Aerosol Influences on Cloud Modification and Rainfall Suppression in the South Asian Monsoon Region

Abstract

The South Asian monsoon (SAM) is critical to the agricultural economy of the region. Large-scale processes affecting the monsoon have been traditionally been well studied; however, much less is known about the role of local processes, especially one governed by levels of atmospheric aerosols, or the mix of pollutant particles and dust. This chapter delineates the aerosol influence on cloud modification and short-term rainfall inhibition over India, employing observational data for the period of 2000–2009. Aerosols can change cloud-related properties in contrasting ways in years of deficient and abundant monsoon. In deficient years, increased levels of atmospheric aerosols correlate with smaller cloud drops, shallower cloud heights, and less cloud-ice formation. In contrast, in abundant rainfall years, higher levels of aerosols correlate with larger cloud droplet size, taller clouds, and greater ice-cloud formation. Further, causality was established in high aerosol and low rainfall regions spanning across the Indian subcontinent, wherein enhancement in aerosol levels caused suppression of daily precipitation anomaly multiple times in a monsoon season with lags of a few days. The studies also suggest aerosol effects on increases in the frequency and length of monsoon breaks, with implications for rainfall deficit and food grain production. This work makes important linkages between enhanced air pollution and perturbations in the timing and short-term suppression of regional monsoon rainfall, implying the need for greater synergy in policies addressing air pollution and climate change.