Physical modelling of a complex forest area in a wind tunnel-comparison with field data

The impact of biogenic trace gases on tropospheric chemistry, air quality, and the formation of secondary products affecting our climate on a regional and global scale are far from being understood. A considerable lack of knowledge exists concerning a typical forest stand as a net source of reactive trace compounds into the troposphere. The concept of this study was to combine field experiments, laboratory experiments investigating emission and uptake of trace compounds by the plants, and modelling experiments simulating the chemistry of biogenic trace gases and the dynamics of a forest stand under well-defined conditions. The chosen site was the forest area surrounding the Forschungszentrum Juelich (Juelich Research Centre, Germany). In order to simulate the dynamical properties, the forest area was modelled to a scale of 1:300 and studied in the large boundary layer wind tunnel at the Meteorological Institute of Hamburg University. An area 3150 m long and 1200 m wide was replicated, including the upwind edges of the forest. The model of the forest must reproduce the resistance to the wind generated by this porous environment. Rings of metallic mesh were used to represent the trees following preliminary tests to find an arrangement of these rings that provided the appropriate aerodynamic characteristics for a forest. The turbulence properties of the flow were measured in the wind tunnel within and above the canopy. Subsequently, they were compared with field data obtained at the Juelich Research Centre, in order to test the quality of the modelling concept. The comparison showed a good agreement and results were consistent with previous studies. Tracer-gas experiments were carried out in the field within the canopy, which were then replicated in the wind tunnel. The order of magnitude of the dimensionless concentration downwind of the point source was in agreement.