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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union

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Atmos. Meas. Tech., 10, 869-880, 2017
http://www.atmos-meas-tech.net/10/869/2017/
doi:10.5194/amt-10-869-2017
© Author(s) 2017. This work is distributed
under the Creative Commons Attribution 3.0 License.
Research article
09 Mar 2017
Flux calculation of short turbulent events – comparison of three methods
Carsten Schaller1,2,a, Mathias Göckede2, and Thomas Foken1,3 1University of Bayreuth, former Department of Micrometeorology, 95440 Bayreuth, Germany
2Max-Planck-Institute for Biogeochemistry, Dept. Biogeochemical Systems, 07745 Jena, Germany
3University of Bayreuth, Bayreuth Center of Ecology and Environmental Research (BayCEER), 95440 Bayreuth, Germany
anow at: University of Münster, Institute of Landscape Ecology, Climatology Group, Heisenbergstr. 2, 48149 Münster, Germany
Abstract. The eddy covariance method is commonly used to calculate vertical turbulent exchange fluxes between ecosystems and the atmosphere. Besides other assumptions, it requires steady-state flow conditions. If this requirement is not fulfilled over the averaging interval of, for example, 30 min, the fluxes might be miscalculated. Here two further calculation methods, conditional sampling and wavelet analysis, which do not need the steady-state assumption, were implemented and compared to eddy covariance. All fluxes were calculated for 30 min averaging periods, while the wavelet method – using both the Mexican hat and the Morlet wavelet – additionally allowed us to obtain a 1 min averaged flux.

The results of all three methods were compared against each other for times with best steady-state conditions and well-developed turbulence. An excellent agreement of the wavelet results to the eddy covariance reference was found, where the deviations to eddy covariance were of the order of  < 2 % for Morlet as well as  < 7 % for Mexican hat and thus within the typical error range of eddy covariance measurements. The conditional sampling flux also showed a very good agreement to the eddy covariance reference, but the occurrence of outliers and the necessary condition of a zero mean vertical wind velocity reduced its general reliability. Using the Mexican hat wavelet flux in a case study, it was possible to locate a nightly short time turbulent event exactly in time, while the Morlet wavelet gave a trustworthy flux over a longer period, e.g. 30 min, under consideration of this short-time event.

At a glance, the Mexican hat wavelet flux offers the possibility of a detailed analysis of non-stationary times, where the classical eddy covariance method fails. Additionally, the Morlet wavelet should be used to provide a trustworthy flux in those 30 min periods where the eddy covariance method provides low-quality data due to instationarities.


Citation: Schaller, C., Göckede, M., and Foken, T.: Flux calculation of short turbulent events – comparison of three methods, Atmos. Meas. Tech., 10, 869-880, doi:10.5194/amt-10-869-2017, 2017.
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Short summary
The eddy covariance (EC) method allows for measuring and calculating vertical turbulent exchange fluxes between ecosystems and the atmosphere. It fails in non-steady-state flow conditions, e.g. in Arctic regions. Two alternative calculation methods, conditional sampling and wavelet analysis, were implemented and compared to EC. Wavelet analysis for allows calculating a trustworthy flux even in non-stationary times and offers new possibilities for exact flux calculation in difficult environments.
The eddy covariance (EC) method allows for measuring and calculating vertical turbulent exchange...
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