System driven changes in aerosol-cloud interactions and its impact on the life-cycle of a monsoon depression

Abstract

Abstract This study examines the aerosol effect on the life cycle of a monsoon depression (‘MD’ hereafter), particularly whether it could suppress the system as suggested by previous modeling studies. Airborne aerosol and cloud microphysical observations collected during an MD (24 and 25, August 2009) are analyzed and the interactions between aerosols, cloud microphysics and MD dynamics are discussed. The growth of warm rain processes during the MD life-cycle was also investigated. An absence of rainfall due to the SW-NE orientation of rain belt exposed the northwest region of India to dry conditions and the transport of dust and anthropogenic aerosols through westerlies and north-westerlies heavily increased the aerosol concentration (NAERO) over the region which was favorable to suppress the convection. The results show an increase in regional convergence of zonal wind associated with high NAERO that led to an early transport of moisture to this region. The high NAERO under strong wind shear and dry to humid transition significantly affected the convective activity over the land during the initial phase of the MD. Over Central India, the combination of humid air and aerosols lead to the suppression and later redistribution of rainfall associated with the MD. However, these effects did not sustain for long as the continuous moisture transport part of the mesoscale convective system revitalized the system which is evident from surface latent heat flux. The strong low-level moisture transport supported by large-scale convergence from the Arabian Sea also weakened the aerosol effect helping in the good rainfall. The study also highlights that giant Cloud Condensation Nuclei (CCN) could play a positive role while the smaller CCN contributes to the suppression of rainfall.