The high-frequency response correction of eddy covariance fluxes - Part 1 : An experimental approach and its interdependence with the time-lag estimation

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dc.contributor.author Peltola, Olli
dc.contributor.author Aslan, Toprak
dc.contributor.author Ibrom, Andreas
dc.contributor.author Nemitz, Eiko
dc.contributor.author Rannik, Ullar
dc.contributor.author Mammarella, Ivan
dc.date.accessioned 2021-08-31T13:26:01Z
dc.date.available 2021-08-31T13:26:01Z
dc.date.issued 2021-07-28
dc.identifier.citation Peltola , O , Aslan , T , Ibrom , A , Nemitz , E , Rannik , U & Mammarella , I 2021 , ' The high-frequency response correction of eddy covariance fluxes - Part 1 : An experimental approach and its interdependence with the time-lag estimation ' , Atmospheric Measurement Techniques , vol. 14 , no. 7 , pp. 5071-5088 . https://doi.org/10.5194/amt-14-5071-2021
dc.identifier.other PURE: 168024044
dc.identifier.other PURE UUID: 68817d36-9da4-4f10-99c7-6a3ca3a2a1d4
dc.identifier.other WOS: 000678717400001
dc.identifier.other Scopus: 85111782497
dc.identifier.other ORCID: /0000-0002-8516-3356/work/99269579
dc.identifier.other ORCID: /0000-0002-1744-6290/work/99271061
dc.identifier.uri http://hdl.handle.net/10138/333827
dc.description.abstract The eddy covariance (EC) technique has emerged as the prevailing method to observe the 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 low-pass filters on the time series of the scalar chi (e.g. CO2, H2O), and this induces a time lag (t(l)(pf)) between vertical wind speed (omega) and scalar chi 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 t(l)(pf) and hence overestimates the travel time in the sampling line. This results in a phase shift between the time series of omega and chi, which distorts the measured cospectra between omega and chi and hence has an effect on the correction for the dampening of the EC flux signal at high frequencies. This distortion can be described with a transfer function related to the phase shift (H-p) 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 H-p 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 the 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. en
dc.format.extent 18
dc.language.iso eng
dc.relation.ispartof Atmospheric Measurement Techniques
dc.rights cc_by
dc.rights.uri info:eu-repo/semantics/openAccess
dc.subject 114 Physical sciences
dc.title The high-frequency response correction of eddy covariance fluxes - Part 1 : An experimental approach and its interdependence with the time-lag estimation en
dc.type Article
dc.contributor.organization Institute for Atmospheric and Earth System Research (INAR)
dc.contributor.organization Micrometeorology and biogeochemical cycles
dc.description.reviewstatus Peer reviewed
dc.relation.doi https://doi.org/10.5194/amt-14-5071-2021
dc.relation.issn 1867-1381
dc.rights.accesslevel openAccess
dc.type.version publishedVersion

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