Enhanced Scalar Concentrations and Fluxes in the Lee of Forest Patches: A Large-Eddy Simulation Study

摘要

By means of large-eddy simulation, we investigate the transport of a passive scalar in the lee of forest patches under neutral atmospheric conditions in flat terrain. We found a pronounced local enhancement of scalar concentration and scalar flux in the lee zone of the forest, while further downstream above the unforested surface, the scalar transport adjusted to an equilibrium with the underlying surface conditions. By means of a term-by-term analysis of the scalar transport equation, we determined the local accumulation of the scalar to be caused by the convergence of: (1) mean and turbulent streamwise transport, (2) mean vertical transport. However, the relative importance of each transport mechanism for the accumulation process was found to depend strongly on forest density. Based on systematic parameter changes, we found concentrations to significantly increase with increasing forest density and with decreasing wind speed, while fluxes were invariant to wind speed and showed a similar relation to forest density as for the concentrations. Despite the scalar sources-ground and/or canopy sources-a local flux enhancement was present in the lee zone. Finally, we provide a first step towards localizing enhanced concentrations and fluxes at micrometeorological sites.