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Measuring and Modeling NEE over Complex Terrain

Measuring and Modeling NEE over Complex Terrain. Chuixiang (Tree) Yi School of Earth and Environmental Sciences Queens College, City University of New York. Acknowledgements to Russ Monson, University of Colorado Dean Anderson, USGS Brian Lamb, Washington State University

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Measuring and Modeling NEE over Complex Terrain

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  1. Measuring and Modeling NEE over Complex Terrain Chuixiang (Tree) Yi School of Earth and Environmental Sciences Queens College, City University of New York

  2. Acknowledgements to Russ Monson, University of Colorado Dean Anderson, USGS Brian Lamb, Washington State University John Zhai, University of Colorado Andrew Turnipseed, NCAR Sean Burn, University of Colorado

  3. Outline • Advection measurements • Superstable layer

  4. Advection measurements

  5. Advection Problem 1-D NEE measurement Advection flux • Idealized conditions: • A perfectly flat topography • Homogenous vegetation

  6. Eddy Flux Tower CO2H2O CH4 T CO2H2O CH4 T CO2H2O CH4 T Not problem in daytime

  7. Canopy flows are separated by asuperstable layer in calm nights. Courtesy of Jielun Sun Super-stable layer, flow separation (Yi et al., 2005) The sensors cannot properly account for scalars advected away by drainage flows below the superstable layer.

  8. Problem: nighttime NEE is negative in calm conditions Davis et al. 2004

  9. Examining advection issues at AmerifluxNiwot Ridge site

  10. 3-D measurement system North tower East tower West tower South tower

  11. Wind Speed Air Temperature h 0 Zr Vertical exchange zone Super-stable layer - Horizontal exchange zone Zs + Yi et al., 2008 Vertical advection Nighttime dc/dz is always negative

  12. Lee’s assumption Zr Zs Super-stable layer Yi et al., 2008 Vertical advection

  13. Lee’s assumption Zr Zs Super-stable layer Yi et al., 2008 Vertical advection

  14. Horizontal CO2 gradient Horizontal CO2 gradient Horizontal CO2 gradient Horizontal advection calculation Yi et al., 2008

  15. The CO2 gradient that is closer to wind direction is first choice to use. For example, the angle can be limited by Horizontal CO2 gradient Horizontal advection calculation Yi et al., 2008

  16. V Wind direction V CO2 gradient U U Another horizontal advection calculation Projection of CO2 gradient into wind direction dc/dr is CO2 gradient along a pair towers Projection of CO2 gradient into x and y directions x-y coordinate system is fixed on an instrument Yi et al., 2008

  17. Comparison between two methods Good agreement between Two algorithms Udc/dx+Vdc/dy udc/drcos(q) Yi et al., 2008

  18. Advection fluxes are functions of u* Vertical advection is opposite to horizontal advection Yi et al., 2008

  19. 6-year cumulative NEE NEE+Fhadv NEE+Ftadv NEE+u*filter NEE NEE+Fvadv Yi et al., 2008

  20. Comparison to u* filter at 0.2 Horizontal advection Vertical advection u* correction Yi et al., 2008

  21. Advection flux is not a function of soil temperature Advection flux Yi et al., 2008 Soil temperature (oC)

  22. Turbulence Bacterial Nocturnal NEE Advection correction Biological correction

  23. Summary Advection fluxes have no correlation to drivers of biological activity. Conversely, the biological correction (u* filter) is based on climatic responses of organisms, and has no physical connection to aerodynamic processes. Conclusion We must accept the fact that the biological correction has no physical bases

  24. Superstable layer

  25. Canopy flows are separated by asuperstable layer in calm nights. Courtesy of Jielun Sun Super-stable layer, flow separation (Yi et al., 2005) The sensors cannot properly account for scalars advected away by drainage flows below the superstable layer.

  26. A super stable layer Wind Speed Air Temperature h Vertical exchange zone Horizontal exchange zone (1) Slow mean airflow; (2) Maximum drag elements; (3) Minimum vertical exchange; (4) Maximum horizontal CO2 (or other scalar) gradient; (5) Maximum ratio of wake and shear production rate.

  27. The existence of a super-stable layer was validated by SF6experiments Yi et al. 2005

  28. Horizontal CO2gradients observed at Niwot Ridge AmeriFlux site in summer nights of 2002 Horizontal CO2 gradients are maximum at the super stable layer Yi et al. 2008

  29. -22 -24 -26 dR-canopy (‰) -28 -30 NWR Canopy Inlets Bowling et al. -32 0 1 2 3 4 -23 VPD (kPa) -24 dR (‰) -25 -22 -26 -24 -27 0.05 0.1 0.15 0.2 q (m3 m-3) -26 -22 dR-ground (‰) -28 -24 -26 dR (‰) -30 -28 NWR Ground Inlets Niwot Ridge -32 -30 0.05 0.1 0.15 0.2 Bowling et al. -32 0 1 2 3 4 θ (m3 m-3) VPD (kPa) A superstable layer is useful for understanding C-13 data problem Whole canopy layer After dividedby Super-stable layer Above SSL Super-stable layer Below SSL (Schaeffer et al., 2008)

  30. Reminding When you are puzzled with canopy layer data in clam nights, please don’t forget the superstable layer.

  31. Thank You!

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