Satellite-Based Analysis of Size-Segregated Aerosols and Their Effects on Warm Cloud Properties over the Northern Indian Ocean

Aerosol-cloud interactions contribute to 75–80% of the total radiative effect of aerosols and remain a major source of uncertainty in predicting future climate. Aerosols significantly influence the warm cloud properties by serving as cloud condensation nuclei (CCN). An increase in CCN leads to the formation of more numerous and smaller cloud droplets, suppresses warm rain by reducing the efficiency of collision and coalescence processes, and extends the cloud lifetime, and liquid water path (LWP) and/or cloud fraction (CF). The activation of a CCN into a cloud droplet is strongly influenced by its size and chemical composition, which subsequently affects the size distribution of cloud droplets and other cloud properties. Although the physical processes of nucleation are well documented for individual particles, the impact of aerosol size on cloud properties is often underestimated because both fine and coarse aerosols co-exist together. To bridge this gap, this study aims to address the impact of size-differentiated aerosols on warm cloud properties over the Northern Indian Ocean (NIO) by utilizing ~20 years of multi-satellite observation data.

The Arabian Sea (AS) and the Bay of Bengal (BoB) in the NIO were chosen in this study as these regions experience a continuous load of aerosols from natural and anthropogenic sources with high seasonal variations. Comparative analysis of size-segregated aerosol optical depth (AOD) revealed the dominance of coarse mode particles (c-AOD) over AS, and fine mode (f-AOD) over BoB. However, a significant increasing trend in the mean f-AOD, particularly during the post-monsoon (ON) and winter (DJF) seasons, is observed over both the AS (0.05/decade) and BoB (0.045/decade) from 2000 to 2021, primarily driven by rising anthropogenic emissions. Further, a climatological analysis of warm cloud CF during these seasons reveals a corresponding increasing trend over the AS (0.07/decade) and BoB (0.05/decade). A correlation analysis of c-AOD and f-AOD with warm CF was conducted, which revealed a stronger annual positive correlation of warm CF with c-AOD (AS: r = 0.56, BOB: r = 0.41) compared to f-AOD (AS: r = 0.37, BOB: r = 0.27). To further investigate the impact of f-AOD and c-AOD on cloud effective radius (CER) for a fixed LWP, an additional correlation analysis was performed. For low LWP (up to 70 gm^-2), an increase in CER was observed with both c-AOD and f-AOD, with a more pronounced increase in CER associated with c-AOD over both the AS and BoB regions. However, as LWP increased, f-AOD exhibited a faster decrease in CER over the BoB compared to the AS. In contrast, c-AOD consistently showed an increasing CER with rising LWP, indicating a contrasting effect relative to f-AOD. These results indicate the dominant radiative effect of fine mode aerosols on cloud formation against the classical microphysical effect of coarse mode aerosols.  Further analysis, incorporating meteorological parameters such as relative humidity and atmospheric stability, is essential to better understand these relationships and enhance the robustness of this study.