Temporal Dynamics of Aerodynamic Canopy Height Derived From Eddy Covariance Momentum Flux Data Across North American Flux Networks

Chu, Housen; Baldocchi, Dennis D.; Poindexter, Cristina; Abraha, Michael; Desai, Ankur R.; Bohrer, Gil; Arain, M. Altaf; Griffis, Timothy; Blanken, Peter D.; O'Halloran, Thomas L.; Thomas, R. Quinn; Zhang, Quan; Burns, Sean P.; Frank, John M.; Christian, Dold; Brown, Shannon; Black, T. Andrew; Gough, Christopher M.; Law, Beverly E.; Lee, Xuhui; Chen, Jiquan; Reed, David E.; Massman, William J.; Clark, Kenneth; Hatfield, Jerry; Prueger, John; Bracho, Rosvel; Baker, John M.; Martin, Timothy A.

doi:10.1029/2018GL079306

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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