Understanding Cloud Formation in Eastern Mediterranean Mountainous Environments

Understanding aerosol properties is essential for assessing their impacts on clouds, precipitation and climate. These interactions depend strongly on the aerosol levels present as well as the dynamical forcing (vertical velocity) that drive supersaturation development and droplet formation. Datasets that span the wide range of conditions found throughout the atmosphere are much needed to help constrain models and to characterize cloud susceptibility to aerosol.

 

High-altitude mountain stations, offer an exciting opportunity to study aerosol-cloud interactions because clouds often form at their peaks. The aerosol that acts as precursors of droplet formation can originate from near ground (i.e., within the planetary boundary layer) or long-range sources (i.e., through free-tropospheric transport). Being able to unravel the periods during which clouds are influenced by each air type can vastly expand the scientific value and relevance of aerosol-cloud studies at mountain tops.

 

The Demokritos Helmos Hellenic Atmospheric Aerosol and Climate Change ((HAC)²) station in Greece (2314 m a.s.l.) is the only high-altitude station in the eastern Mediterranean, a region highly sensitive to climate change. It is located at the crossroads of different air masses and is therefore very well-suited for aerosol-cloud interaction studies. To enhance understanding of the processes driving the formation and evolution of warm and mixed-phase clouds, the CALISHTO (Cloud-Aerosol InteractionS in the Helmos Background TropOsphere) and CHOPIN (Cleancloud Helmos OrograPhic sIte experiment) campaigns were conducted at Mount Helmos during the autumn-winter periods of 2021–2022 and 2024–2025, respectively. During these campaigns, in-situ and remote sensing measurements at a number of sites, located at the Kalavrita Ski Center and the (HAC)² station, were used to characterize the influence of the PBL at the (HAC)² and also the concentration of cloud droplets when a cloud forms at the station. We use these measurements, together with a state-of-the-art cloud droplet formation parameterization to predict the concentrations of CCN, and cloud droplet number that form throughout the year at the (HAC)². Using established metrics, we separate the periods of BL and FT influence and thus determine the susceptibility of clouds to aerosol in each airmass type and class. The calculations are also confirmed using in-situ measurements of cloud droplet number obtained through a PVM-100.