Insights into Cloud Processes from In Situ UAV-Based Cloud Observations and Aerosol-Aware Numerical Simulations

Understanding the complex interactions between aerosols, cloud microphysics, and dynamics is essential for accurately predicting cloud behavior and its impacts on the climate system. One of the key open questions concerns how cloud liquid water path responds to changes in cloud droplet number concentration. Turbulent mixing, initiated by radiative cooling near the cloud top, plays a central role in this feedback by modifying cloud microphysical properties. This mechanism has been suggested as a primary explanation for the observed reduction in cloud liquid water content with increasing aerosol concentration over the global oceans.

In this study, we provide new insights based on observations of subarctic low-level clouds combined with model-assisted analysis of the coupling between boundary-layer dynamics and cloud microphysical processes. The work benefits from unique measurement capabilities at Pallas, Finland, where unmanned aerial vehicle (UAV) systems can be operated up to 2000 m agl and beyond the visual line of sight, supported by ACTRIS cloud and aerosol measurement facilities. We present high-frequency surface-based and airborne in situ datasets collected using multiple cloud droplet sensors and compare them with surface-based remote sensing products. In addition, we employ UCLALES-SALSA, a large-eddy simulation model with sectional aerosol–cloud–precipitation microphysics, to investigate the processes controlling both mean cloud properties and the spatial variability of cloud droplet size distributions in relation to cloud dynamics. This combined observational and modeling approach improves our understanding of differences between surface-based and airborne in situ observations and provides high-quality reference data for the validation of remote sensing products.