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

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 destroyed a third of the Landes forest in 2009, is considered. Fifteen summer days with clouds over the Landes are simulated and show that the damage leads to a decrease in cloudiness, in agreement with the results of a previous study based on satellite image analysis. At the regional scale, recent models can thus be used to study and quantify the impact of deforestation on cloudiness. Key Points: A mesoscale model successfully simulates cloud enhancement by a temperate forest and provides realistic cloud fields at hectometric resolutionA comparison of the physical processes over forested and non‐forested areas shows that sensible heat flux and surface roughness are the key drivers of cloudinessThe model helps to understand and quantify the substantial decrease in forest‐induced cloudiness that was observed after storm Klaus [ABSTRACT FROM AUTHOR]