Physical Modeling of Biogenic Emissions from a Finite Forest Area

Within the scope of the German Atmospheric Research Program AFO2000, an extensive field campaign on biogenic emissions from deciduous forests was carried out. Based on field measurements and systematic wind tunnel experiments, the exchange of biogenic emissions and tracer substances between the forest and the atmosphere was studied in the collaborative project 'ECHO'. The wind tunnel experiments were performed in the Environmental Wind Tunnel Laboratory of the Meteorological Institute at Hamburg University.

In a first step, a 1:300 scaled model of the field site 'Stetternicher Forst' was built. An improved method for physical modeling of deciduous forest was developed based on an extensive set of preliminary tests for defining a proper porosity of the model forest. As a reference, representative wind measurements from within the forest at the field site were used. For boundary layer flow modeling, a specific arrangement of turbulence generators and floor roughness upstream of the forest model was utilized. Finally, a good agreement of mean wind profiles and turbulence characteristics in and above the forest could be achieved.

After a careful validation of the wind tunnel model based on a reference field experiment (SF6 release from a point source) a series of specific tests was carried out in the wind tunnel in support of the field campaigns. For example, footprint experiments have been performed in order to specify the origin of tracers/samples taken at different field measurement points. Another task was to specify the traveling time of air masses from different emission points to various sampling points since biogenic compounds may have been altered due to the fast chemical reactions taking place. A third major topic covered by extensive wind tunnel experiments was the turbulent mass transfer from the treetops into the lower atmosphere. By means of simultaneous measurements of turbulent wind- and tracer-fluctuations, the turbulent mass fluxes could be specified. Measuring turbulent fluxes in a dense grid of measurement points above the forest also enabled the horizontal and vertical inhomogeneity of fluxes to be visualized and quantified. Transient transport phenomena like the 'sweeps' and 'ejections' observed during field measurements could be reproduced in the wind tunnel model at the similar time scales, supporting the confidence in a proper physical modeling of the complex flow and dispersion phenomena in and above a forest area.

The talk will outline the physical modeling concepts applied. Illustrated examples of the results will be presented and the usefulness of combined field/windtunnel-experiments will be discussed.