Preprints
https://doi.org/10.5194/amt-2020-478
https://doi.org/10.5194/amt-2020-478

  23 Dec 2020

23 Dec 2020

Review status: a revised version of this preprint is currently under review for the journal AMT.

The high frequency response correction of eddy covariance fluxes. Part 1: an experimental approach for analysing noisy measurements of small fluxes

Toprak Aslan1, Olli Peltola2, Andreas Ibrom3, Eiko Nemitz4, Üllar Rannik1, and Ivan Mammarella1 Toprak Aslan et al.
  • 1Institute for Atmospheric and Earth System Research (INAR)/Physics, Faculty of Science, University of Helsinki, P.O. Box 68, 00014 Helsinki, Finland
  • 2Climate Research Programme, Finnish Meteorological Institute, P.O. Box 503, 00101 Helsinki, Finland
  • 3Dept. Environmental Engineering, Technical University of Denmark (DTU), Lyngby, Denmark
  • 4UK Centre for Ecology and Hydrology (UKCEH), Edinburgh Research Station, Penicuik, Bush Estate, EH26 0QB, UK

Abstract. Fluxes measured with the eddy covariance (EC) technique are subject to flux losses at high frequencies (low-pass filtering). If not properly corrected for, these result in systematically biased ecosystem-atmosphere gas exchange estimates. This loss is corrected using the system's transfer function which can be estimated with either theoretical and experimental approaches. In the experimental approach, commonly used for closed-path EC systems, the low-pass filter transfer function (H) can be derived from the comparison of either (i) the measured power spectra of sonic temperature and the target gas mixing ratio or (ii) the cospectra of both entities with vertical wind speed. In this study, we compare the power spectral approach (PSA) and cospectral approach (CSA) in the calculation of H for a range of attenuation levels and signal-to-noise ratios (SNRs). For a systematic analysis, we artificially generate a representative dataset from sonic temperature (T) by attenuating it with a first order filter and contaminating it with white noise, resulting in various combinations of time constants and SNRs. For PSA, we use two methods to account for the noise in the spectra: the first is the one introduced by Ibrom et al. (2007a) (PSAI07), where the noise and H are fitted in different frequency ranges and the noise is removed before estimating H. The second is a novel approach that uses the full power spectrum to fit both H and noise simultaneously (PSAA20). For CSA, we use three different methods: (1) a plain version of Lorentzian equation describing the H (CSAH), (2) a square-root of the H (CSAH), and (3) a square-root of the H with shifted vertical wind velocity time series via cross-covariance maximisation (CSAH,sync). PSAI07 tends to overestimate the time constant when low-pass filtering is low, whilst the new PSAA20 successfully estimates the expected time constant regardless of the degree of attenuation and SNR. CSAH underestimates the time constant with decreasing accuracy as attenuation increases due to the omission of the quadrature spectrum. CSAH overestimates, but its accuracy increases with time-lag correction in the CSAH,sync. We further examine the effect of the time constant obtained with the different implementations of PSA and CSA on cumulative fluxes using estimated time constants in frequency response correction. For our example time series, the fluxes corrected using time constants derived by PSAI07 show a bias of ±2 %. PSAA20 showed a similar variation, yet slightly better accuracy. CSAH underestimated fluxes by up to 4 %, while CSAH overestimated them by up to 3 %, a bias which was mostly eliminated with time-lag correction in the CSAH,sync (−2 % to 1 % ). The accuracies of both PSA and CSA methods were not affected by SNR level, instilling confidence in EC flux measurements and data processing in setups with low SNR. Overall we show that, when using power spectra for the empirical estimation of parameters of H for closed-path EC systems the new PSAA20 outperforms PSAI07, while when using cospectra the CSAH,sync approach provides the most accurate results. These findings are independent of the SNR value and attenuation level.

Toprak Aslan et al.

 
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Status: final response (author comments only)
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Toprak Aslan et al.

Toprak Aslan et al.

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Short summary
Vertical turbulent fluxes of gases measured by the eddy covariance (EC) technique are subject to high frequency losses. There are different methods used to describe this low-pass filtering effect and to correct the measured fluxes. In this study, we analyzed the systematic uncertainty related to this correction for various attenuation and signal-to-noise ratios. A new and robust transfer function method is finally proposed.