Nocturnal Respiration: Can we use the Temporal Dynamics of Advection to Derive an Alternative to the u*-Threshold Filtering Technique?
Even moderately complex topography can lead to significant horizontal and vertical advection and a consequent underestimation of nocturnal CO2 fluxes derived from eddy covariance measurements on a single tower. The presence of advective transport is identified using the time series of CO2 concentrations. Given a continuous input of carbon (soil-, wood- and leaf respiration) into a control volume (CV) we expect concentrations in that control volume to increase through time if there is no flux through the upper lid and no advective transport. Time series measurements usually show a strong increase in carbon concentration once the atmosphere is stably stratified, but after this initially strong increase the temporal build-up of concentrations is slower and may even remain relatively constant. As this steady-state situation is not associated with fluxes through the top of the CV we can assume that advection is draining CO2 away. We have analysed data from 24 Fluxnet sites where turbulent flux (Fc) and change of storage (Sc) of CO2 are routinely measured. Analysis of Sc confirms the observation by Aubinet et al.  that a common feature in canopy flows is that a maximum rate of change of CO2 occurs in the early evening. This suggests that after this maximum, the advection terms in the mass balance become non negligible. Based on this observation we have developed an alternative to the u*-threshold filtering technique . We have shown that applying this alternative approach leads to good agreement between eddy covariance derived estimates of respiration and independent chamber measurements on a study site in a medium dense Eucalyptus forest in south-east Australia. In this contribution we discuss the applicability of this approach for several sites ranging from the tropics to subpolar latitudes and located in terrain that varies from almost ideally flat to very complex. We compare results from three different estimates of respiration and discuss differences between them.  Aubinet, M., Berbigier, P., Bernhofer, Ch., Cescatti, A., Feigenwinter, C., Granier, A., Gruenwald, Th., Havrankova, K., Heinesch, B., Longdoz. B., Marcolla, B., Monagnani, L. and Sedlak, P., 2005, Comparing CO2 storage and advection conditions at night at different Carboeuroflux sites. Boundary-Layer Meteorol., 116, 63-94.  van Gorsel, E., Leuning, R., Cleugh, H. A., Keith, H. and Suni, T. 2007. Nocturnal carbon efflux: reconciliation of eddy covariance and chamber measurements using an alternative to the u*-threshold filtering technique. Tellus, 59B, 397-403.
AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- 0428 Carbon cycling (4806)