1. 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. 3

5. 5

6. Eddy Flux Tower CO 2 H 2 O CH 4 T CO 2 H 2 O CH 4 T CO 2 H 2 O CH 4 T

7. Super-stable layer, flow separation (Yi et al., 2005) Courtesy of Jielun Sun Canopy flows are separated by a superstable layer in calm nights.

8. Davis et al. 2004

10. West tower Southtower North tower East tower

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

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

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

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

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

16. Another horizontal advection calculation Projection of CO2 gradient into wind direction Projection of CO2 gradient into x and y directions UV CO 2 gradient Wind direction U V dc/dr is CO2 gradient along a pair towers x-y coordinate system is fixed on an instrument Yi et al., 2008

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

19. NEE+F hadv NEE+F tadv NEE+u*filter NEE NEE+F vadv

20. Yi et al., 2008 Vertical advection Horizontal advection u* correction

21. Soil temperature ( o C) Advection flux Yi et al., 2008

22. Advection correction Biologicalcorrection Turbulence Bacterial

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. 24

25. Super-stable layer, flow separation (Yi et al., 2005) Courtesy of Jielun Sun Canopy flows are separated by a superstable layer in calm nights.

26. h Wind Speed Air Temperature (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. Vertical exchange zone Horizontal exchange zone

27. Yi et al. 2005

28. Yi et al. 2008 Horizontal CO2 gradients are maximum at the super stable layer

29. VPD (kPa) 01234 -32 -30 -28 -26 -24 -22 Niwot Ridge Bowling et al.  R (‰) 0.050.10.150.2 -27 -26 -25 -24 -23  (m 3 m -3 )  R (‰) Whole canopy layer (Schaeffer et al., 2008) 01234 -32 -30 -28 -26 -24 -22 VPD (kPa) NWR Canopy Inlets Bowling et al.  R-canopy (‰) 0.050.10.150.2 -32 -30 -28 -26 -24 -22 NWR Ground Inlets  R-ground (‰) θ (m 3 m -3 ) Super-stable layer After divided by Super-stable layer Above SSL Below SSL

30. Reminding