Sensitivity of marine low-level clouds to above-cloud aerosol-layer characteristics of smoke and dust over the tropical Atlantic Oceans

Absorbing aerosols, such as smoke and dust, play a crucial role in influencing the Earth’s radiative budget and cloud dynamics. These aerosols absorb radiation and can warm the surrounding atmospheric layer, modulating the temperature and moisture profiles and consequently influencing the cloud distribution. This rapid adjustment or response of clouds to these aerosol-induced changes in heating and moisture distribution is commonly known as the semidirect effect (SDE). Whereas smoke particles often have smaller sizes (< 1 µm), thereby interacting primarily with shortwave radiation, dust particles can interact with both shortwave and longwave radiation because of their wide particle sizes range (0.1-100 µm). Given this difference, it is unclear how low-level cloud responses to semi-direct effects differ when smoke or dust is above the cloud. To address this, we compared two regions in the tropical Atlantic Ocean, characterized by frequently occurring low-level clouds: the northeast Atlantic Ocean with transported dust layers from the Sahara deserts and the southeast Atlantic Ocean with transported smoke layers from the Central African wildfires. We used aerosol-layer properties and cloud parameters from CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations), along with radiative fluxes from CloudSat-CALIPSO combined products and radiative transfer simulations, to analyze how the altitude, geometrical, and optical thickness of these dust and smoke layers affect underlying cloud properties. We find that above-cloud smoke and dust layers enhance atmospheric warming, increasing lower-tropospheric stability and consequently increasing low-level cloud cover. However, the magnitude of the cloud cover response differs for dust and smoke aerosols and also depends on the aerosol-layer characteristics. Specifically, we find a significant negative cloud response to the dust layer, whereby a geometrically thicker dust layer (regardless of base altitude) is associated with smaller increases in cloud cover. In contrast, we find a negligible or small positive cloud response to the smoke layer, whereby variations in smoke-layer geometric thickness (regardless of base altitude) do not significantly alter the cloud cover. Because the smoke layer often contains elevated moisture content, our analysis also reveals that increasing the smoke layer thickness produces moisture-induced longwave effects similar to dust. We conclude that dust-driven or moisture-driven longwave effects reduce low-level cloud-top cooling and partially counteract the stability-enhancing shortwave effects. Our analysis underscores the importance of improving the representation and understanding of absorbing aerosols in climate models to better account for cloud responses to these aerosols and accurately quantify their semidirect effect.