The impact of Saharan Air Layers on atmospheric stability and clouds during the BOWTIE campaign

Abstract. Clouds over the tropical Atlantic remain under-represented in ground-based observational studies, despite their key role in the regional radiative and hydrological budgets. This observational gap contributes to substantial uncertainties in the representation of these clouds in climate models. The recent BOWTIE (German: Beobachtung von Ozean und Wolken – Das Trans ITCZ Experiment) campaign aimed to address this deficiency through a six-week, ship-borne measurement campaign across the tropical Atlantic, spanning from Cape Verde to Barbados. This study investigates the influence of Saharan Air Layers (SALs) on cloud properties using vertically pointing remote sensing observations collected during the research cruise. Cloud types are classified and combined with Cloudnet-derived microphysical products to assess both macrophysical and microphysical cloud characteristics. The analysis reveals a shift in the cloud characteristics toward less convective regimes during SAL presence, accompanied by changes in cloud base height (CBH) and cloud top height (CTH). Microphysically, the suppression of convection associated with SAL leads to a greater vertical separation of cloud layers, meaning that droplet and ice effective radii (DER and IER) differ across layers. However, hydrometeor populations across all observed layers are comparable to those observed in marine cloud regimes reported in previous studies. Overall, the thermodynamic structure of the SAL emerges as the dominant driver of the observed changes in cloud properties. These findings highlight the importance of accounting for SAL influences in understanding cloud development over the tropical Atlantic and improving their representation in atmospheric models.