ED Sciences et environnements
Impacts of spatial landscape heterogeneities on microclimate.
by Myrtille GRULOIS (ISPA - Interaction Sol-Plante-Atmosphère)
The defense will take place at 14h00 - Salle de conférence ISPA Bâtiment C1 INRAE 71 avenue Edouard Bourlaux 33140 Villenave d'Ornon
in front of the jury composed of
- Sylvain DUPONT - Directeur de recherche - Université de Bordeaux - Directeur de these
- Fleur COUVREUX - Ingénieure en Chef des Ponts des Eaux et des Forêts - Météo-France - Rapporteur
- Anne VERHOEF - Professeure des universités - University of Reading - Rapporteur
- Laurent PERRET - Professeur - École Centrale de Nantes - Rapporteur
- Jérôme OGéE - Directeur de recherche - Université de Bordeaux - CoDirecteur de these
- Fabienne LOHOU - Professeure des universités - Université de Toulouse III - Examinateur
Heterogeneities in mixed agricultural and forestry landscapes generate strong spatial variations in microclimate (wind, temperature, humidity), with areas where summer extremes such as heatwaves and droughts can be mitigated, protecting plants and enhancing biodiversity. With such events becoming more frequent and intense, it becomes urgent to better understand the role played by these landscape heterogeneities on the microclimate in order to improve climate models and accompany agroecology and conservation policies. To evaluate management scenarios and identify the role of each landscape heterogeneity independently of the others, it is necessary to combine micrometeorological measurements with numerical simulations at scales that incorporate the landscape heterogeneity of interest and its impact on the atmospheric boundary layer above it. In the context of summer extremes, this implies domain sizes of several kilometres in width and height, with a spatial resolution of momentum, heat and moisture sources and sinks of a few meters to be able to resolve the microclimate fields around the landscape heterogeneity. During this PhD, two types of landscape heterogeneity were studied: a crop-forest transition (edge) and a small canyon (or gully) within a forested landscape. Using the model ARPS-MuSICA that couples (1) a Large-Eddy Simulation meteorological model adapted to fine spatial scales and explicitly simulating turbulent structures and (2) a multi-layer model estimating mass (water, CO2 ) and energy fluxes in forest and agricultural canopies, we simulated the microclimate and turbulent structures around the two types of landscape heterogeneities, under neutral to unstable atmospheric conditions for a typical summer day. A field experiment was conducted at an edge site to compare the simulated edge effect with in situ observations. The simulations clarified for the first time the impact of thermal stability on the micrometeorology of edge flows, with consequences for the microclimate of the forest understorey and the micrometeorology of the atmospheric boundary layer. Field measurements confirmed the edge-transition effects on the microclimate of the understorey, as well as certain simulated characteristics of the edge flow. Simulations around the forest gully allowed to identify a circulation pattern induced by the gully, that caused a drop in understorey temperature from the plateau to the bottom of the gully. The size and shape of this circulation pattern, and the amplitude of the temperature drop, were strongly sensitive to the type of vegetation (oak vs pine, with or without regeneration understorey) inside and around the gully, as well as to the wind intensity. Overall, this modeling approach offers a way to improve the representation of land-atmosphere exchanges over heterogeneous surfaces in climate models. It helps to understand the respective roles played by different landscape heterogeneities (change in vegetation, micro-topography) on the microclimate variability, and enables the study of different land management scenarios in order to mitigate climatic extremes in an applied context of agroecology and conservation policies.