BENGALURU: Scientists at the Indian Institute of Science (IISc) have estimated the Critical Cloud Fraction (CCF) value over the Bay of Bengal region. For the first time, they have demonstrated that it changes with the nature and type of aerosols present above the clouds.
“CCF indicates when aerosols above the clouds will change from cooling to heating the earth,” explained Prof S K Satheesh, professor at the Centre for Atmospheric and Oceanic Studies and Divecha Centre for Climate Change at IISc, who conducted the study along with J Srinivasan, professor and chairman of Divecha Centre for Climate Change.
The paper has been published in the Atmospheric Science Letters. The scientists said previous investigators had argued that CCF is the same all over the globe. “We have shown that this is not the case and it varies on account of changes in the chemical composition of aerosols,” Srinivasan said.
Aerosols such as smoke or dust suspended in the lower atmosphere can either heat up the planet by trapping solar radiation or cool it by reflecting sunlight back into space.
Earlier, research had shown that absorption of radiation by aerosols can significantly heat up the atmosphere over the Bay of Bengal. As this region greatly influences the Indian summer monsoon, any change in the effect of aerosols can have a critical impact on local climate, a press note stated. Little is known, however, about how aerosols that are present above the clouds contribute to this warming. Their contribution could depend on the amount of cloud cover below them, studies show.
The researchers found that CCF shifted from a higher value during the post-monsoon period (September-November) to less than half of that value during winter (December-February).
They also found that this shift corresponded with a change in the nature of aerosols over the region during those periods. In the post-monsoon season, the aerosols present were those that absorb less sunlight, such as sea salt aerosols. In their presence, a larger cloud coverage would be needed for the aerosols to have a warming effect.
But in winter, the aerosol composition shifted to those that absorb more solar radiation, such as black carbon and sulfate emissions, likely originating from India, China and Myanmar, leading to a lower warming threshold or CCF value.
The analysis used data collected over four years from multiple satellites.