Influence of Mineral Dust on the Microphysics of Low-Level Warm Clouds Over the North Atlantic Ocean

Mineral dust, one of the most abundant aerosol species, is among the most significant atmospheric components whose role in modulating Earth’s climate is still poorly understood. They absorb light, thereby reducing the surface-reaching solar radiation, resulting in modulation of atmospheric temperature and moisture profile. In addition, while freshly emitted dust particles are non-hygroscopic and not significant contributors to cloud condensation nuclei (CCN), their hygroscopicity can increase during long-range transport as they mix with other aerosols and gases in the atmosphere. This change in dust hygroscopicity during transport can influence warm clouds by changing cloud droplet size and overall cloud albedo, known as, aerosol first indirect effects (FIE). However, a clear relationship between dust properties and low-level warm cloud microphysics, including the associated first indirect effect, remains unclear. In this study we used satellite-based observations of aerosol and cloud properties, including aerosol optical depth, cloud droplet size, along with reanalysis products to investigate the microphysical effects of dust on low-level clouds over North Atlantic between 2003-2012. Our analysis reveals that dust aerosols significantly influence FIE over north Atlantic Ocean. Specifically, we found that, on high-dust days, the FIE is lower compared to low-dust days. We attribute these decreases to the enhanced droplet size resulting from dust-induced warming, which shifts the cloud drop size distribution towards larger particle radii. We further hypothesize that dust-induced warming leads to the evaporation of smaller droplets, increasing the mean droplet size. In addition, our results are supported by simulations using the WRF-Chem (Weather Research and Forecasting – Chemistry) model, which also shows that more absorbing dust is associated with larger droplets for the same liquid water path. Our study emphasizes the importance of accurately representing dust absorption in climate models to better estimate radiative forcing associated with aerosol-radiation and aerosol-cloud interactions.