Abstract
Horizontal heterogeneity in either the source-sink distribution or the wind field results in streamwise advection of momentum and scalars, which must be accounted for whenever we deduce surface exchange from micrometeorological measurements on flux towers. This Chapter focuses on the second of these causes, addressing scalar advection in topography covered with uniformforest canopies rather than that generated by heterogeneity in the land cover. After defining advection and its relationship with modeling we discuss flow over forested hills by looking first at the wind field, next at the transfer of a generic scalar and finally at the implications for measuring photosynthesis on a two-dimensional ridge. Using analytic approaches as far as is possible, we show that both the turbulent wind field and scalar flow and transport in the canopy on a hill have a two-layer asymptotic structure with an upper canopy layer, coupled by turbulent transfer to the surface-layer flow above, and a lower canopy layer, that is driven by the pressure gradient produced as the wind field is deflected over the hill. The dynamics of these two layers are quite different and their matching through the upper canopy leads to strong modulation of turbulent transport over the hill and substantial advective flux divergence, even on gentle hills. The effect of the hill-induced perturbations on photosynthesis is calculated numerically and is shown to be small, being of order of the hill slope. In contrast, their e ect on the net ecosystem exchange that would be deduced from eddy-flux measurements on a single flux tower is large, being of order one.
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Finnigan, J. (2004). Advection and Modeling. In: Lee, X., Massman, W., Law, B. (eds) Handbook of Micrometeorology. Atmospheric and Oceanographic Sciences Library, vol 29. Springer, Dordrecht. https://doi.org/10.1007/1-4020-2265-4_10
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DOI: https://doi.org/10.1007/1-4020-2265-4_10
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