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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union
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Short summary
Gas fluxes measured by the eddy covariance (EC) technique are subject to filtering due to unideal instrumentation. For linear first-order systems this filtering causes also a time-lag between vertical wind speed and gas signal which is additional to the gas travel time in the sampling line. The effect of this additional time-lag on EC fluxes is ignored in current EC data processing routines. Here we show that this oversight biases EC fluxes and hence propose an approach to rectify this bias.
Preprints
https://doi.org/10.5194/amt-2020-479
https://doi.org/10.5194/amt-2020-479

  11 Jan 2021

11 Jan 2021

Review status: this preprint is currently under review for the journal AMT.

The high frequency response correction of eddy covariance fluxes. Part 2: the empirical approach and its interdependence with the time-lag estimation

Olli Peltola1, Toprak Aslan2, Andreas Ibrom3, Eiko Nemitz4, Üllar Rannik2, and Ivan Mammarella2 Olli Peltola et al.
  • 1Climate Research Programme, Finnish Meteorological Institute, P.O. Box 503, 00101 Helsinki, Finland
  • 2Institute for Atmospheric and Earth System Research (INAR)/Physics, Faculty of Science, University of Helsinki, P.O. Box 68, 00014 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. The eddy covariance (EC) technique has emerged as the prevailing method to observe ecosystem - atmosphere exchange of gases, heat and momentum. EC measurements require rigorous data processing to derive the fluxes that can be used to analyse exchange processes at the ecosystem - atmosphere interface. Here we show that two common post-processing steps (time-lag estimation via cross-covariance maximisation, and correction for limited frequency response of the EC measurement system) are interrelated and this should be accounted for when processing EC gas flux data. These findings are applicable to EC systems employing closed- or enclosed-path gas analysers which can be approximated to be linear first-order sensors. These EC measurement systems act as a low-pass filters on the time-series of the scalar χ (e.g. CO2, H2O) and this induces a time-lag (tlpf) between vertical wind speed (w) and scalar χ time series which is additional to the travel time of the gas signal in the sampling line (tube, filters). Time-lag estimation via cross-covariance maximisation inadvertently accounts also for tlpf and hence overestimates the travel time in the sampling line. This results in a phase shift between the time-series of w and χ, which distorts the measured cospectra between w and χ and hence has an effect on the correction for dampening of EC flux signal at high frequencies. This distortion can be described with a transfer function related to the phase shift (Hp) which is typically neglected when processing EC flux data. Based on analyses using EC data from two contrasting measurement sites, we show that the low-pass filtering induced time-lag increases approximately linearly with the time constant of the low-pass filter, and hence the importance of Hp in describing the high frequency flux loss increases as well. Incomplete description of these processes in EC data processing algorithms results in flux biases of up to 10 %, with the largest biases observed for short towers due to prevalence of small scale turbulence. Based on these findings, it is suggested that spectral correction methods implemented in EC data processing algorithms are revised to account for the influence of low-pass filtering induced time-lag.

Olli Peltola et al.

Status: open (until 08 Mar 2021)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on amt-2020-479', Marc Aubinet, 15 Jan 2021 reply

Olli Peltola et al.

Olli Peltola et al.

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Short summary
Gas fluxes measured by the eddy covariance (EC) technique are subject to filtering due to unideal instrumentation. For linear first-order systems this filtering causes also a time-lag between vertical wind speed and gas signal which is additional to the gas travel time in the sampling line. The effect of this additional time-lag on EC fluxes is ignored in current EC data processing routines. Here we show that this oversight biases EC fluxes and hence propose an approach to rectify this bias.
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