Large-Eddy Simulation of Convective Boundary-Layer Mixing
of Decaying Scalars Emitted by a Forest Canopy
Edward G. Patton1, Kenneth J. Davis1, Mary C. Barth2, Chin-Hoh Moeng2
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In an attempt to further our understanding of turbulent mixing on atmospheric chemistry occurring above and within a plant canopy, we present an investigation of the transport of decaying scalars released from a forest canopy mixing through the Convective Boundary Layer (CBL). Of particular interest are biogenic hydrocarbons, such as isoprene, which play important roles in photochemistry, but whose atmospheric lifetimes are short. Observations have indicated that these species are not consistently well mixed within the CBL. Typically, the atmospheric layers within forest canopies are stably stratified and thus biogenic compounds can be retained within the canopy. Intermittent passage of canopy scale coherent structures may be a major source of the variability in the CBL profiles of these compounds. CBL photochemical models typically assume the CBL is well mixed for simplicity, while at the same time, canopy-meteorologists typically study conserved scalars. We make an attempt to investigate the relationship between turbulent mixing and chemical loss.
In order to resolve the effects of the plant canopy, we use the newly developed grid-nesting abilities in the LES (Sullivan et al., 1996). Following Patton (1997), the canopy is implemented as a horizontally homogeneous distribution of drag elements with a vertical variation similar to a deciduous forest. The canopy only exists in the fine mesh. Since the fine mesh answers are filtered to the coarse mesh resolution, the coarse mesh is fully aware of the presence of the canopy. The scalars are introduced to be similar to Moeng and Wyngaard (1984), with the vertically integrated source from the plants equaling the sources prescribed in Moeng and Wyngaard (1984). We have not allowed for any uptake of the species by the plants or the soil surface. To provide a tool for the investigation of the relationship between turbulent mixing and chemical loss, we impose a simple exponential decay for a number of the scalars.
It is thought that the combination of horizontal heterogeneity in canopy emissions (largely due to spatial distributions of emitting species) and sub-crown storage is responsible for the unmixed profiles seen in observations well above vegetation. To investigate these two possible incluences, two runs have been performed, one with horizontally homogeneous scalar emissions and a second with a dynamically identical canopy but with patchy scalar sources. The effects of these two mechanisms will be illustrated by comparisons of mean profiles, statistical moments and spectra. In addition, an investigation of the relationship between simple decay chemistry, sub-canopy storage, and intermittently occurring turbulent structures will be presented