Competing aerosol effects in triggering deep convection over the Indian Region

Contrasting direct and feedback effects of aerosols on deep convection in the Gangetic Plain of Indian subcontinent have been analyzed by employing observation and modeling framework during the Ganges Valley Aerosol Experiment conducted in 2011. The key question addressed in the current investigation is: while the aerosol direct radiative effect leads to reduced surface shortwave flux and possible stabilization of the lower atmosphere, how is the deep convection favored in a heavily polluted atmosphere? The composites of aerosol, cloud and meteorological parameters during the formation of deep convective clouds (DCCs) under both light- and heavy- pollution conditions are contrasted to elucidate the aerosol effects. An enhanced low-level warming is discernible due to the presence of heavily polluting absorbing aerosols. A seminal role of aerosols in increasing the moist static energy (MSE) through enhancement of internal energy is identified as a primary feature that increases the convection potential of the lower atmosphere. Concomitantly, the greater moisture convergence facilitated by dynamical feedback of enhanced low-level heating destabilizes the atmospheric stratification. Additionally, the microphysical invigoration is examined for any possible role, nevertheless, only a weakened invigoration could be observed possibly due to dominance of meteorology. The observed results are well supported by model simulation of a deep convective event under high- and low-aerosol backgrounds. Besides highlighting the dominance of radiative processes over microphysical processes under heavily polluted environment, the significant correlation between aerosol and high clouds observed during DCC formation under such conditions suggests that the aerosol direct effect in weakening convection could be overcome primarily through enhancement in MSE and concomitant moisture convergence.