Abstract
The aim of this large-eddy simulation study is to improve the yet scarce understanding of the scalar (e.g. \(\hbox {CO}_{2}\)) transport in forest-edge flows. In order to contribute to a basic knowledge on the scalar transport, we focus on idealized neutral flows across a clearing-to-forest transition, with a passive scalar released from a horizontally homogeneous source at the clearing and at the forest floor. Corresponding to previous studies, we found pronounced peaks in scalar concentration and flux downstream of the forest edge, where the flux peak significantly exceeded the given surface flux. We investigated for the first time those transport mechanisms that steer the scalar accumulation, by analyzing the terms in the scalar transport equation. The analysis reveals that the accumulation is accomplished by the streamwise convergence of the mean and turbulent transport. In order to investigate how the concentration and flux enhancement behaves under different conditions, we performed a series of simulations with varying forest density and wind speed. We demonstrate a strong dependence of the peak location and magnitude on forest density. With increasing density, concentration and flux peaks were found closer to the forest edge and peak values increased significantly. Decreasing the wind speed caused an increase of the concentration peak, while the flux peak remained unaffected. For an adequate interpretation and design of micrometeorological measurements near forest edges, it is necessary to identify the regions (fetches) with enhanced concentrations and fluxes. We therefore analyzed different approaches for a proper fetch estimation.
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The code can be accessed under http://palm.muk.uni-hannover.de/browser?rev=874.
The corresponding animation is available under http://dx.doi.org/10.5446/14297. It was created with VAPOR, a product of the Computational Information Systems Laboratory at the National Center for Atmospheric Research, www.vapor.ucar.edu.
The related VAPOR animation is available at http://dx.doi.org/10.5446/14311.
The NCAR Command Language (Version 6.1.2) [Software]. (2013). Boulder, Colorado: UCAR/NCAR/CISL/VETS. http://dx.doi.org/10.5065/D6WD3XH5.
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Acknowledgments
This study was supported by the German Research Foundation (DFG) under grant RA 617/23-1. All simulations were performed on the SGI Altix ICE at The North-German Supercomputing Alliance (HLRN) in Hannover and Berlin. NCLFootnote 4 and VAPOR have been used for data analysis and visualization. We thank the two anonymous reviewers for their detailed comments that helped to improve this manuscript.
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Kanani-Sühring, F., Raasch, S. Spatial Variability of Scalar Concentrations and Fluxes Downstream of a Clearing-to-Forest Transition: A Large-Eddy Simulation Study. Boundary-Layer Meteorol 155, 1–27 (2015). https://doi.org/10.1007/s10546-014-9986-3
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DOI: https://doi.org/10.1007/s10546-014-9986-3