Ibrom, Andreas

Online Research Database In Technology

                       General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.  • Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal   If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.     Downloaded from orbit.dtu.dk on: Dec 17, 2017Uncertainty of carbon dioxide flux estimates introduced by different high-pass filteringmethodsIbrom, AndreasPublished in:Geophysical Research AbstractsPublication date:2009Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Ibrom, A. (2009). Uncertainty of carbon dioxide flux estimates introduced by different high-pass filtering methods.Geophysical Research Abstracts, 11, EGU2009-3979-1.Geophysical Research Abstracts,Vol. 11, EGU2009-3979-1, 2009EGU General Assembly 2009© Author(s) 2009Uncertainty of carbon dioxide flux estimates introduced by differenthigh-pass filtering methodsA. Ibrom and the IMECC NA5 TeamRisø National Laboratory for Sustainable Energy, Technical University of Denmark (DTU), Roskilde, Denmark(anib@risoe.dtu.dk/+4546774160)Accuracy and precision of eddy covariance flux measurements depend among other factors on the method of high-pass filtering (HPF). Even if the time series are not filtered by linear detrending or recursive filtering, measurementsare high-pass filtered as a consequence of the limited sampling interval. This makes correction for HPF necessary.Although some recommendations exist, different methods are used by different groups and affect the flux estimatesin a different way.We investigated annual CO2 flux data sets from 8 different European flux sites comprising a wide range of surfaceproperties, climates and technical setups for the effects of linear detrending, recursive filtering and block averagingon systematic and random errors of the flux estimates. To correct for HPF effects, we used the spectral transferfunction approach based on a common model spectra parameterisation.Results showed comparably large absolute effects of HPF correction (Tab. 1), especially above rough surfaces (4to 15% of the annual C uptake). Above smooth surfaces, like the cropland the grassland and the wetland sites, HPFeffects were small. The higher the attenuation of the filter, i.e. linear detrending and recursive filtering with smalltime constants, the higher was the reduction of both the random error and the absolute flux signal. The HPF effectswere the larger the higher measurement height and the smaller wind velocity which were two main effects leadingto different accuracies at the different sites in this comparison.Tab. 1 Effects of high-pass filtering correction on annual flux estimates and their standard deviation (g C m−2yr−1). Treatments: BA, block averaged, LD, linear detrended, RM, recursive filter with time constants (τ ) equal todifferent fractions of sampling time (Ts)surface type BA LD RM τ=1/8 Ts RM τ=1/4 Ts RM τ=1/2 Tssmooth (n=3) -1(±0.2) -1(±0.5) -2(±0.8) -1(±0.4) -1(±0.2)rough (n=5) -8(±2.1) -16(±4.0) -27(±6.6) -15(±3.6) -7(±1.8)However, like also found earlier, the systematic effect was largely reduced, but not fully compensated by thecorrection (Tab. 2). This effect introduced systematic bias to the flux estimates caused by the choice of the HPFfilter method. Block averaging was used as a reference, but it is not clear how much the corrected flux after blockaveraging is biased by an incomplete correction. With one exception the residual differences were much smallerthan not correcting the flux for HPF.Tab. 2: Differences between high-pass filtering corrected annual flux estimates after linear detrending or recursivefiltering compared to block averaging and their standard deviation.surface type BA LD RM τ=1/8 Ts RM τ=1/4 Ts RM τ=1/2 Tssmooth (n=3) reference 2(±1.3) 3(±1.0) 1(±0.4) -1(±0.4)rough (n=5) reference 3(±0.6) 5(±1.4) 5(±0.6) 7(±1.5)The investigation raises the question, if our understanding of turbulent fluxes is yet complete, i.e. whether low-frequency flux contributions should be accounted for in plot scale flux studies or not and how these flux contribu-tions can be determined and realistically represented by model spectra. The quantitative results indicate that thisquestion is practically more relevant for forest studies than for cropland or grassland studies. With our presentationwe aim at raising a discussion about the usefulness of the suggested spectral correction approach and want to raiseawareness on accuracy and precision issues with regard to high pass filtering among users of the eddy covariancetechnique.Acknowledgements:We gratefully acknowledge funding by the EU-IMECC (Infrastructure for Measurements of the European CarbonCycle) project.

Uncertainty of carbon dioxide flux estimates introduced by different high-pass filtering methods

http://orbit.dtu.dk/ws/files/12289950/EGU2009_3979_1.pdf

Uncertainty of carbon dioxide flux estimates introduced by different high-pass filtering methods

Abstract

Similar works

Full text

Available Versions

Online Research Database In Technology