de Mattos Zeri, LM*; Kutsch, W; Foken, T; Kolle, O; Rebmann, C; Schulze, ED: Accounting of advective fluxes in a complex terrain in Thuringia, Germany
Poster, Flux Measurements in Difficult Conditions, a Specialist Workshop, Boulder, CO, U.S.A.: 2006-01-26 - 2006-01-28

Eddy covariance was used in the past years to measure the CO2 flux over a spruce site in a low mountain range in Thuringia, Germany. New measurements have been done in order to check if the wind field disturbance caused by the hill, as described in Finnigan and Belcher (2004) [1], may be related to the CO2 balance at the site. The Wetzstein site is located on a plateau, at 785 m above sea level. The plateau is oriented in the SW-NE direction. Mean tree age is around 50 years and its height approximately 20 m. For the year 2002, the effective leaf area index was 4 m2m-2 and annual NEE was near zero, as estimated by Anthoni et al. (2004) [2]. Anthoni et al. (2004) [2] pointed out that independent soil respiration (Rs) measurements, performed in the summer of 2003, made possible a comparison between Rs and the nigh-time eddy covariance respiration (Reco). This comparison resulted in a Rs/Reco -ratio of 0.25, which is quite low when compared to a spruce site at a similar elevation (Waldstein/Weidenbrunnen, 780 m a.s.l.), where the before mentioned ratio was approximately 50 % (Rebmann et al. 2005 [3]). The authors suggested that advection could be related to these high respirative night-time fluxes. Analysis with the data collected at the tower show that high respirative fluxes occur at turbulent conditions and when the wind blows from southwest, the main wind direction. A wind model was used to simulate the general flow pattern over the topography. According to the model, strong winds coming from the main wind direction create downwind and upwind areas in the tower footprint. These patterns may be related to local circulations that promote up-hill scalars flows. In 2004 an experiment was carried out in order to calculate the horizontal advection. The vertical advection was calculated using the existing CO2 vertical profile and wind measurements at the top of the tower. The results showed that both vertical and horizontal advection terms are opposed to each other during daytime. However, after the sunset they have the same sign and contribute to reduce the total flux. New measurements were made with an additional eddy system just above the canopy, 6 m below the eddy at the top. Preliminary results show some differences between the two heights, especially during nighttime. Also, the mean vertical velocities in the two heights seem to vary differently according to wind direction.

[1] Finnigan, J.J. and S.E. Belcher, 2004, Quarterly Journal of the Royal Meteorological Society, 130(596), 1-29
[2] Anthoni, P.M., et al., 2004, Global Change Biology, 10(12), 2005-2019
[3] Rebmann, C., et al., 2005, Theoretical and Applied Climatology, 80(2-4), 121-141 <7html>

last modified 2006-01-23