Abstract
We applied three approaches to estimate the zero-plane displacement \(d\) through the aerodynamic measurement height \(z\) (with \(z = z_{m}- d\) and \(z_{m}\) being the measurement height above the surface), and the aerodynamic roughness length \(z_{0}\), from single-level eddy covariance data. Two approaches (one iterative and one regression-based) were based on the universal function in the logarithmic wind profile and yielded an inherently simultaneous estimation of both \(d\) and \(z_{0}\). The third approach was based on flux–variance similarity, where estimation of \(d\) and consecutive estimation of \(z_{0}\) are independent steps. Each approach was further divided into two methods differing either with respect to the solution technique (profile approaches) or with respect to the variable (variance of vertical wind and temperature, respectively). All methods were applied to measurements above a large, growing wheat field where a uniform canopy height and its frequent monitoring provided plausibility limits for the resulting estimates of time-variant \(d\) and \(z_{0}\). After applying, for each approach, a specific data filtering that accounted for the range of conditions (e.g. stability) for which it is valid, five of the six methods were able to describe the temporal changes of roughness parameters associated with crop growth and harvest, and four of them agreed on \(d\) to within 0.3 m most of the time. Application of the same methods to measurements with a more heterogeneous footprint consisting of fully-grown sugarbeet and a varying contribution of adjacent harvested fields exhibited a plausible dependence of the roughness parameters on the sugarbeet fraction. It also revealed that the methods producing the largest outliers can differ between site conditions and stability. We therefore conclude that when determining \(d\) for canopies with unknown properties from single-level measurements, as is increasingly done, it is important to compare the results of a number of methods rather than rely on a single one. An ensemble average or median of the results, possibly after elimination of methods that produce outliers, can help to yield more robust estimates. The estimates of \(z_{0}\) were almost exclusively physically plausible, although \(d\) was considered unknown and estimated simultaneously with the methods and results described above.
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Acknowledgments
This study was enabled by the Deutsche Forschungsgemeinschaft (DFG) in the framework of the project “Links between local scale and catchment scale measurements and modelling of gas exchange processes over land surfaces” (GR2687/3-1 and SCHU 2350/2-1). Additional synergies were provided by the Helmholtz investment initiative TERENO (instrument availability) and the DFG Collaborative Transregional Research Centre 32 “Patterns in Soil–Vegetation–Atmosphere–Systems”, particularly subproject C3, which provided their canopy-height measurements for comparison and densification of our own observations. We would like to thank Martin Lennefer, University of Bonn, and Bernhard Pospichal, now University of Leipzig, for assistance in the set-up and regular maintenance of the stations, and Dirk Schüttemeyer, University of Bonn (now European Space Agency) for support in the set-up as well as helpful comments on the manuscript. We are indebted to the three anonymous reviewers for suggestions that helped to greatly improve the manuscript.
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Graf, A., van de Boer, A., Moene, A. et al. Intercomparison of Methods for the Simultaneous Estimation of Zero-Plane Displacement and Aerodynamic Roughness Length from Single-Level Eddy-Covariance Data. Boundary-Layer Meteorol 151, 373–387 (2014). https://doi.org/10.1007/s10546-013-9905-z
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DOI: https://doi.org/10.1007/s10546-013-9905-z