Consistent signal of aerosol indirect and semi-direct effect on water clouds in the oceanic regions adjacent to the Indian subcontinent

Abstract

Abstract Quantifying aerosol-cloud interaction is one of the most challenging tasks in climate science. Previous studies in the oceanic regions adjacent to the Indian subcontinent providing evidence of aerosol impacts on cloud properties were mostly confined in the winter season. Here we analyze 15 years (March 2000–February 2015) of MODIS aerosol and cloud products and 10 years (October 2004–February 2015) of OMI absorbing aerosol index (AI) over the Arabian Sea (AS), Bay of Bengal (BoB) and South Indian Ocean (SIO) to examine the sensitivity of marine water cloud characteristics to changing aerosol loading. Amidst a monotonous increase in Reff (cloud effective radius) with an increase in LWP (liquid water path) in clean and polluted conditions (an indicator of the meteorological influence), a smaller Reff at higher AOD (aerosol optical depth) bins demonstrates that aerosol signal is detectable and consistent in LWP range of 150–350 g m−2. Furthermore, a faster rate of Reff growth with increasing LWP in clean condition and the pattern of change in cloud fraction (fc) with an increase in AOD that cannot be explained by meteorology alone also provide evidence of the consistency of aerosol indirect effect. Evidence of aerosol semi-direct (decrease of LWP with an increase in AI) is also conspicuous in the AS and BoB. Our results suggest improved representations of aerosol and cloud processes in the models are required to reduce uncertainty in regional climate forcing.