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Gap Filling Comparison Workshop, September 18-20, 2006, Jena, Germany

Eddy covariance measurements and their shortcomings for the determination of the net ecosystem exchange of carbon dioxide. Corinna Rebmann Olaf Kolle Max-Planck-Institute for Biogeochemistry Jena, Germany. Gap Filling Comparison Workshop, September 18-20, 2006, Jena, Germany. Outline.

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Gap Filling Comparison Workshop, September 18-20, 2006, Jena, Germany

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  1. Eddy covariance measurements and their shortcomings for the determination of the net ecosystem exchange of carbon dioxide Corinna Rebmann Olaf Kolle Max-Planck-Institute for Biogeochemistry Jena, Germany Gap Filling Comparison Workshop, September 18-20, 2006, Jena, Germany

  2. Outline • Introduction of measurement site and advection experiment • Reasons for data gaps • Special features of open path analyser • Consequences for final flux data • Summary

  3. Measurement Site:Wetzstein, Thuringia, Germany,flux measurements established end of 2001 tower A main tower tower B tower C tower D ADVEX’06 (April 11– June 19, 2006)flux measurements for t, H, lE, CO2 CO2, wind and temperature profiles measuring heights: Main tower: 30.0m Tower C: 29.4m

  4. The ADVEX Experiment Advection experiment CarboEurope-IP: 4 towers around the main tower: A, B, C, D: profiles of [CO2], T, u‘, v‘, w‘, T‘, tower B with CO2-fluxes below canopy, tower C and main tower with CO2-fluxes above canopy 60m

  5. Why care about advection? Eddy covariance theory is derived from tracer conservation equation with many simplifications which are only valid under homogeneous conditions

  6. Data gaps are due to • Maintenance interruptions, power failures, ice coating • Instrumental problems • Non-turbulent conditions • Unfavoured wind directions (tower effects, heterogeneous terrain) • Precipitation, fog events (open path analyser) • high wind speeds

  7. Wetzstein, main towerdata gaps (closed-path analyser)Jan 1 – Aug 24, 2006

  8. Wetzstein, main tower and tower CApr 11 – Jun 19, 2006data gaps caused by maintenance, power failures etc.

  9. Wetzstein, main tower and tower CApr 11 – Jun 19, 2006data gaps caused by maintenance, power failures etc.

  10. Wetzstein, main tower and tower CApr 11 – Jun 19, 2006time series of CO2-fluxes after pre-selection (eg Vickers & Mahrt 1997, JAOT14)

  11. Wetzstein, main tower and tower CApr 11 – Jun 19, 2006data gaps after pre-selection 30.2%

  12. Wetzstein, main tower and tower Cwhich data are rejected in case of open path-analyser? April 30 – May 12, 2006, dry period 24 of 624 half-hours (3.8%) rejected

  13. Wetzstein, main tower and tower Cwhich data are rejected in case of open path-analyser? May 13 – 28, 2006, rainy period 263 of 630 half-hours (41.7%) rejected!!!

  14. Wetzstein, main tower and tower CApr 11 – Jun 19, 2006consequences for dependencies on meteorological variables Michalis-Menten-relationship: see Falge et al. 2001, AFM107 NEE: net ecosystem exchange (µmol CO2 m−2 s−1) PPFD: photosynthetic photon flux density (µmol quantum m−2 s−1) a: ecosystem quantum yield (µmol CO2) / (µmol quantum) FGPP,sat: gross primary productivity at saturating light (µmol CO2 m−2 s−1) Rday: ecosystem respiration during the day (µmol CO2 m−2 s−1)

  15. Wetzstein, main tower and tower CApr 11 – Jun 19, 2006consequences for dependencies on meteorological variables Michalis-Menten-relationship: see Falge et al. 2001, AFM107 NEE: net ecosystem exchange (µmol CO2 m−2 s−1) PPFD: photosynthetic photon flux density (µmol quantum m−2 s−1) a: ecosystem quantum yield (µmol CO2) / (µmol quantum) FGPP,sat: gross primary productivity at saturating light (µmol CO2 m−2 s−1) Rday: ecosystem respiration during the day (µmol CO2 m−2 s−1)

  16. Wetzstein, main tower and tower Ctime series of CO2-fluxes with stationarity tests May 8 – 14, 2006

  17. Wetzstein, main tower and tower CApr 11 – Jun 19, 2006When do instationaries occur? Instationarities occur mainly at low or zero radiation conditions

  18. Wetzstein, main tower and tower CApr 11 – Jun 19, 2006data gaps summary

  19. Do we have perfect data now?Are these data reliable as input for gap filling procedures? Still missing:advective processes night flux treatment reliability check

  20. HainichDrainage/advective fluxes Data from W. Kutsch

  21. Night-flux problem • Weak turbulence • Instrumental problems, large footprints, gravity waves • Turbulent flux is influenced by other transport/storage processes →Site dependent see eg: Lee, 1998 Aubinet et al, 2003, 2005 Staebler and Fitzjarrald, 2004 Feigenwinter et al, 2004

  22. Night-flux corrections Empirical: Separate calm and turbulent periods, remove calm periods, fill the gap u*-criterion mostly used

  23. NEEnight versus u* Aubinet et al. AER30, 2000

  24. WetzsteinNEE 2005, unrealistic high night-time fluxes

  25. Wetzsteinwhen do high fluxes occur? • u*>0.4m s-1 • wind direction between 200° and 280° or 30° and 40° • neutral atmospheric conditions: stability parameter: -0.0625<ζ<0.0625 (determined by M. Zeri) → turbulent upwind mixing from the valley

  26. WetzsteinNEE 2005after application of MZ criteria for 2005: 72% data available 58% data available

  27. WetzsteinNEE 2005after application of MZ criteria

  28. Wetzsteinnight-time NEE 2005after application of MZ criteria R10=3.9 R10=3.0

  29. WetzsteinNEE comparison during advection experimentafter application of MZ criteria

  30. Summary Amount of data gaps strongly depending on: • site • type of quality check (still no common agreement in CarboEurope-IP!) • type of analyser, weather pattern • threshold criteria for u* (have to be objective, Gu et al. AFM128, 2005) Derived dependencies on meteorological variables vary with data left after selection →biased datasets Reliability has to be tested against chamber and biometric measurements

  31. Thanks for your attention! Questions?

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