Effects of Saharan air layers on clouds over the tropical Atlantic during BOWTIE

The radiative effect of clouds is determined by their development, lifetime and microphysical characteristics. The processes influencing cloud properties are diverse and require further investigation to gain a more profound understanding. However, process studies cannot be conducted by modelling alone, as observations are rarely available, particularly over the oceans. This leads to uncertainties in global weather modelling and climate forecasts. In order to disentangle the effects of various influencing factors on clouds in the intertropical convergence zone (ITCZ), recent observations from the BOWTIE campaign are being analysed with regard to the effects of Saharan dust. During the BOWTIE (Beobachtung von Ozean und Wolken – das Trans-ITCZ Experiment) campaign, which was part of the ORCESTRA (Organized Convection and EarthCARE Studies over the Tropical Atlantic) campaign, the research vessel (RV) Meteor crossed the tropical Atlantic from Cape Verde to Barbados in August and September 2024. The proximity to the African continent and the Sahara led to some trajectories of the Saharan air layers (SAL) traversing the atmosphere above the RV Meteor. Three episodes of SAL lasting between two and three days were observed at altitudes spanning from one to five km. The dry and dust-loaden character of the SAL is determined by their origin in the Saharan desert. Analysis of the Raman lidar observations provides information about the time and altitude of the SAL. In synergy with other instruments, such as radiosondes and a motion-stabilised 94GHz cloud radar, the effect on clouds can be investigated. The stabilisation of the cloud radar, which was monitored by STARPAS (STAbilized Radar Platform Alignment Sensor), ensures reliable vertically pointing cloud observations. The findings indicate that SAL reduces cloud vertical development and suppresses weak convection. This behaviour is due to the thermodynamic structure of the SAL with low relative humidity, nearly dry adiabatic temperature gradients and inversions at the top and bottom. Particle-specific properties of acting as cloud condensation nuclei or ice nucleating particles are of secondary order.