Schröter, J; Lüers, J; Ruppert, J; Thomas, C; Foken, T*: Advective influence on the CO2-exchange of a forest site
Poster, Flux Measurements in Difficult Conditions, a Specialist Workshop, Boulder, CO, U.S.A.: 2006-01-26 - 2006-01-28

The underestimation of the nocturnal NEE (Net Ecosystem Exchange) by measurements of the turbulent flux and the storage flux occurs frequently at the flux station ‘WE’ Waldstein Weidenbrunnen as at many other forest sites. As the Weidenbrunnen site is situated on an extensive slope, it was suspected that an undetected advective transport of CO2 caused by gravity flows (drainage flows) in the trunk space may account for the nocturnal flux deficit as found by other researchers [1].
To test this hypothesis, measurements of horizontal and vertical CO2-advection were performed during an intensive measurement campaign of 20 day duration in July 2003 within the framework of the WALDATEM 2003 Experiment [4]. Vertical advection was measured on the main tower (height: 32 m, canopy height 19 m) as proposed by Lee [3]. Horizontal advection in the trunk space was determined by measuring wind speed and CO2 gradients in two heights along and across the slope. Both wind field and CO2 gradients showed a dependence on stability, which suggests a systematic direction of advection. Vertical and horizontal advective fluxes showed a typical mean daily course but with opposite sign. Horizontal advection was negative at night, which implies a transport of CO2 into the control volume, while vertical advection in contrast was positive, implying a flow of CO2 out of the control volume. Both fluxes were on the same order of magnitude as the nocturnal eddy covariance (EC) flux. During the day, vertical advection tended to zero, whereas horizontal advection showed small but positive values. Horizontal advection was caused by gravity flows down the slope as well as by the mean wind across the slope.
The total advective budget was dominated by the positive nocturnal vertical advection resulting, in terms of figures, in a mean advective flux of 0.59 gCm-2d-1 out of the control volume. This is in line with the idea of a nocturnal flux deficit caused by advection. However, the measured advective fluxes showed a large day to day variability and were affected with large inaccuracies due to technical problems and simplifications in the temporal and spatial resolution of data sampling. But the results, similar to those found in other studies [1, 2] indicate that advection plays a significant role in the CO2 exchange of the Weidenbrunnen site.

[1] M. Aubinet, B. Heinesch, M. Yernaux,, 2003, Boundary-Layer Meteorol., 108, 397–417.
[2] C. Feigenwinter, C. Bernhofer, R. Vogt, 2004, Boundary-Layer Meteorol., 113, 201–224.
[3] X. Lee, 1998,. Agric. For. Meteorol., 91, 39–50.
[4] C. Thomas, J. Ruppert, J. Lüers, J. Schröter, J.-C. Mayer, T. Bertolini, 2005, Arbeitsergebnisse, Universität Bayreuth, Abt. Mikrometeorologie, Print ISSN 1614-8916, 27, 46pp.

last modified 2006-01-23