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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union
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Volume 9, issue 10 | Copyright
Atmos. Meas. Tech., 9, 5163-5181, 2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 21 Oct 2016

Research article | 21 Oct 2016

Random uncertainties of flux measurements by the eddy covariance technique

Üllar Rannik, Olli Peltola, and Ivan Mammarella Üllar Rannik et al.
  • Department of Physics, P.O. Box 48, University of Helsinki, 00014 Helsinki, Finland

Abstract. Large variability is inherent to turbulent flux observations. We review different methods used to estimate the flux random errors. Flux errors are calculated using measured turbulent and simulated artificial records. We recommend two flux errors with clear physical meaning: the flux error of the covariance, defining the error of the measured flux as 1 standard deviation of the random uncertainty of turbulent flux observed over an averaging period of typically 30min to 1h duration; and the error of the flux due to the instrumental noise. We suggest that the numerical approximation by Finkelstein and Sims (2001) is a robust and accurate method for calculation of the first error estimate. The method appeared insensitive to the integration period and the value 200s sufficient to obtain the estimate without significant bias for variety of sites and wide range of observation conditions. The filtering method proposed by Salesky et al. (2012) is an alternative to the method by Finkelstein and Sims (2001) producing consistent, but somewhat lower, estimates. The method proposed by Wienhold et al. (1995) provides a good approximation to the total flux random uncertainty provided that independent cross-covariance values far from the maximum are used in estimation as suggested in this study. For the error due to instrumental noise the method by Lenschow et al. (2000) is useful in evaluation of the respective uncertainty. The method was found to be reliable for signal-to-noise ratio, defined by the ratio of the standard deviation of the signal to that of the noise in this study, less than three. Finally, the random uncertainty of the error estimates was determined to be in the order of 10 to 30% for the total flux error, depending on the conditions and method of estimation.

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Short summary
We review available methods for the random error estimation of turbulent fluxes that are widely used by the flux community. Flux errors are evaluated theoretically as well as via numerical calculations by using measured and simulated records. We recommend two flux random errors with clear physical meaning: the total error resulting from stochastic nature of turbulence, well approximated by the method of Finkelstein and Sims (2001), and the error of the flux due to the instrumental noise.
We review available methods for the random error estimation of turbulent fluxes that are widely...