Dust–Cloud Vertical Configurations Influence the Effective Radius of Low-Level Warm Clouds over Marine and Continental Environments

Guoqing Gong, Adeyemi A Adebiyi 

Aerosol–cloud interactions remain a major source of uncertainty in climate forcing estimates, partly because cloud responses depend on the location of aerosols relative to clouds; yet for mineral dust, which accounting for about two-thirds of all aerosol mass, the effects of dust–cloud vertical configuration on cloud droplet effective radius remain unclear. Using multi-year observations from the Atmospheric Radiation Measurement sites at the Eastern North Atlantic (ENA; marine) and Southern Great Plains (SGP; continental), we quantify the sensitivity of low-level warm-cloud effective radius to dust-layer optical depth—the dust aerosol–cloud interaction (dust-ACI) index—for dust located above (Top), within (Middle), and below (Bottom) clouds. These configurations modulate the sign and magnitude of the dust-ACI index. At ENA, the index is strongly positive for the Top configuration (0.130 ± 0.065), weakly positive for the Middle (0.080 ± 0.029), and slightly negative for the Bottom (−0.026 ± 0.021) configuration, whereas the Middle (−0.021 ± 0.057) and Bottom (0.086 ± 0.041) responses reversing signs at SGP. After controlling for meteorological covariability, dust-ACI indices become consistently positive across sites and configurations, revealing that thermodynamic changes associated with vertical configuration can mask an underlying dust microphysical brightening tendency. We propose potential mechanisms whereby dust-induced semi‑direct heating dominates in the Top configuration, enhancing stability and suppressing entrainment to yield smaller droplets, whereas the Middle and Bottom configurations reflect competing microphysical and semi‑direct pathways whose relative strengths differ between marine and continental regimes. These results show that dust effects on clouds cannot be inferred from dust loading alone, but require accurate representation of dust–cloud vertical configuration and environmental regime.