Marine Stratocumulus to Land Fog Transition in the Coastal Mountain-Range of Atacama Desert

Abstract. The advection of marine stratocumulus clouds (Sc) from the Southeast Pacific into the Atacama Desert forms extensive, semi-permanent fog banks at the top of the coastal mountain range. These fog banks provide the sole water input for xeric ecosystems and serve as a freshwater resource for human activities. To improve our understanding of this relevant system, we conducted a field experiment in July 2024 at the coastal Atacama called StraToFog to measure vertical profiles and near-surface parameters related to the marine boundary layer (MBL) state during the marine Sc-to-land fog transition. In this study, we aim to describe the campaign and present some of the first results from observations of the main processes at non-local and local scales during the Sc-fog transition. The vertical profile measurements reveal that the cloud-topped MBL frequently reaches ~15 km inland, with negligible thermal changes but a significant decrease in humidity, suggesting fog and water vapour retention on the coastal mountains. We found that maximum fog collection occurs in the afternoon, driven by a sea-to-land breeze that transports higher marine humidity, thereby increasing liquid water content. During the night, fog collection decreases due to lower wind speed and liquid water content resulting from lower air temperature. At the surface, our observations suggest that negative latent heat flux is produced by dewfall at dawn, water vapour adsorption during midday, and fog deposition over the afternoon. The StraToFog field experiment revealed key acting processes, linking spatial scales to understand atmospheric water pathways in this hyperarid environment.