Mumbai: The Indian monsoon is influenced by many complex factors, from large-scale atmospheric motion to local and regional physical processes. While large-scale processes have been traditionally well studied, the role of local processes, especially those related to aerosols, or the mix of atmospheric pollution particles and dust, have been largely overlooked. A study carried out at the Interdisciplinary Programme in Climate Studies at IIT Bombay found that atmospheric aerosols WERE present over the monsoon region altered cloud properties in diametrically opposite ways during years of deficient and abundant rainfall. The research, which appeared on 24 March 2017 in the journal Scientific Reports of the Nature Publishing Group (NPG), was carried out by PhD students Mr. Nitin Patil and Mr. Prashant Dave and Prof. Chandra Venkataraman.
Using a 10 year dataset, from 2000-2009, of satellite and ground-based observations of aerosol abundance and cloud properties, over the core monsoon region of India, the study found that in deficient rainfall years, higher levels of atmospheric aerosols correlated with smaller cloud drop sizes, shallower cloud heights and less cloud-ice formation. In contrast, in abundant rainfall years, higher levels of aerosols correlated with larger cloud drop sizes, taller clouds and greater cloud-ice formation.
Deficient rainfall years typically have lower moisture availability, less upward wind motion or convection, along with lower cloud fractions or cloud coverage. The changes in cloud properties found in these years with increased aerosol levels, could further inhibit cloud development and subsequent rainfall. However, when overall higher vertical wind and moisture transport, in abundant rainfall years, coincided with increased aerosol levels, the changes observed in cloud properties could intensify rainfall.
In describing the work, Prof. Chandra Venkataraman said, “The question we asked was whether aerosols affect clouds in similar or dissimilar ways, under broadly different meteorological fields, which occur in deficient and abundant monsoon years. The opposing influences found here confirm that local physical processes exert important effects.”
Observations of monsoon trends in the last 50 years show decreases in frequency of moderate and low rainfall but increases in frequency of higher intensity rainfall. More work is needed to link relationships observed in this study to consequent changes in rainfall development. “Understanding aerosol-cloud-rainfall interactions could help improve the physics of climate models for future climate prediction,” added Prof. Venkataraman.