Forest edge flows and fluxes under unstable atmospheric conditions.

Microclimate within the rural landscape is difficult to predict due to its strong spatial heterogeneity resulting from the juxtaposition landscape elements contrasted in terms of biophysical parameters. Canopy edge is one of the dominant heterogeneities in rural areas, whose influences on microclimate and turbulent structures have mainly been described under neutral thermal stratification.

This study focuses on the micrometeorology over and within a crop-forest transition under unstable atmospheric conditions, when shear and buoyancy have a combined action on turbulence motions within the atmospheric boundary layer (ABL). This study is based on both field and numerical experiments. The field experiment was carried out in Lannemezan (South of France) from March 2023 to March 2024 as part of the MOSAI project (Model and  Observation for Surface-Atmosphere Interactions, https://mosai.aeris-data.fr/). High-frequency sensors were used to measure wind speed and direction, air temperature and humidity, and CO 2 mixing ratio at varying distances from the forest edge. The numerical experiment was performed using a Large Eddy Simulation (LES) atmospheric model, coupled to a one-dimensional multi-layer soil and canopy energy and gas exchanges model. The novelty of this simulation is to resolve both within- and above-canopy turbulence, allowing the characterization of turbulent exchanges during free convection.

We will show how the forest edge flow differs depending on the thermal stability, and the influence of canopy- and ABL-scale motions on the canopy-atmosphere turbulent exchanges. This study is a first step towards simulating micrometeorology over heterogeneous landscapes in unstable conditions. Such detailed simulations should contribute to a better understanding of surface-atmosphere exchanges, and in particular to a better account of surface heterogeneity in meteorological models.