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A widely used assumption in boundary layer meteorology is the z independence of turbulent scalar fluxes Fs throughout the atmospheric surface layer, where z is the distance from the boundary. This assumption is necessary for the usage of Monin-Obukhov Similarity Theory and for the interpretation of eddy covariance measurements of Fs when using them to represent emissions or uptake from the surface. It is demonstrated here that the constant flux assumption offers intrinsic constraints on the third-order turbulent transport of Fs in the unstable atmospheric surface layer. When enforcing z independence of Fs on multilevel Fs measurements collected above different surface cover types, it is shown that increasing instability leads to a novel and universal description of (i) the imbalance between ejecting and sweeping eddy contributions to Fs and (ii) the ratio formed by a dimensionless turbulent transport of Fs and a dimensionless turbulent transport of scalar variance. When combined with structural models for the turbulent transport of Fs, these two findings offer a new perspective on “closing” triple moments beyond conventional gradient diffusion schemes. A practical outcome is a diagnostic of the constant flux assumption from single-level Fs measurements.
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Effects of Gentle Topography on Forest‐Atmosphere Gas Exchanges and Implications for Eddy‐Covariance Measurements
Abstract The interpretation of tower‐based eddy‐covariance (EC) turbulent flux measurements above forests hinges on three key assumptions: (1) steadiness in the flow statistics, (2) planar homogeneity of scalar sources or sinks, and (3) planar homogeneity in the flow statistics. Large eddy simulations (LESs) were used to control the first two so as to explore the break‐down of the third for idealized and real gentle topography such as those encountered in Amazonia. The LES runs were conducted using uniformly distributed sources inside homogeneous forests covering complex terrain to link the spatial patterns of scalar turbulent fluxes to topographic features. Results showed strong modulation of the fluxes by flow features induced by topography, including large area with negative fluxes compensating “chimney” regions with fluxes almost an order of magnitude larger than the landscape flux. Significant spatial heterogeneity persisted up to at least two canopy heights, where most eddy‐covariance measurements are performed above tall forests. A heterogeneity index was introduced to characterize and contrast different scenarios, and a topography categorization was shown to have predictive capabilities in identifying regions of negative and enhanced fluxes.
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- Award ID(s):
- 1644375
- PAR ID:
- 10453964
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Atmospheres
- Volume:
- 125
- Issue:
- 11
- ISSN:
- 2169-897X
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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