Eddy-Covariance Flux Measurements in the Complex Terrain of an Alpine Valley in Switzerland |
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| Authors: | Rebecca Hiller Matthias J Zeeman Werner Eugster |
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| Institution: | (1) Institute of Geography, University of Bern, Bern, Switzerland;(2) Present address: University of Minnesota, Saint Paul, MN 55108, USA;(3) Institute of Plant Sciences, ETH Zurich, LFW C55.2, Universitaetsstrasse 2, 8092 Zurich, Switzerland |
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| Abstract: | We measured the surface energy budget of an Alpine grassland in highly complex terrain to explore possibilities and limitations
for application of the eddy-covariance technique, also for CO2 flux measurements, at such non-ideal locations. This paper focuses on the influence of complex terrain on the turbulent energy
measurements of a characteristic high Alpine grassland on Crap Alv (Alp Weissenstein) in the Swiss Alps during the growing
season 2006. Measurements were carried out on a topographic terrace with a slope of 25◦ inclination. Flux data quality is assessed via the closure of the energy budget and the quality flag method used within the
CarboEurope project. During 93% of the time the wind direction was along the main valley axis (43% upvalley and 50% downvalley
directions). During the transition times of the typical twice daily wind direction changes in a mountain valley the fraction
of high and good quality flux data reached a minimum of ≈50%, whereas during the early afternoon ≈70% of all records yielded
good to highest quality (CarboEurope flags 0 and 1). The overall energy budget closure was 74 ± 2%. An angular correction
for the shortwave energy input to the slope improved the energy budget closure slightly to 82 ± 2% for afternoon conditions.
In the daily total, the measured turbulent energy fluxes are only underestimated by around 8% of net radiation. In summary,
our results suggest that it is possible to yield realistic energy flux measurements under such conditions. We thus argue that
the Crap Alv site and similar topographically complex locations with short-statured vegetation should be well suited also
for CO2 flux measurements. |
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| Keywords: | Alpine grasslands CarboEurope project Complex terrain Energy budget closure Footprint model Quality control Valley wind system |
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