Flow angles measured at a fetch-limited beech forest with clearings
Ebba Dellwik1, Jakob Mann2, Ferhat Bingöl2, Klaus Larsen3
1 Wind Energy Division, Risø-DTU
2 Wind Energy Division, R
3 Bio Sysytems Division, Risø-DTU
2 Wind Energy Division, R
3 Bio Sysytems Division, Risø-DTU
P6 in Poster presentations
Flow angles were measured with a Gill Solent R2 and a Metek USA-1 sonic anemometer. The Solent sonic was mounted at 43m height and the USA-1 at 31m height above a 26-27m high beech forest. The beech forest is characterized by inhomogeneous surface cover with clearings and several plantations of around 15m tall Noway Spruce. Further, in the direction where the forest is most homogeneous, the distance to the upwind forest edge from the mast corresponds to only 18 canopy heights. The measured flow angles were analyzed in relation to the terrain. It was possible to relate local minima and maxima to influence from the clearings or the limited fetch. During summer time, flow angles taken with the two different anemometers agreed well especially after flow distortion corrections were applied. The flow angles were also analyzed as a function of atmospheric stability. Both the stable and unstable data showed similar dependence on wind direction as the near-neutral data. The difference between the near-neutral and the stable flow angles depended on what sonic anemometer was analyzed and what flow distortion correction was applied. Since the distribution of attack angles on the sonic anemometer depended on atmospheric stability, the difference between near-neutral and stable data could be significantly biased by imperfect flow distortion corrections. The difference between flow angles measured during near-neutral, unstable and stable stratification should be a reflection of the fact that the flow reacts to the combined effect of surface inhomogeneities and atmospheric stability. It is argued that the vertical advection caused by the heterogeneous surface cover should not be included in the net ecosystem exchange since this term can only be considered representative for a relatively small area close to the mast and not for the forest as a whole.
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fp_P6 (07.12.2009 14:04)