Gaëtan Noual will defend his PhD thesis titled “Land use and regional climate: the impact of forested areas on cloud enhancement in south-west France“, next Tuesday 14th of March at 2pm. The defence is open to everyone and will take place in the conference room of the CNRM campus in Toulouse, and also via visio-conference on the following link: More details about it below.


Land cover changes such as deforestation are known to have an impact on cloud cover and precipitation. However, contradictory results have been obtained depending on latitude and scale, highlighting our poor 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 a mesoscale atmosphere-surface model (Meso-NH coupled to SURFEX). Based on observed data, the model configuration is first optimised. A horizontal resolution of 500 m allows to successfully simulate the higher summer cloud cover observed over the forest, compared to its surroundings. The physical processes leading to cloud formation are then studied 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 environment modifies the local circulations by significantly reinforcing the sea breeze and by 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 of the Landes forest, is considered. Over fifteen representative convective summer days, the model succeeds to simulate the decrease in summer cloudiness reported in a previous study based on satellite
observations. As a complementary tool, the mesoscale simulations thus make it possible to quantify the impacts of Klaus on the whole diurnal cycle of the boundary layer. Finally, a sensitivity study to an idealized evolution of the forest cover shows a non-linear correlation of the cloud cover with the forest surface fraction.

Keywords: boundary-layer clouds, forest, surface-atmosphere interactions, mesoscale modelling

Jury: Mme Frédérique CHERUY, Chargée de recherche CNRS; M. Joan CUXART, Professeur Université des Baléares; M. Lionel JARLAN, Directeur de recherche IRD; M. Adriaan TEULING, Professeur associé Wageningen University; Mme Nathalie DE NOBLET, Directrice de recherche CEA; M. Yves BRUNET, Directeur de recherche INRAE; Mme Christine LAC, Ingénieur CNRM, Co-directrice des Ponts, des Eaux et des Forêts; M. Patrick LE MOIGNE, Ingénieur de la Météorologie CNRM.

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