Simulating the Effects of Regional Forest Cover and Windthrow‐Induced Cover Changes on Mid‐Latitude Boundary‐Layer Clouds

• Published in: Journal of geophysical research. Atmospheres 2023-07, Vol.128 (13), p.n/a
• Main Authors: Noual, G., Brunet, Y., Le Moigne, P., Lac, C.
• Format: Article
• Language: English
• Subjects: Atmosphere; Atmospheric models; Boundary layers; boundary‐layer clouds; Cloud cover; Cloud formation; Cloudiness; Clouds; Comparative analysis; Damage; Deforestation; Deforestation effects; Diurnal cycle; Diurnal variations; Earth Sciences; Enthalpy; forest cloud cover; Forests; Geophysics; Heat flux; Heat transfer; Image analysis; Image processing; Land cover; land use; land‐atmosphere interaction; Massifs; mesoscale modeling; Mesoscale phenomena; Meteorology; Modelling; Precipitation; Roughness; Satellite imagery; Satellite observation; Sciences of the Universe; Sensible heat; Sensible heat flux; Sensible heat transfer; Simulation; Storms; Summer; temperate forests; Vertical velocities; Windthrow
• ISSN: 2169-897X; 2169-8996
• DOI: 10.1029/2023JD038477

Evidence has been provided that land‐cover changes such as deforestation can have an impact on cloudiness and precipitation. However, conflicting results have been obtained at different scales and places, highlighting our poor understanding of the physical processes involved. Here we focus on mesoscale summer cloudiness in a temperate region, as influenced by a large forest massif (the Landes forest in France). Our study is based on an up‐to‐date atmosphere‐surface mesoscale model (Meso‐NH coupled with SURFEX). Based on observational data, we first optimize the model configuration for our purpose, and show that with a 500 m horizontal resolution we can successfully simulate the higher summer cloud cover observed over the forest, compared to its surroundings. Second, we investigate the physical processes leading to cloud formation in a representative case study. Based on a comparative analysis of diagnostics and budgets over forest and non‐forest areas, we find that the larger sensible heat flux over the forest and its higher roughness are the main drivers of cloudiness, enhancing vertical velocity and boundary‐layer mixing. Third, we simulate the impact of the 2009 Klaus storm that led to the loss of about one third of the trees. Considering 15 representative convective summer days, we show that the model simulates well the resulting decrease in summer cloudiness that was reported in a previous study based on satellite observations. As a complementary tool, the mesoscale simulations allow to quantify the impacts of the Klaus storm windthrow on the diurnal cycle of the boundary layer. Plain Language Summary Previous studies have shown that land‐cover changes such as deforestation can have an impact on cloud cover and precipitation. But the results can be contradictory depending on the region of the world studied. Here, we focus on the temperate Landes forest in southwest France, during the summer period with boundary‐layer clouds. The surface and the atmosphere are represented by two coupled models operating at a 500 m horizontal resolution. We show that, when properly configured, the system can represent boundary‐layer clouds forming over the forest. We then investigate the link between the forest and the generated clouds, highlighting the prominent role of the larger sensible heat flux and roughness over forested areas. These key factors generate vertical motions and mixing that enable cloud formation. Finally, the damage caused by Klaus storm, which dest