Effects of Topography on Residence Time and Export Fraction of Gases Emitted within Forests
Abstract
The exchange of reactive and non-reactive gases across the top of a vegetated canopy is of significance to a plethora of applications in air quality, climate sciences, ecology, and hydrology. Particularly, this exchange problem requires the determination of the fraction of air parcels that make contact with the canopy sources and sinks then escape into the overlying planetary boundary layer (PBL). This "export fraction" results from a competition between turbulent transport and in-canopy sinks (e.g. deposition and chemical/biological loss). The few studies that address this question are based on idealized conditions, including the assumption of flat topography. It has been known for quite some time now that moderate topographical features can introduce substantial changes in the mean flow and turbulence structures within and above the canopy, thereby altering residence time and export fraction. The inquiry into the effects of topography on residence time and export fraction is addressed here using a suite of idealized LES runs for a neutral PBL. The vegetation canopy is represented using a standard drag model and idealized sinusoidal topographic features are modeled using the immersed boundary method (IBM). Air parcels are tracked using an offline Lagrangian approach driven by the LES derived flow field. A series of simulations in which the amplitudes of the topographical features are systematically increased relative to the canopy height are then used to quantify the modulations in the turbulence and their impacts on residence time and export fraction. By increasing the topography height, the local mean pressure gradient is increased in magnitude and finally leads to changes of the diffusive properties of turbulence as well as the canonical relaxation time scales of eddies that carry air parcels coherently. The results show that even small topographic features can have a significant influence on residence time and export fraction of reactive gases. The topography also leads to a nonuniform transport across the canopy top, even for a uniform source distribution. These results are also relevant in interpreting and scaling-up tower measurements of reactive and non-reactive gas fluxes.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.A53B..07C
- Keywords:
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- 0315 Biosphere/atmosphere interactions;
- ATMOSPHERIC COMPOSITION AND STRUCTUREDE: 3307 Boundary layer processes;
- ATMOSPHERIC PROCESSESDE: 3322 Land/atmosphere interactions;
- ATMOSPHERIC PROCESSESDE: 3379 Turbulence;
- ATMOSPHERIC PROCESSES