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Boundary-Layer Meteorology

, Volume 169, Issue 2, pp 163–184 | Cite as

Ejective and Sweeping Motions Above a Peatland and Their Role in Relaxed-Eddy-Accumulation Measurements and Turbulent Transport Modelling

  • Gabriel Katul
  • Olli PeltolaEmail author
  • Tiia Grönholm
  • Samuli Launiainen
  • Ivan Mammarella
  • Timo Vesala
Research Article
  • 321 Downloads

Abstract

The three turbulent velocity components, water vapour (\(\text {H}_2\text {O}\)), carbon dioxide (\(\text {CO}_{2}\)), and methane (\(\text {CH}_{4}\)) concentration fluctuations are measured above a boreal peatland and analyzed using conditional sampling and quadrant analysis. The overarching question to be addressed is to what degree lower-order cumulant expansion methods describe transport efficiency and the relative importance of ejections and sweeps to momentum, \(\text {CH}_{4}\), \(\text {CO}_{2}\) and \(\text {H}_2\text {O}\) fluxes across a range of atmospheric flow regimes. The patchy peatland surface creates distinctly different source and sink distributions for the three scalars in space and time thereby adding to the uniqueness of the set-up. The measured and modelled fractional contributions to the momentum flux show that sweep events dominate over ejections in agreement with prior studies conducted in the roughness sublayer. For scalar fluxes, ejections dominate the turbulent fluxes over sweeps. While ejective motions persist longer for momentum transport, sweeping events persist longer for all three scalars. Third-order cumulant expansions describe many of the results detailed above, and the results are surprising given the highly non-Gaussian distribution of \(\text {CH}_{4}\) turbulent fluctuations. Connections between the asymmetric contributions of sweeps and ejections and the flux-transport term arising in scalar turbulent-flux-budget closure are derived and shown to agree reasonably well with measurements. The proposed model derived here is much simpler than prior structural models used to describe laboratory experiments. Implications of such asymmetric contributions on, (i) the usage of the now proliferating relaxed-eddy-accumulation method in turbulent flux measurements, (ii) the constant-flux assumption, and (iii) gradient-diffusion closure models are presented.

Keywords

Closure models Cumulant expansion Ejections and sweeps Methane flux Relaxed eddy accumulation 

Notes

Acknowledgements

G.K. acknowledges support from the National Science Foundation (NSF-EAR-1344703, NSF-DGE-1068871), and the U.S. Department of Energy (DOE) through the office of Biological and Environmental Research (BER) Terrestrial Ecosystem Science (TES) Program (DE-SC0011461). T.V., O.P., and T.G. acknowledge support from the Academy of Finland Center of Excellence (Project Nos. 272041 and 118780) and Academy Professor projects (Nos. 1284701 and 1282842), ICOS-Finland (Project No. 281255) and CARB-ARC (Project No. 286190) funded by the Academy of Finland and the AtMath project funded by University of Helsinki. S.L. acknowledges support from the Academy of Finland Academy Research Fellow Project (Nos. 296116 and 307192).

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© Springer Nature B.V. 2018

Authors and Affiliations

  1. 1.Nicholas School of the EnvironmentDuke UniversityDurhamUSA
  2. 2.Department of Civil and Environmental EngineeringDuke UniversityDurhamUSA
  3. 3.Institute for Atmospheric and Earth System Research/Physics, Faculty of ScienceUniversity of HelsinkiHelsinkiFinland
  4. 4.Natural Resources Institute FinlandEnvironmental Impacts of ProductionHelsinkiFinland
  5. 5.Institute for Atmospheric and Earth System Research/Forest Sciences, Faculty of Agriculture and ForestryUniversity of HelsinkiHelsinkiFinland

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