this study finds that under high aerosol loading the clouds grow taller and hit the tropopause more often. The thermal capping at the tropopause in combination with tropical easterly jet stream (fast flowing wind) causes the cloud tops (anvil) to spread horizontally, forming thicker strati form clouds,” says Dr. Chandan Sarangi from IIT Kanpur and first author of the paper published in Nature Communications.

The anvil is thin and contains fewer ice crystals when there is less pollution. “But when aerosol loading is higher, the anvil contains more number of smaller ice particles and these ice particles tend to reflect the shortwave radiation from the top of the cloud leading to increased cooling of the earth surface. The cooling by shortwave radiation surpasses the warming by longwave radiation leading to net reduction in day time temperature during summer monsoon,” says Prof. Tripathi.

Published in The Hindu on September 14, 2018