On the Role of Land-Atmosphere Coupling in Boundary Layer Cloud Development Over a Mixed Forest in Eastern Canada

Forests regulate moisture and heat fluxes in the lower atmosphere, which are inextricably linked to fair weather shallow cumulus formation within the boundary layer. Forest enhanced cloud shading thereby affects the earth’s surface radiation budget, a phenomenon that has been reasonably well studied through modelling and large-scale satellite studies. However, there is a lack of surface-based observational studies linking surface fluxes to cloud formation. By combining flux tower and ceilometer measurements in a mixed Acadian (Atlantic Canadian) forest near Fredericton, New Brunswick, we gain a unique opportunity to study land-cloud coupling using these local, surface-based flux observations. Analysis of 30-minute averaged surface-based tower measurements reveals fair weather summertime shallow cumulus formation over a 3-year period (2023-2025). Shallow cumuli occur on 160 days in total, exhibiting a clear seasonal cycle with a pronounced peak between June and August. We employ machine learning to determine the importance of environmental drivers and surface fluxes on daytime cloud fraction and cloud base height on days with shallow cumuli, with surface moisture exhibiting the strongest influence. Additionally, we show the response of shallow cumulus to extreme surface conditions by examining the period between August and September 2025. Fredericton experienced anomalously dry conditions, receiving only ~15% and ~40% of normal precipitation in August and September, respectively, during which soil moisture falls to 30% typical late-summer values. We find a significant reduction in shallow cumulus formation during the dry conditions, which we hypothesize is caused by the shift of surface flux partitioning from latent to sensible heating, and the concurrent enhancement of daytime lifting condensation level growth.