Uni-Bayreuth grafik-uni-bayreuth



Turbulence parameter inside and above a tall spruce site

Tobias Biermann1, Katharina Staudt1, Andrei Serafimovich1, Thomas Foken1
1 Mikrometeorologie, Universität Bayreuth,

P 1.5 in Funktion von Ökosystemen

Integral turbulence characteristics, the normalized standard deviation of a turbulent quantity, can be used to describe the structure of turbulence. A comparison between measured and predicted values of integral turbulence characteristics shows whether turbulence is fully developed and is therefore used in quality assessment. In order to improve the data quality assessment of flux data different parameterization approaches for integral turbulence characteristics were tested, since there is no uniform theory for parameterizations inside a forest available. 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 September and October 2007. In the EGER project, different physical, chemical and biological processes in the soil-vegetation-boundary-layer system were investigated. The field experiments were performed at the BayCEER research site Weidenbrunnen (Waldstein), 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 parameterizations showed that in order to parameterize the integral turbulence characteristics of the wind components inside the roughness sub layer under different stratification conditions a dimensionless height z* = hc L-1 should be used as scaling parameter instead of z = z L-1, which is used above short vegetation. Since Profiles of integral turbulence characteristics from different ecosystems show that the decrease inside the roughness sub layer is similar but that parameterizations of profiles can not be generalized due to different stand structures the parameterizations 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 usually overestimated above the canopy and underestimated inside the trunk space and is therefore no alternative for the parameterizations. 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.

Letzte Änderung 25.03.2009