Moisture Aggregation and the Cloud Transition Zone Shape Mesoscale Aerosol Properties

Abstract Mesoscale moisture aggregation and cloud organization in shallow convection have gained much attention regarding their role in Earth's energy budget and cloud feedbacks on climate. Because aerosol particles swell in humid air, mesoscale moisture patterns are expected to be accompanied by corresponding variations in aerosol optical depth (AOD). However, observing such patterns is challenging because space‐borne imagers cannot reliably retrieve AOD in the aerosol–cloud transition zone (CTZ), where radiative transfer is different from both clear‐sky and cloudy conditions. By linking the concepts of moisture aggregation and the CTZ, we ask whether mesoscale patterns organize AOD patterns on the mesoscale, and what are the structures and components of the CTZ in different cloud morphologies. We address these questions using space‐borne images complemented by high‐resolution airborne LiDAR acquired during the EUREC 4 A field campaign, which sampled a range of cloud patterns and aerosol concentrations. We show that small cumulus clouds contribute to the CTZ through local moistening of the cloud layer. As moisture aggregates to support larger clouds, thin detrained layers near the trade‐wind inversion spread at the top of the cloud layer over tens of kilometers and increase AOD while often manifesting as optically thin clouds that would often not be detected by satellites. Such perturbations reverse during dust events, when precipitation‐processed air is cleaner than the background aerosol profile. Our results indicate that expected mesoscale AOD gradients are missing from satellite retrievals and suggest that mesoscale dynamics affect Earth's energy budget through shaping aerosol‐cloud‐radiation interactions.