Eddy Covariance Flux Measurements in Nocturnal conditions

1. Eddy Covariance Flux Measurements in Nocturnal conditions Marc Aubinet Aubinet, Ecol. Appl., in prep. Open Science Conference. The GHG Cycle in the Northern Atmosphere

2. Content • Quantitative vs qualitative approach • Drainage flows • Intermittent turbulence • Representativeness in well developed turbulence • Conclusions

3. The problems that challenge night eddy covariance measurements(Massman and Lee, 2002) • High frequencies not captured • Insufficient resolution for small scale turbulence • Too small averaging times • Advection is significant at the sites • Measurement system decoupled from surface • Footprint problems • Similarity relations not valid • Stationarity criteria not filled

4. Qualitative approach: Description of the processes at work in the stable boundary layer • Turbulent ramps (TR) (Cava et al.,2004) • Small scale turbulence (SST) (Mahrt and Vickers, 2005) • Gravity waves (GW) (Cava et al., 2004) • Drainage flows (DF) (Aubinet et al., 2003) • Land breezes (LB) (Sun et al., 1998) • Intermittent turbulence (IT) (Coulter and Doran,2002)

5.  : No problem ! : Not critical problem !!! : Critical problem TR : Turbulent ramps SS : Small scale turbulence GW : Gravity waves DF : Drainage flow LB : Land breeze IT : Intermittent turbulence

6.  : No problem ! : Not critical problem !!! : Critical problem TR : Turbulent ramps SS : Small scale turbulence GW : Gravity waves DF : Drainage flow LB : Land breeze IT : Intermittent turbulence

7.  : No problem ! : Not critical problem !!! : Critical problem TR : Turbulent ramps SS : Small scale turbulence GW : Gravity waves DF : Drainage flow LB : Land breeze IT : Intermittent turbulence

8.  : No problem ! : Not critical problem !!! : Critical problem TR : Turbulent ramps SS : Small scale turbulence GW : Gravity waves DF : Drainage flow LB : Land breeze IT : Intermittent turbulence

9.  : No problem ! : Not critical problem !!! : Critical problem TR : Turbulent ramps SS : Small scale turbulence GW : Gravity waves DF : Drainage flow LB : Land breeze IT : Intermittent turbulence

10.  : No problem ! : Not critical problem !!! : Critical problem TR : Turbulent ramps SS : Small scale turbulence GW : Gravity waves DF : Drainage flow LB : Land breeze IT : Intermittent turbulence

11.  : No problem ! : Not critical problem !!! : Critical problem TR : Turbulent ramps SS : Small scale turbulence GW : Gravity waves DF : Drainage flow LB : Land breeze IT : Intermittent turbulence

12. Content • Introduction • Quantitative vs qualitative approach • Drainage flows • Intermittent turbulence • Representativeness in well developed turbulence • Conclusions

13. Drainage flows / Land breezes • Very common (slope < 1%) • Apparition of sublayers (→ decoupling) • Induce advection • Different patterns according to topography and land cover

14. Drainage flow typology • Momentum equation (i.e. Staebler and Fitzjarrald, 2005): • Implication for mass flow in stationary conditions:

15. Drainage flow typology

16. Drainage flow typology

17. Drainage flow typology

18. Drainage flow typology

19. Simple CO2 budget model (Aubinet et al., 2005): 2 Dimensions Transport only due to advection Mass conservation + tracer Negative vertical [CO2] gradients   Conclusion of the model In convergence flows, horizontal advection sign depends on source heterogeneities. Drainage flow typology

20. Drainage flow typology

21. Drainage flow typology

22. Drainage flow typology Vielsalm (Aubinet, 2003) Tharandt ? (Feigenwinter, 2004)

23. Tharandt Vielsalm Feigenwinter et al., BLM, 2004 Aubinet et al., BLM, 2003 • Impact on CO2 balance not clear (net advection < uncertainty)

24. Drainage flow typology Vielsalm (Aubinet, 2003) Tharandt ? (Feigenwinter, 2004) Browns River ? Prince Albert ? (Lee, 1998) Hyytialla

25. Hyytialla (Vesala poster, this session)

26. Drainage flow typology Vielsalm (Aubinet, 2003) Tharandt ? (Feigenwinter, 2004) Browns River ? Prince Albert ? (Lee, 1998) Hyytialla Renon ? (Marcolla, 2005)

27. Renon (Marcolla et al., AFM, 2005)

28. Drainage flow typology Vielsalm (Aubinet, 2003) Tharandt (Feigenwinter, 2004) Niwot Ridge ? (Turnipseed, 2003) Browns River ? Prince Albert ? (Lee, 1998) Hyytialla Renon ? (Marcolla, 2005)

29. Niwot Ridge(Turnipseed et al., 2003)

30. Drainage flow typology Vielsalm (Aubinet, 2003) Tharandt (Feigenwinter, 2004) Hesse (Aubinet 2005) Wetzstein (Feigenwinter talk) Niwot Ridge ? (Turnipseed, 2003) Browns River ? Prince Albert ? (Lee, 1998) Hyytialla Renon ? (Marcolla, 2005)

31. Hesse (Aubinet et al., 2005)

32. Norunda !!! (Feigenwinter, this session)

33. Drainage flow typology Vielsalm (Aubinet, 2003) Tharandt ? (Feigenwinter, 2004) Hesse (Aubinet 2005) Wetzstein Niwot Ridge ? (Turnipseed, 2003) Browns River ? Prince Albert ? (Lee, 1998) Hyytialla (Vesala poster) Renon ? (Marcolla, 2005) Harvard ? Staebler and Fitzjarrald (2003) Morgan Monroe? Froelich (2005) Norunda

34. Plan • Introduction • Quantitative vs qualitative approach • Drainage flows • Intermittent turbulence • Representativeness in well developed turbulence • Conclusions

35.  : No problem ! : Not critical problem !!! : Critical problem TR : Turbulent ramps SS : Small scale turbulence GW : Gravity waves DF : Drainage flow LB : Land breeze IT : Intermittent turbulence

36. Intermittent turbulence • Multiple causes: • Low level jet • Density currents • Kelvin Helmholtz waves • Advection • Produces strong turbulence during limited period • Stationarity conditions not filled

37. Intermittent turbulence • Destroys flow pattern that establishes during calm periods (drainage flow, land breeze) • Removes CO2 stored during calm periods • Impact on the measurements: • The flux is the most important during IT periods (50% of the flux during 20% of the time – Coulter and Doran, 2002) • The processes are non stationary (skipped by quality control tests) • Turbulent transport only?

38. Intermittent turbulence • Are we able to measure fluxes correctly during intermittent turbulence ? • What is the representativeness of these fluxes ?

39. Case study : Neustift (Wohlfhart et al., AFM, 2005)

40. Plan • Introduction • Quantitative vs qualitative approach • Drainage flows • Intermittent turbulence • Representativeness in well developed turbulence • Conclusions

41.  : No problem ! : Not critical problem !!! : Critical problem TR : Turbulent ramps SS : Small scale turbulence GW : Gravity waves DF : Drainage flow LB : Land breeze IT : Intermittent turbulence

42. Chequamegon-Nicolet National Forest (Northern Wisconsin) (Cook et al., 2004)

43. Wetzstein site Kolle (pers. Comm.)

44. Conclusions • Night flux error results from several different processes. • Main processes are advection and intermittent turbulence. • If site is affected by advection, the u* correction remains the best way to correct the fluxes. • If site is affected by intermittent turbulence, stationary criterion is needed. • These processes should be identified at the sites in order to allow implementing an adapted night flux correction at each site.

45. Thank you !