Turbulence parameter inside and above a tall spruce site
Tobias Biermann1, Katharina Staudt1, Andrei Serafimovich1, Thomas Foken1
1 Abt. Mikrometeorologie, Universität Bayreuth
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For most flux measurements it is crucial that the turbulent flow field is well developed. Integral turbulence characteristics, the normalized standard deviation of a turbulent quantity, can be used to describe the structure of turbulence therefore a comparison between measured and predicted values is used in quality assessment. Since there is no uniform theory for the parameterizations of integral turbulence characteristics inside a forest different approaches were tested. Observations of turbulence structure were obtained by a vertical profile of sonic anemometers covering all parts of the forest up to the lower part of the roughness sub layer during the EGER (ExchanGE processes in mountainous Regions) project in fall 2007 at the BayCEER research site Weidenbrunnen, a 23 m tall spruce site located in the Fichtelgebirge Mountains in North-Eastern Bavaria. Field observations are complemented by simulations of ACASA model (Advanced Canopy-Atmosphere-Soil Algorithm). Profiles of the integral turbulence characteristics show a strong dependency on stratification and inside a forest on the structure of the stand. The comparison of different approaches showed that the wind components should be parameterized with a dimensionless height z* = hc L-1 instead of z = z L-1, which is used above short vegetation. The comparison also showed a great dependency on the stand structure, the parameterizations therefore also need to be adjusted to the different measurement levels inside the stand. Selecting the profiles of the integral turbulence characteristics by coupling situations between the atmosphere above and inside the stand did not reveal a significant different behavior than a selection according to stratification above the canopy. A comparison between the measured values and model results from the ACASA model showed a fairly good agreement for the normalized wind speed but the integral turbulence characteristics of the wind components were not well represented. For the quality control of flux data inside and above a forest a combined parameterization which accounts for stability and stand structure dependency is recommended an can not be replaced by a model.
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