Impact of regional forest cover on boundary-layer clouds in southwest France.

Land cover changes such as deforestation are known to have an impact on cloudiness and precipitation. However, contradictory results have been obtained depending on the latitude and scale considered, highlighting our limited understanding of the physical processes involved.

This work focuses on summer cloudiness at mesoscale in a temperate region, under the influence of a large forest (the Landes forest in France). It is based on atmosphere-surface mesoscale modeling (Meso-NH coupled to SURFEX).

Based on observed data, the model configuration at 500 m horizontal resolution is first optimized. It successfully simulates the higher summer cloud cover observed over the forest, compared to its surroundings. Then, the physical processes leading to cloud formation are characterized on a representative case. A comparative analysis of diagnostics and budgets over forest and non-forest areas shows that sensible heat flux and roughness length, both higher over the forest, are the main drivers of cloud convection. The difference in heating between the forest and its surroundings modifies the local circulations by significantly strengthening the sea breeze, as well as generating a forest breeze. In a third step, the impact of the 2009 Klaus storm, responsible for the loss of about one third of the trees in the Landes forest, is considered. On fifteen representative convective summer days, the model reproduces the decrease in summer cloud cover reported in a previous study based on satellite observations. As a complementary analysis tool, the mesoscale simulations allow to quantify the impacts of Klaus on the whole diurnal cycle of the boundary layer.